Climate change in the United States

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United States projected Köppen climate classification map for 2071 to 2100

U.S. temperature record from 1950 to 2009 according to the National Oceanic and Atmospheric Administration (NOAA)

Climate change in the United States refers to historical changes in the climate of the United States, as well as the regional climactic, economic, and cultural responses to global warming.

The current effects of global warming in the United States are widespread and varied. In 2012, the United States experienced its warmest year on record. As of 2012, the thirteen warmest years for the entire planet have all occurred since 1998, transcending those from 1880. Different regions experience widely different climatic changes. Changes in climate in the regions of the United States appear significant. For example, drought conditions appear to be worsening in the southwest while improving in the northeast. Some research has warned against possible problems due to American climate changes such as the spread of invasive species and possibilities of floods as well as droughts.^[4] Climate change is seen as a national security threat to the United States.

The United States is among the most significant emitters of greenhouse gasses in the world. In terms of both total and per capita emissions, it is among the largest contributors.

As of April 2019, 69% of Americans think that climate change is happening and 55% think that it is mostly human caused.

In 2015, according to The New York Times and others, oil companies knew that burning oil and gas could cause global warming since the 1970s but, nonetheless, funded deniers for years. 2016 was a historic year for billion-dollar weather and climate disasters in U.S.

Greenhouse gas emissions by the United States

The United States emitted 5.4 billion metric tons carbon dioxide equivalent of greenhouse gas in 2018, the second largest in the world after greenhouse gas emissions by China. This is over 15 tonnes per person and, amongst the top ten emitters, is the second highest country by greenhouse gas emissions per person after Canada. Because coal-fired power stations are gradually shutting down, in the 2010s emissions from electricity generation fell to second place behind transportation.

Current and potential effects of climate change in the United States

The United States Environmental Protection Agency's (EPA) website provides information on climate change: EPA Climate Change. Climate change is a problem that is affecting people and the environment. Human-induced climate change has, e.g., the potential to alter the prevalence and severity of extreme weathers such as heat waves, cold waves, storms, floods and droughts.^[] A report released in March 2012 by the Intergovernmental Panel on Climate Change (IPCC) confirmed that a strong body of evidence links global warming to an increase in heat waves, a rise in episodes of heavy rainfall and other precipitation, and more frequent coastal flooding.^[15][16] The U.S. had its warmest March–May on record in 2012.^[17] (See March 2012 North American heat wave)

According to the American government's Climate Change Science Program, "With continued global warming, heat waves and heavy downpours are very likely to further increase in frequency and intensity. Substantial areas of North America are likely to have more frequent droughts of greater severity. Hurricane wind speeds, rainfall intensity, and storm surge levels are likely to increase. The strongest cold season storms are likely to become more frequent, with stronger winds and more extreme wave heights."^[18]

National temperature, sea level, and precipitation

This graph shows average drought conditions in the contiguous 48 states, according to the EPA, with yearly data going from 1895 to 2011. The curve is a nine-year weighted average.

The general effect of climate changes has been found in the journal Nature Climate Change to have caused increased likelihood of heat waves and extensive downpours.^[19] Concerns exist that, as stated by a National Institutes of Health (NIH) study in 2003, increasing "heat and humidity, at least partially related to anthropogenic climate change, suggest that a long-term increase in heat-related mortality could occur." However, the report found that, in general, "over the past 35 years, the U.S. populace has become systematically less affected by hot and humid weather conditions" while "mortality during heat stress events has declined despite increasingly stressful weather conditions in many urban and suburban areas." Thus, as stated in the study, "there is no simple association between increased heat wave duration or intensity and higher mortality rates" with current death rates being largely preventable, the NIH deeply urging American public health officials and physicians to inform patients about mitigating heat-related weather and climate effects on their bodies.^[20]

In terms of U.S. droughts, a study published in Geophysical Research Letters in 2006 about the U.S. reported, "Droughts have, for the most part, become shorter, less frequent, and cover a smaller portion of the country over the last century." It also stated that the "main exception is the Southwest and parts of the interior of the West" where "drought duration and severity... have increased."^[3] Sea level rise has taken place in the U.S. for decades, going back to the 19th century. As stated in research published by the Proceedings of the National Academy of Sciences, west coast sea levels have increased by an average of 2.1 millimeters annually. In English notation, that equates to 0.083 inches per year and 0.83 inches per decade.^[21]

As shown in the adjacent image, wet and rainy conditions versus moments of drought in the U.S. have varied significantly over the past several decades. Average conditions for the 48 contiguous states flashed into extreme drought in the mid-1930s 'dust bowl' era as well as during the turn of the 20th century. In comparison, the mid-2000s decade and mid-1890s experienced only slight drought and had mitigating rainy periods.^[22] The National Drought Mitigation Center has reported that financial assistance from the government alone in the 1930s dry period may have been as high as $1 billion (in 1930s dollars) by the end of the drought.^[23]

Climate scientists have hypothesized that the stratospheric polar vortex jet stream will gradually weaken as a result of global warming and thus influence U.S. conditions.^[24][25][26] This trend could possibly cause changes in the future such as increasing frost in certain areas. The magazine Scientific American noted in December 2014 that ice cover on the Great Lakes had recently "reached its second-greatest extent on record", showing climate variability.^[25]

In July 2012, the National Oceanic and Atmospheric Administration (NOAA) reported that the 12-month period July 2011 to June 2012 was the warmest 12-month period on record in the continental United States, with average temperature 3.23 °F above the average for the 20th century.^[27] Earlier it was reported that exceptionally warm months between January and May 2012 had made the 12 month previous to June 2012 the warmest 12-month block since record keeping began,^[28] but this record was exceeded by the July 2011 to June 2012 period. NOAA stated that the odds of the July 2011 to June 2012 high temperatures occurring randomly was 1 in 1,594,323.^[27]

Other effects of both general climatic change and man-made climate change combine with the temperature precipitation effects of climate change to exacerbate conditions.

Locations in the US with low altitude above sealevel

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  Western United States
  
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  San Francisco
  
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  Southeastern United States
  
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  New Orleans
  
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  Northeastern United States
  
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  Washington D.C.
  
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  New York City
  
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  Boston
  

Extreme weather events

Warmer air can contain more water vapor than cooler air. Global analyses show that the amount of water vapor in the atmosphere has in fact increased due to human-caused warming. This extra moisture is available to storm systems, resulting in heavier rainfalls.

The number and severity of high-cost extreme weather events has increased in the 21st century in the United States. By August 2011 alone, the NOAA had registered nine distinct extreme weather disasters for that year, each totalling $1 billion or more in economic losses. Total losses for 2011 were evaluated as more than $35 billion before Hurricane Irene.^[29]

Though the costs and frequency of cyclones have increased on the east coast, it remains unclear whether these effects have been driven primarily by climate change.^[19][30] When correcting for this, a comprehensive 2006 article in Geophysical Research Letters found "no significant change in global net tropical cyclone activity" during past decades, a period when considerable warming of ocean water temperatures occurred. However, the study found major regional shifts, including a general rise of activity in the North Atlantic area, including on the U.S. eastern coast.^[31]

From 1898 through 1913, there have been 27 cold waves which totalled 58 days. Between 1970 and 1989, there were about 12 such events. From 1989 until January 6, 2014, there were none. The one on the latter date caused consternation because of decreased frequency of such experiences.^[32]

Looking at the lack of certainty as to the causes of the 1995 to present increase in Atlantic extreme storm activity, a 2007 article in Nature used proxy records of vertical wind shear and sea surface temperature to create a long-term model. The authors found that "the average frequency of major hurricanes decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s." As well, they also found that "hurricane activity since 1995 is not unusual compared to other periods of high hurricane activity in the record and thus appears to represent a recovery to normal hurricane activity, rather than a direct response to increasing sea surface temperature." The researches stated that future evaluations of climate change effects should focus on the magnitude of vertical wind shear for answers.^[33]

The frequency of tornadoes in the U.S. have increased, and some of this trend takes place due to climatological changes though other factors such as better detection technologies also play large roles. According to a 2003 study in Climate Research, the total tornado hazards resulting in injury, death, or economic loss "shows a steady decline since the 1980s". As well, the authors reported that tornado "deaths and injuries decreased over the past fifty years". They state that addition research must look into regional and temporal variability in the future.^[34]

Human effects: health, economy and agriculture

The 2018 the Fourth National Climate Assessment notes that regional economies dominated by agriculture or tourism may have additional vulnerabilities from climate change.^[35] Joseph Stiglitz, Nobel prize-winning economist, notes that climate-related disasters in 2017 cost the equivalent of 1.5% of GDP.^[36]

Crop and livestock production will be increasingly challenged. Threats to human health will increase.^[37]

A 2012 report in Nature Climate Change stated that there is reason to be concerned that American climate changes could increase food insecurity by reducing grain yields, with the authors noting as well that substantial other facts exist influencing food prices as such as government mandates turning food into fuel and fluctuating transport costs. The researchers concluded that U.S. corn price volatility would moderately increase with American warming with relatively modest rises in food prices assuming that market competition and integration partly mitigated climate affects. They warned that biofuels mandates would, if present, widely increase corn price sensitivity to U.S. warming.^[38]

Environmental Racism

There is a growing Climate Gap and prevalence of Environmental Racismin the U.S. Climate change will change the United States' current systems/opportunities for low paying jobs typically held by BIPOC and low SES people, such as agriculture and tourism. Job opportunities in these fields are expected to decrease and become more taxing on workers due to harsher conditions. Based on a study done on Environmental inequities in California, people of color and people of low socioeconomic status populate the cities with the worst air quality in the state, putting these groups at increased risk for being exposed to harmful air pollutants. Additionally, many low SES individuals may not have adequate access to healthcare to rectify their disproportionate exposure, causing long term health issues. Racialized families spend higher percentages of their income on basic necessities, and will be disproportionately challenged by rising food prices and other basic necessities, further widening the wealth gap between social classes in the US. One of the most prevalent contributors to health inequity in the food system is climate change.  Higher food prices will continue to contribute to low SES individuals experiencing an increase in food insecurity, and diets that contribute to malnutrition and obesity. The food system will also see an increase in the spread of diseases such as cholera and filariasis.

Based on a study done on California's population, low SES neighborhoods and neighborhoods occupied by people of color experience heatwaves more frequently, and they effects there are most harsh due to being situated in the middle of cities, a phenomenon called the Heat Island Effect. Additionally, racialized individuals are less likely to have access to air conditioning and transportation to relief stations, doubling the African American mortality rate caused by heat waves in Los Angeles.

Policy

Federal, state, and local governments have all debated climate change policies, but the resulting laws vary considerably. The U.S. Congress has not adopted a comprehensive greenhouse gas emissions reduction scheme, but long-standing environmental laws such as the Clean Air Act have been used by the executive branch and litigants in lawsuits to implement regulations and voluntary agreements.

The federal government has the exclusive power to regulate emissions from motor vehicles, but has granted the state of California a waiver to adopt more stringent regulations. Other states may choose to adopt either the federal or California rules. Individual states retain the power to regulate emissions from electrical generation and industrial sources, and some have done so. Building codes are controlled by state and local governments, and in some cases have been altered to require increased energy efficiency. Governments at all levels have the option of reducing emissions from their own operations such as through improvements to buildings, purchasing alternative fuel vehicles, and reducing waste; and some have done so.

Political opponents to emissions regulations argue that such measures reduce economic activity in the fossil fuel industry (which is a substantial extractive industry in the United States), and impose unwanted costs on drivers, electricity users, and building owners. Some also argue that stringent environmental regulations infringe on individual liberty, and that the environmental impact of economic activity should be driven by the informed choices of consumers. Regulatory proponents argue that the economy is not a zero-sum game, and that individual choices have proven insufficient to prevent damaging and costly levels of global warming. Some states have financed programs to boost employment in green energy industries, such as production of wind turbines. Areas heavily dependent on coal production have not taken such steps and are suffering economic recession due to both competition from now lower-priced natural gas and environmental rules that make generation of electricity from coal disadvantageous due to high emissions of CO₂ and other pollutants compared to other fuels.

History of federal policy and international agreements

The United States, although a signatory to the 1997 Kyoto Protocol, under President Clinton, neither ratified nor withdrew from the protocol. In 1997, the U.S. Senate voted unanimously under the Byrd–Hagel Resolution that it was not the sense of the senate that the United States should be a signatory to the Kyoto Protocol, and in March 2001, the Bush Administration announced that it would not implement the treaty, saying it would create economic setbacks in the U.S. and does not put enough pressure to limit emissions from developing nations. In February 2002, Bush announced his alternative to the Kyoto Protocol, by bringing forth a plan to reduce the intensity of greenhouse gasses by 18 percent over 10 years. The intensity of greenhouse gasses specifically is the ratio of greenhouse gas emissions and economic output, meaning that under this plan, emissions would still continue to grow, but at a slower pace. Bush stated that this plan would prevent the release of 500 million metric tons of greenhouse gases, which is about the equivalent of 70 million cars from the road. This target would achieve this goal by providing tax credits to businesses that use renewable energy sources.

In 2007, the U.S. Supreme Court ruled in the case Massachusetts v. Environmental Protection Agency that EPA regulation of carbon dioxide is required under the Clean Air Act.

President Barack Obama proposed a cap-and-trade program as part of the 2010 United States federal budget, but this was never adopted by Congress.

President Obama committed in the December 2009 Copenhagen Climate Change Summit to reduce carbon dioxide emissions in the range of 17% below 2005 levels by 2020, 42% below 2005 levels by 2030, and 83% below 2005 levels by 2050. Data from an April 2013 report by the Energy Information Administration (EIA), showed a 12% reduction in the 2005 to 2012 period. Just over half of this decrease has been attributed to the recession, and the rest to a variety of factors such as replacing coal-based power generation with natural gas and increasing energy efficiency of American vehicles (according to a Council of Economic Advisors analysis). Executive Order 13514 set various requirements for energy efficiency in federal buildings and operations, including goals for 2015. That year, Executive Order 13693 set requirements for federal operations generally.

In an address to the U.S. Congress in June 2013, the President detailed a specific action plan to achieve the 17% carbon emissions cut from 2005 by 2020, including measures such as shifting from coal-based power generation to solar and natural gas production. Some Republican and Democratic lawmakers expressed concern at the idea of imposing new fines and regulations on the coal industry while the U.S. still tries to recover from the world economic recession, with Speaker of the House John Boehner saying that the proposed rules "will put thousands and thousands of Americans out of work". Christiana Figueres, executive director of the UN's climate secretariat, praised the plan as providing a vital benchmark that people concerned with climate change can use as a paragon both at home and abroad.

After not participating in previous climate international treaties, the United States signed the Paris Agreement on April 22, 2016 during the Obama administration. Though this agreement does not mandate a specific reduction for any given country, it sets global goals, asks countries to set their own goals, and mandates reporting.

The U.S. submitted its action plan in March, 2015, ahead of the treaty signing. Reaffirming the November 2014 announcement it made with China, the United States declared it would reduce greenhouse gas emissions to 26-28% below 2005 levels by 2025. This is to be accomplished by several executive actions:

• Clean Power Plan - regulating sources of electricity (put on hold by the Supreme Court in February, 2016, pending the outcome of a lawsuit)
• New emission standards for heavy-duty vehicles, finalized by EPA in March, 2016
• Department of Energy efficiency standards for commercial buildings, appliances, and equipment
• Various actions to reduce emissions of greenhouse gases other than carbon dioxide, including regulation and voluntary efforts related to methane from landfills, agriculture, coal mines; and hydrofluorocarbons (HFCs) reduction through domestic regulation and amendment of the Montreal Protocol

In June 2017, President Donald Trump announced United States withdrawal from the Paris Agreement, although the exit process specified by the treaty (which Trump said the U.S. would follow) will last until at least November 4, 2020. Trump states that dropping out the agreement will create more job opportunities in the United States, but it may actually have the opposite effect by stifling the renewable energy industries. At the same time, Trump administration shut down the United States Environmental Protection Agency's climate change web pages and removed mentions of the topic elsewhere on the site. In April 2018, the Trump administration cancelled NASA's Carbon Monitoring System (CMS) program, which helped with the monitoring of CO₂ emissions and deforestation in the United States and in other countries. The Trump administration has also moved to increase fossil fuel consumption and roll back environmental policies that are considered to be burdensome to businesses.

For offsetting the dismantlement of the Clean Power Plan approximately 10 billion trees would need to be planted. Activists try to plant this number of trees.

In January 2020 Trump announced that the USA would join the Trillion Tree Campaign. Climate activists critiqued the plan for ignoring the root causes of climate change.. House Natural Resources Committee Chairman Raul Grijalva critiqued the plan as “a feel-good participatory gesture” without a broader portfolia of environmental actions surrounding it.

In June 2020, Democrats proposed a plan for climate action in USA aiming to not sell greenhouse gas emitting cars by 2035, reach zero emissions from the energy sector by 2040 and reduce to zero all the greenhouse gas emission of the country by 2050. The plan includes some actions to improve environmental justice. In 2016, 38% of adults in United States thought that stopping climate change are a top priority, in 2020 52% think like that. Many Republicans share this opinion.

Role of the US military

The US military is an unequivocal validator of climate science, and its current efforts to value true costs and benefits of energy conservation and increased use of renewables can serve as drivers of change, according to a 2014 study from the University of Pennsylvania Legal Studies Department.

A 2014 report described the projected climate change as a “catalyst for conflict”. The DOD had issued a Fiscal Year 2012 Climate Change Adaptation Roadmap, in which it outlined its vulnerabilities, yet the Government Accountability Office (GAO) found, that installation officials rarely proposed projects with climate change adaptation, because the processes for approving and funding military construction do not include climate change adaptation in the ranking criteria for projects.

State and regional policy

Across the country, regional organizations, states, and cities are achieving real emissions reductions and gaining valuable policy experience as they take action on climate change. According to the report of America's Pledge, 65% of the American population, 51% of the GHG emissions and 68% of the GDP, are now part of different coalitions that support climate action and want to fulfill the commitments of USA in the Paris Agreement. The coalitions include We Are Still In, US Climate Alliance, Climate Mayors and more.

These actions include increasing renewable energy generation, selling agricultural carbon sequestration credits, and encouraging efficient energy use. The U.S. Climate Change Science Program is a joint program of over twenty U.S. cabinet departments and federal agencies, all working together to investigate climate change. In June 2008, a report issued by the program stated that weather would become more extreme, due to climate change. States and municipalities often function as "policy laboratories", developing initiatives that serve as models for federal action. This has been especially true with environmental regulation—most federal environmental laws have been based on state models. In addition, state actions can significantly affect emissions, because many individual states emit high levels of greenhouse gases. Texas, for example, emits more than France, while California's emissions exceed those of Brazil. State actions are also important because states have primary jurisdiction over many areas—such as electric generation, agriculture, and land use—that are critical to addressing climate change.

Many states are participating in Regional climate change initiatives, such as the Regional Greenhouse Gas Initiative in the northeastern United States, the Western Governors' Association (WGA) Clean and Diversified Energy Initiative, and the Southwest Climate Change Initiative.

Inside the ten northeastern states implementing the Regional Greenhouse Gas Initiative, carbon dioxide emissions per capita decreased by about 25% from 2000 and 2010, as the state economies continued to grow while enacting various energy efficiency programs.

Cost and consequences

In 2013 there were 11 weather and climate disaster events with losses over $1 billion each in the United States. In total these 11 events losses were over $110 billion. 2013 was the warmest year ever in the contiguous United States and about one-third of all Americans experienced 10 days or more of 100-degree heat.

These increasingly common and severe weather events have put pressure on existing disaster-relief efforts. For instance, the increasing rate of wildfires, the increasing length of the fire season, and increasing severity have put pressure on national and international resources. In the US, federal firefighting efforts surpassed $2 billion a year for the first time in 2017, and this expense was repeated in 2018. At the same time, internationally shared capital, such as firefighting planes, has experienced increasing demand, requiring new investment. 

Impact on agriculture

Increasing floods (such as the 2019 Midwestern U.S. floods), heat waves, and droughts have brought economic problems to farmers business and increased product prices.

USDA research indicates that "climate change is likely to drive down yields for some crops, harm milk production, and lead to a drop in nutrient density for key crops like rice and wheat."

Public response

Public opinion about climate change

In April 2019, 69% of Americans thought that climate change is happening and 55% think that it is mostly human caused. In September 2019 approximately 75% thought that climate change is real and man made. In November 2016, 69% of registered voters said that USA should remain in Paris Agreement. 13% said that it should leave the agreement.

Education

At least three US high schools have objected to mention of climate change in 2019 graduation speeches by students.

Political ideologies

Historical support for environmental protection has been relatively non-partisan. Republican Theodore Roosevelt established national parks whereas Democrat Franklin D. Roosevelt established the Soil Conservation Service. This non-partisanship began to change during the 1980s when the Reagan administration stated that environmental protection was an economic burden. Views over global warming began to seriously diverge among Democrats and Republicans when ratifying the Kyoto Protocol was being debated in 1998. Gaps in opinions among the general public are often amplified among the political elites, such as members of Congress, who tend to be more polarized. A 2017 study by the Center for American Progress Action Fund of climate change denial in the United States Congress found 180 members who deny the science behind climate change; all were Republicans.

Beyond politicians, there is a variety of views by each political party. In March 2014, Gallup found that among Democrats, 45% say they worry a great deal about the quality of the environment while the number drops to 16% for Republicans.

Political disagreement is also strongly rooted in our potential solutions to addressing climate change. Strategies such as a Cap and Trade system are still a heated argument.

On January 20, 2017, within moments of Donald Trump's inauguration, all references to climate change were removed from the White House website. The U.S. has been considered the most authoritative researcher of this information, and there is concern amongst the scientific community as to how the Trump administration will prioritize the issue.

In early indications to news media of the first federal budget process under Donald Trump's administration, there are signs that most efforts under the Obama administration to curb U.S. greenhouse gas emissions would effectively be rolled back.

In July 2018 the Trump Administration released its Draft Environmental Impact Statement from the NHTSA. In it was the prediction that on our current course the planet will warm a disastrous seven degrees Fahrenheit (or about 3.9 degrees Celsius) by the end of this century.

Many pages were created to examine and compare the views of the candidates in the presidential election 2020 on climate change. The League of Conservation Voters create a special site, entirely dedicated to the issue called: "Change the Climate 2020". Similar pages were created in the site of NRDC, Ballotpedia, Boston CBS, the Skimm

Business community

In 2015, according to The New York Times and others, oil companies knew that burning oil and gas could cause global warming since the 1970s but, nonetheless, funded deniers for years.

A review, published in 2016, of academic literature that explores the potential for greenhouse emissions related liability, calculated that climate change related liability could reach trillions of dollars based on lost revenue from nations that would forced to evacuate because of sea level rise.

Climate of the United States

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Climate_of_the_United_States

  

Köppen climate types of the U.S. (50 states, District of Columbia, and 5 inhabited U.S. territories)

 

The climate of the United States varies due to changes in latitude, and a range of geographic features, including mountains and deserts. Generally, on the mainland, the climate of the U.S. becomes warmer the further south one travels, and drier the further west, until one reaches the West Coast. 

West of the 100th meridian, much of the U.S. has a cold semi-arid climate in the interior upper western states (Idaho to Dakotas), to warm to hot desert and semi-arid climates in the southwestern U.S. East of the 100th meridian, the climate is humid continental in northern areas (locations roughly above 40 north latitude, Northern Plains, Midwest, Great Lakes, New England), transitioning into a humid temperate climate from the Southern Plains and lower Midwest east to the Middle Atlantic states (Virginia to southern Connecticut).

A humid subtropical climate is found along and south of a mostly east-west line from the Virginia/Maryland capes (north of the greater Norfolk, Virginia area), westward to approximately northern Oklahoma, north of the greater Oklahoma City area. Along the Atlantic seaboard, the humid subtropical climate zone extends southward into central Florida. A Mediterranean climate prevails along most of the California coast, while southern Florida has a tropical climate, the warmest region on the US mainland. Hawaii and the U.S. territories also have tropical climates.

Higher-elevation areas of the Rocky Mountains, the Wasatch Range, Sierra Nevada, and Cascade Range are alpine. Coastal areas of Oregon and Washington have an oceanic climate. The state of Alaska, on the northwestern corner of the North American continent, is largely dominated by a subarctic climate, but with a subpolar oceanic climate in the southeast (Alaska Panhandle), southwestern peninsula and Aleutian Islands, and a polar climate in the north.

The primary drivers of weather in the contiguous United States are the seasonal change in the solar angle, the migration north/south of the subtropical highs, and the seasonal change in the position of the polar jet stream.

In the Northern Hemisphere summer, the subtropical high pressure systems move northward and closer to the United States mainland. In the Atlantic Ocean, the Bermuda High creates a south-southwest flow of tropical air masses over the southeastern, south-central and central United States - resulting in warm to hot temperatures, high humidity and frequent intense (but usually brief) showers and/or thunderstorms as the heat builds in the afternoon. In the Northern Hemisphere summer, high pressure in the Pacific Ocean builds toward the California coast, resulting in a northwesterly airflow, creating the cool, dry, and stable weather conditions prevalent along the West Coast in summer.

In the Northern Hemisphere winter, the subtropical highs retreat southward. The polar jet stream (and associated conflict zone between cold, dry air masses from Canada and warm, moist air masses from the Gulf of Mexico) drops further southward into the United States - bringing more frequent periods of stormy weather, with rain, ice and snow, and much more variable temperatures, with rapid temperature rises and falls not uncommon. Areas in the southern U.S. (Florida, the Gulf Coast, the Desert Southwest, and southern California) however, often have more stable weather, as the polar jet stream's impact does not usually reach that far south.

Weather systems, be they high-pressure systems (anticyclones), low-pressure systems (cyclones) or fronts (boundaries between air masses of differing temperature, humidity and most commonly, both) are faster-moving and more intense in the winter/colder months than in the summer/warmer months, when the belt of lows and storms generally moves into southern Canada.

The Gulf of Alaska is the origination area of many storms that enter the United States. Such "North Pacific lows" enter the U.S. through the Pacific Northwest, then move eastward across the northern Rocky Mountains, northern Great Plains, upper Midwest, Great Lakes and New England states.  Across the central states from late fall to spring, "Panhandle hook" storms move from the central Rockies into the Oklahoma/Texas panhandle areas, then northeast toward the Great Lakes. They generate unusually large temperature contrasts, and often bring copious Gulf moisture northward, resulting sometimes in cold conditions and possibly-heavy snow or ice north and west of the storm track, and warm conditions, heavy rains and potentially-severe thunderstorms south and east of the storm track - often simultaneously. 

Across the northern states in winter usually from Montana eastward, "Alberta clipper" storms track east and bring light to moderate snowfalls from Montana and the Dakotas across the upper Midwest and Great Lakes states to New England, and often, windy and severe Arctic outbreaks behind them. When winter-season Canadian cold air masses drop unusually far southward, "Gulf lows" can develop in or near the Gulf of Mexico, then track eastward or northeastward across the Southern states, or nearby Gulf or South Atlantic waters. They sometimes bring rain, but can bring snow or ice across the South, mostly in interior or northern areas. 

In the cold season (generally November to March), most precipitation occurs in conjunction with organized low-pressure systems and associated fronts. In the summer, storms are much more localized, with short-duration thunderstorms common in many areas east of the 100th meridian and south of 40 latitude.

In the warm season, storm systems affecting a large area are less frequent, and weather conditions are more solar {sun} controlled, with the greatest chance for thunderstorm and severe weather activity during peak heating hours, mostly between 3 PM and 9 PM local time. From May to August especially, often-overnight mesoscale-convective-system (MCS) thunderstorm complexes, usually associated with frontal activity, can deliver significant to flooding rainfall amounts from the Dakotas/Nebraska eastward across Iowa/Minnesota to the Great Lakes states. 

From late summer into fall (mostly August to October), tropical cyclones (hurricanes, tropical storms and tropical depressions) sometimes approach or cross the Gulf and Atlantic states, bringing high winds, heavy rainfall, and storm surges (often topped with battering waves) to Gulf and Atlantic lowlands and coastal areas. 

Record one day precipitation by county between 1979 and 2011.

 

 

A map of the average annual high temperatures in the United States.

Regional overview

Southwest

The Southwest has a hot desert climate, at lower elevations. Cities like Phoenix, Las Vegas, Yuma, and Palm Springs have average highs over 100 °F (38 °C) during the summer months and lows in the 70s or even 80s. In winter, daily temperatures in the southwest are cooler with highs in the 50s and 60s F, and lows in the 40s F.

In Phoenix, Las Vegas and similar Southwestern desert areas, on average June is the driest month, after Pacific-originating winter storms have concluded and before the Southwestern summer "monsoon" begins. The Southwest and the Great Basin are affected by said monsoon from the Gulf of California from July–September. This results in some increase in humidity and cloud cover, bringing higher nighttime low temperatures and localized thunderstorms to the region, which can result in flash flooding. Further eastward in the desert Southwest (Tucson, AZ eastward toward El Paso, TX), winter-season precipitation decreases, while the summer monsoon increasingly provides a summer precipitation maximum. For example, El Paso and Albuquerque, NM have a pronounced July to September precipitation maximum. Still, drought has been frequent in the region, often lasting for periods of years or longer. Forest fires across the Western United States (especially the southwest) occur many years, and can be severe to extreme in especially hot, dry summer seasons. 

Sonoran Desert at Saguaro National Park

 

Northern Arizona and New Mexico, central and northern Nevada and most of Utah (outside higher mountain areas) have a temperate semi-desert to desert climate, but with colder and snowier winters than in Phoenix and similar areas, and less-hot summers (as at Salt Lake City, Utah). Summer high temperatures often reach the 90s, but low temperatures drop into the low 60s and even 50s. As in other temperate desert climates, the dry air results in large differences (sometimes over 40 degrees) between daytime high and nighttime low temperatures. Precipitation, though scarce, often falls year-round, influenced both by summer thunderstorms brought by the Southwestern monsoon (primarily in southern areas), and by winter-season storms from the Pacific Ocean.

The coast of California has a Mediterranean climate. Daily high temperatures range from 70 to 80 °F (21 to 27 °C) in the summer to 50 to 65 °F (10 to 16 °C) in winter, with low temperatures from the 60 °F (16 °C)s in summer to the mid 40s F in winter. Like most Mediterranean climates, much of coastal California has a wet winter and dry summer. Early summers can often bring cool, overcast weather (fog and low stratus clouds) to coastal California. As such, the warmest summer weather is delayed until August, even September in many areas of the California coast; on average, September is the warmest month in San Francisco, CA. Upwelling of cold Pacific waters also contributes to the frequent cool spring and early summer weather in coastal California. In California's inland river valleys (Bakersfield, Sacramento areas), the wet-winter, dry-summer pattern remains, but winters are cooler and more prone to occasional frost or freeze, while summers are much hotter, with blazing sunshine and daytime high temperatures not uncommonly in the 90s °F to over 100 °F (38 °C).

Gulf Coast/Lower Mississippi Valley/South Atlantic states

The Gulf and South Atlantic states have a humid subtropical climate with mostly mild winters and hot, humid summers. Most of the Florida peninsula including Tampa and Jacksonville, along with other coastal cities like Houston, New Orleans, Savannah, GA, Charleston, SC and Wilmington, NC all have average summer highs from near 90 to the lower 90s F, and lows generally from 70 to 75 °F (21 to 24 °C); combined with moist tropical air, this creates the sultry summer weather conditions that prevail here.

Swamp in southern Louisiana

In the interior South, in cities like Raleigh, NC, Atlanta, Birmingham, AL, Nashville, TN and Jackson, average summer highs and lows are similar to coastal areas, while some areas of interior eastern and central Texas (i.e. Dallas, Austin and San Antonio areas) have average daily highs in the mid to upper 90s F. In winter, average daily high temperatures range from the 40 °F (4 °C)s (upper South: northern Arkansas, Kentucky and Virginia), to the 60 °F (16 °C)s along the Gulf Coast and South Atlantic coast (Charleston southward), with 70 °F (21 °C)s in central Florida and far southern Texas. Average daily lows in winter range from 20 °F (−7 °C)s north to 40 °F (4 °C)s along the Gulf and far South Atlantic coasts, with 50 °F (10 °C)s in Florida and coastal south Texas.

Much of the interior South (Tennessee, Kentucky and the northern Gulf states) has a winter or spring maximum in precipitation, with December, March or April typically the wettest month, and August to October the driest months - for example, at Birmingham, AL, Huntsville, AL, Tupelo, MS and Memphis, TN. From November to April, these areas commonly experience sharp conflicts between cold, dry air from Canada and warm, moist air from the Gulf of Mexico. These air-mass clashes often bring heavy winter and spring precipitation to the Mid-South. Given the tropical air masses, summer-season thunderstorms can occur throughout the South, but they are heavier and more frequent along the Gulf Coast, South Atlantic coast (Norfolk, VA area southward), and in peninsular Florida. Along most of the Gulf coast (i.e. New Orleans, LA, Mobile, AL and Pensacola, FL areas), and in South Atlantic coastal and sandhills areas (i.e. Columbia, SC, Fayetteville, NC, Raleigh, NC, Wilmington, NC, and Norfolk, VA), July and August are usually the wettest months, and precipitation is fairly evenly distributed the rest of the year. Primarily from August to early October, the coastal Gulf and South Atlantic states are susceptible to being struck by tropical weather systems (tropical depressions, tropical storms, and hurricanes). Even in winter, most precipitation falls as rain. However, occasionally frozen precipitation (snow, sleet and/or freezing rain) can occur (more commonly in interior and northern areas) when southerly-tracking storms throw Gulf or Atlantic moisture over cold air at ground level.

Southern Florida has a tropical climate, with all months having a mean temperature of higher than 65 °F (18 °C), a wet season from May through October, and a dry season from November through April. In cities like Fort Lauderdale, Miami, Key West, Naples, and Palm Beach average daily highs range from the mid 70 °F (21 °C)s in winter to near 90 °F (32-33 °C) in summer. Average overnight lows range from the upper 50 °F (10 °C)s in winter to the mid and upper 70 °F (21 °C)s in summer. Southern Florida is the warmest region of the U.S. mainland in winter.

Southern Plains/Lower Midwest/Middle East Coast

The region from the southern Plains, to the lower Midwest, eastward to the central East Coast (the New York City/coastal Connecticut region southward to Virginia) has a temperate climate climate with cool to cold winters and hot, humid summers. Daytime highs range from 80 to 90 °F (27 to 32 °C) in summer to 35 to 50 °F (2 to 10 °C) in winter. Lows range from the 60 °F (16 °C)s in summer to 25 to 35 °F (−4 to 2 °C) in winter. Cities in this region include Wichita, KS, St. Louis, MO, Springfield, IL, Indianapolis, IN, Columbus, OH, Pittsburgh, PA, Philadelphia, PA, Washington, D.C., Richmond, VA, New York City, NY, New Haven, CT, and Atlantic City, NJ. Precipitation is spread fairly evenly throughout the year, though as one travels from Indiana westward there is an increasingly prominent early-summer concentration, with a May maximum in northern Texas and Oklahoma, and a June maximum increasingly evident from (central/northern) Indiana westward to Kansas. As one travels from east to west across Texas, Oklahoma and Kansas, average annual precipitation steadily decreases. Far western Texas (El Paso area) is desert, and average annual precipitation is less than twenty inches (510 mm) in westernmost Kansas and the Oklahoma Panhandle, where the climate qualifies as semi-arid.

In the lower Midwest (and southern Plains states, especially), temperatures can rise or drop rapidly; winds can be extreme; and clashing air masses, including hot, dry air of Mexican and/or Southwestern origin, warm, moist air from the Gulf of Mexico and cold, dry air from Canada can spawn severe thunderstorms and tornadoes, particularly from April to June. The "dryline," separating hot, dry air of Mexican/Southwestern U.S. origin from warm, moist air from the Gulf of Mexico, often causes severe, occasionally violent, thunderstorms to fire in central and eastern Texas, Oklahoma and Kansas; these sometimes contribute toward the hailstorms and tornado outbreaks that the Southern Plains are well known for. Reflecting these air-mass conflicts, central Oklahoma, including the Oklahoma City and Moore-Norman areas, has the highest frequency of tornadoes per unit land area on planet Earth, with May the highest-risk month for tornadoes throughout "Tornado Alley," from northern Texas north-northeastward toward western and central Iowa.

Northern Great Plains/North-Central/Great Lakes/New England

The northern half of the Great Plains (Nebraska northward), northern Midwest, Great Lakes, and New England states have a humid continental climate. Here there are four distinct seasons, with warm to hot summers, and cold and often-snowy winters. Average daily high temperatures range from 10 °F (−12 °C)s (in North Dakota, and central and northern Minnesota) to 30 °F (−1 °C)s in winter to 70 to 80 °F (21 to 27 °C)s in summer, while overnight lows range from below 0 °F (−18 °C) in winter (in North Dakota and much of Minnesota) to 50 to 60 °F (10 to 16 °C)s in summer. In the New England states, precipitation is evenly distributed around the year, with a slight late fall-early winter (November–December) maximum along the New England coast from Boston, MA northward due to intense early-winter storms. In the Great Lakes states, cold Arctic air in winter crossing the relatively warmer lake waters can result in frequent and sometimes very heavy lake-effect snow, especially on the eastern and southern shores of the Great Lakes (for example, in western Michigan's Lower Peninsula and in the Buffalo, NY area). Cities in this area include Minneapolis, MN, Omaha, NE, Sioux Falls, SD, Fargo, ND, Chicago, IL, Cleveland, OH, Buffalo, NY, Albany, NY, Boston, MA, Concord, NH and Augusta, ME. As one travels from east to west across Nebraska, South Dakota and North Dakota, average annual precipitation steadily decreases, and the westernmost counties of these states have a semi-arid climate, with about or just over 15 inches of precipitation per year, on average (see climate data for Williston, ND, Rapid City, SD and Scottsbluff, NE).

In the upper Midwest and northern Plains states, temperatures may rise or fall rapidly, and winds (from warm-season thunderstorms or larger-scale low-pressure systems) can be strong to extreme. Here, air-mass conflicts primarily involve warm, moist air from the Gulf of Mexico, clashing with cool to cold, dry air from Canada, with only occasional intrusions of hot, dry air from the southwest. The conflicts between Canadian and Gulf air commonly produce severe thunderstorms (including hailstorms, especially on the western Plains) and tornadoes, particularly in May and June. In the northern Plains and North Central states generally, June is the year's wettest month on average, owing to maximum shower and thunderstorm activity. Also, June is the highest-risk month for severe weather throughout North Dakota, South Dakota, Minnesota, Iowa, Wisconsin and northern Illinois.

Pacific Northwest

Cascade Range in Washington

The Pacific Northwest has an oceanic climate. The climate is wet and cool in autumn, winter, and spring, and stable and drier in the summer months, especially July and August. On average, the wettest month is typically November or December; the driest, July. In the summer months, average highs in cities like Seattle and Portland are from 70 to 75 °F (21 to 26 °C) with lows from 50 to 59 °F (10 to 15 °C), while in winter daily highs are from 40 to 45 °F (4 to 9 °C) and overnight lows from 30 to 38 °F (−1 to 4 °C).

In winter, the Pacific Northwest (especially coastal districts and other areas west, i.e. on the prevailing windward side, of the Olympic and Cascade mountain ranges), experiences a mostly overcast, wet and cool climate, but without severe cold like that found in the interior northern U.S. (i.e. Minnesota/North Dakota). At lower elevations, winter precipitation falls mostly as rain. However, snow does occur even at the lowest elevations, primarily when Pacific moisture interacts with cold air intruding into the Pacific Northwest from western Canada (i.e. Alberta and interior British Columbia). Summers in the Pacific Northwest are generally cool, especially along the coastline. The Great Basin and Columbia Plateau (the Intermontane Plateaus) are arid or semiarid regions, with high summer temperatures in the 90s to occasionally over 100 at lower elevations (e.g. at Boise, ID), with annual precipitation averaging less than 15 inches (380 mm) as a result of the rain shadow of the Sierra Nevada and Cascades. Both coastal and interior areas of Oregon and Washington, and southern Idaho, have a wet-winter, dry-summer precipitation pattern, but traveling eastward into Montana and Wyoming, this transitions progressively (for example, at Missoula, MT) toward relatively drier winters and a May and eventually June precipitation maximum, the latter characteristic of the Northern Plains and much of the upper Midwest (i.e. both Dakotas, Nebraska, Iowa and Minnesota).

Alaska

The climate in Juneau and the southeast panhandle is a mid-latitude oceanic climate (Köppen Cfb). The climate in the extreme north of Alaska is what would be expected for an area north of the Arctic Circle — it is an Arctic climate (Köppen ET) with long, very cold winters and short, cool summers. Akclimate.org says the following: "The altitude above sea level influences the climate of a given area [in Alaska]. Lower elevations in interior Alaska, such as the Yukon Flats and the Tanana Valley experience extreme cold in the winter as well as high summertime temperatures."

Hawaii

Hawaii has a tropical climate. Though Hawaii is tropical, Hawaii has several different climates at different altitudes. Snow sometimes occurs at the highest elevations in Hawaii. The following was said about Hawaii's climate:

"Hawaii boasts 11 of the 13 climate zones in the world, each with unique ecosystems and weather characteristics. Factors such as elevation, pressure variations, rainfall, wind and topography combine to create distinctive locations throughout the islands."

Caribbean territories

El Yunque National Forest in Puerto Rico

Puerto Rico has different climatic zones, all tropical. The northeastern part of the territory is very wet, with a tropical rainforest climate (Köppen Af). This supports rainforests like El Yunque. The southern part is drier, mostly a savanna climate (Köppen Aw) with small locations on the southern coast dry enough to have a hot-semi arid climate (Koppen BSh).

 

The U.S. Virgin Islands have a tropical savanna climate, with warm, dry winters, and rainy summers (Köppen Aw), typical of the Caribbean. The wet season is from May to October.

In 2015, the Wall Street Journal reported that U.S. territories such as Puerto Rico are the areas of the U.S. most vulnerable to climate change.

Pacific territories

A view of Ofu Beach on Ofu Island in American Samoa

 

Guam and the Northern Mariana Islands both have a trade-wind tropical rainforest climate (Köppen Af). The dry season is from January to May, and the wet season from July to November.

American Samoa is south of the equator, and therefore its wet/dry seasons are reversed. The wet season is from December-March, and the dry season is from April-September. Rainmaker Mountain on Tutuila traps trade-wind rainclouds, leading to very high rainfall and a tropical rainforest climate (Köppen Af).

Precipitation

Average precipitation

Precipitation (whether by annual amount, annual distribution or characteristic[s]) varies significantly across the United States and its possessions. Late summer and fall extratropical cyclones bring a majority of the precipitation which falls across western, southern, and southeast Alaska annually. During the fall, winter, and spring, Pacific storm systems bring most of Hawaii and the western United States much of their precipitation. Most of Florida has a subtropical monsoon rainfall pattern (wet summer and dry winter).

In the central and upper eastern United States, precipitation is evenly distributed throughout the year, although summer rainfall increases as one moves southeastward. Lake-effect snows add to precipitation potential downwind of the Great Lakes, as well as Great Salt Lake and the Finger Lakes during the cold season. The average snow to liquid ratio across the contiguous United States is 13:1, meaning 13 inches (330 mm) of snow melts down to 1 inch (25 mm) of water. The El Niño-Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics.

During the summer, the Southwest monsoon combined with Gulf of California and Gulf of Mexico moisture moving around the subtropical ridge in the Atlantic Ocean bring the promise of afternoon and evening thunderstorms to the southern tier of the country as well as the Great Plains. Equatorward of the subtropical ridge, tropical cyclones enhance precipitation (mostly from August to October) across southern and eastern sections of the country, as well as Puerto Rico, the United States Virgin Islands, the Northern Mariana Islands, Guam, and American Samoa. Over the top of the ridge, the jet stream brings a summer precipitation maximum to the Great Lakes. Large thunderstorm areas known as mesoscale convective complexes move through the Plains, Midwest, and Great Lakes during the warm season, contributing up to 10% of the annual precipitation to the region.

Extremes

Several different air masses affect the United States.

 

In northern Alaska, tundra and arctic conditions predominate, and the temperature has fallen as low as −80 °F (−62 °C). On the other end of the spectrum, Death Valley, California once reached 134 °F (56.7 °C), officially the highest temperature ever recorded on Earth.

On average, the mountains of the western states receive the highest levels of snowfall on Earth. The greatest annual snowfall level is at Mount Rainier in Washington, at 692 inches (1,758 cm); the record there was 1,122 inches (2,850 cm) in the winter of 1971–72. This record was broken by the Mt. Baker Ski Area in northwestern Washington which reported 1,140 inches (2,896 cm) of snowfall for the 1998-99 snowfall season. Other places with significant snowfall outside the Cascade Range are the Wasatch Range, near the Great Salt Lake and the Sierra Nevada, near Lake Tahoe.

Along the coastal mountain ranges in the Pacific Northwest, rainfall is greater than anywhere else in the continental U.S., with Quinault Ranger Station in Washington having an average of 137 inches (3,480 mm). Hawaii receives even more, with 404 inches (10,262 mm) measured annually, on average, at the Big Bog, in Maui. Pago Pago Harbor in American Samoa is the rainiest harbor in the world (because of the 523 meter Rainmaker Mountain). The Sonoran Desert in the southwest is home to the driest locale in the US. Yuma, Arizona, has an average of 2.63 inches (67 mm) of precipitation each year.

Climate of Hawaii

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Climate_of_Hawaii 

 

Köppen climate types of Hawaii

The American state of Hawaii, which covers the Hawaiian Islands, is tropical but it experiences many different climates, depending on altitude and surrounding. The island of Hawaii for example hosts 4 (out of 5 in total) climate groups on a surface as small as 4,028 square miles (10,430 km²) according to the Köppen climate types: tropical, arid, temperate and polar. When counting also the Köppen sub-categories the island of Hawaii hosts 10 (out of 14 in total) climate zones. The islands receive most rainfall from the trade winds on their north and east flanks (the windward side) as a result of orographic precipitation. Coastal areas are drier, especially the south and west side or leeward sides.

The Hawaiian Islands receive most of their precipitation from October to April. Drier conditions generally prevail from May to September. Due to cooler waters around Hawaii, the risk of tropical cyclones is low for Hawai'i.

Temperature

Temperatures at sea level generally range from highs of 84–88 °F (29–31 °C) during the summer months to 79–83 °F (26–28 °C) during the winter months. Rarely does the temperature rise above 90 °F (32 °C) or drop below 60 °F (16 °C) at lower elevations. Temperatures are lower at higher altitudes. During the winter, snowfall is common at the summits of Mauna Kea and Mauna Loa on Hawaii Island. On Maui, the summit of Haleakalā occasionally experiences snowfall, but snow had never been observed below 7,500 feet (2,300 m) before February 2019, when snow was observed at 6,200 feet (1,900 m) and fell at higher elevations in amounts large enough to force Haleakalā National Park to close for several days. The record low temperature in Honolulu is 52 °F (11 °C) on January 20, 1969.

Temperatures of 90 °F (32 °C) and above are uncommon (with the exception of dry, leeward areas). In the leeward areas, temperatures may reach into the low 90s several days during the year, but temperatures higher than these are unusual. The highest temperature ever recorded on the islands was 100 °F (38 °C) on April 27, 1931 in Pahala. The surface waters of the open ocean around Hawaii range from 75 °F (24 °C) between late February and early April, to a maximum of 82 °F (28 °C) in late September or early October. In the United States, only Florida has warmer surf temperatures.

The Pacific High, and with it the trade-wind zone, moves north and south with changing angle of the sun, so that it reaches its northernmost position in the summer. This brings trade winds during the period of May through September, when they are prevalent 80 to 95 percent of the time. From October through April, the heart of the trade winds moves south of Hawaii; thus there average wind speeds are lower across the islands. Due to Hawaii being at the northern edge of the tropics (mostly above 20 latitude), there are only weak wet and dry seasons unlike many tropical climates.

Winds

Island wind patterns are very complex. Though the trade winds are fairly constant, their relatively uniform air flow is distorted and disrupted by mountains, hills, and valleys. Usually winds blow upslope by day and downslope by night. Local conditions that produce occasional violent winds are not well understood. These are very localized, sometimes reaching speeds of 60 to 100 mph (100 to 160 km/h) and are best known in the settled areas of Kula and Lahaina on Maui. The Kula winds are strong downslope winds on the lower slopes of the west side of Haleakala. These winds tend to be strongest from 2,000 to 4,000 ft (600 to 1,200 m) above mean sea level.

The Lahaina winds are also downslope winds, but are somewhat different. They are also called "lehua winds" after the ʻōhiʻa lehua (Metrosideros polymorpha), whose red blossoms fill the air when these strong winds blow. They issue from canyons at the base of the western Maui mountains, where steeper canyon slopes meet the more gentle piedmont slope below. These winds only occur every 8 to 12 years. They are extremely violent, with wind speeds of 80–100 mph (130–160 km/h) or more.

Road to Hana

Cloud formation

Under trade wind conditions, there is very often a pronounced moisture discontinuity between 4,000 and 8,000 feet (1,200 and 2,400 m). Below these heights, the air is moist; above, it is dry. The break (a large-scale feature of the Pacific High) is caused by a temperature inversion embedded in the moving trade wind air. The inversion tends to suppress the vertical movement of air and so restricts cloud development to the zone just below the inversion. The inversion is present 50 to 70 percent of the time; its height fluctuates from day to day, but it is usually between 5,000 and 7,000 feet (1,500 and 2,100 m). On trade wind days when the inversion is well defined, the clouds develop below these heights with only an occasional cloud top breaking through the inversion.

These towering clouds form along the mountains where the incoming trade wind air converges as it moves up a valley and is forced up and over the mountains to heights of several thousand feet. On days without an inversion, the sky is almost cloudless (completely cloudless skies are extremely rare). In leeward areas well screened from the trade winds (such as the west coast of Maui), skies are clear 30 to 60 percent of the time.

Windward areas tend to be cloudier during the summer, when the trade winds and associated clouds are more prevalent, while leeward areas, which are less affected by cloudy conditions associated with trade wind cloudiness, tend to be cloudier during the winter, when storm fronts pass through more frequently. On Maui, the cloudiest zones are at and just below the summits of the mountains, and at elevations of 2,000 to 4,000 ft (600 to 1,200 m) on the windward sides of Haleakala. In these locations the sky is cloudy more than 70 percent of the time. The usual clarity of the air in the high mountains is associated with the low moisture content of the air.

Precipitation

Hawaii differs from many tropical locations with pronounced wet and dry seasons, in that the wet season coincides with the winter months (rather than the summer months more typical of other places in the tropics). For instance, Honolulu's Köppen climate classification is the rare As wet-winter subcategory of the Tropical wet and dry climate type.

Major storms occur most frequently in October through March. There may be as many as six or seven major storm events in a year. Such storms bring heavy rains and can be accompanied by strong local winds. The storms may be associated with the passage of a cold front, the leading edge of a mass of relatively cool air that is moving from west to east or from northwest to southeast.

Annual mean rainfall ranges from 188 mm (7.4 inches) on the summit of Mauna Kea to 10,271 mm (404 inches) in Big Bog. Windward slopes have greater rainfall than leeward lowlands and tall mountains.

Average Annual Rainfall for the State of Hawai‘i, http://rainfall.geography.hawaii.edu/

On windward coasts, many brief showers are common, not one of which is heavy enough to produce more than 0.01 in (0.25 mm) of rain. The usual run of trade wind weather yields many light showers in the lowlands, whereas torrential rains are associated with a sudden surge in the trade winds or with a major storm. Hana has had as much as 28 in (710 mm) of rain in a single 24-hour period.

Severe thunderstorms, as defined by the National Weather Service (NWS) as tornadoes, hail 1 in (25 mm) or larger, and/or convective winds of at least 58 mph (93 km/h) occur but are relatively uncommon. Nontornadic waterspouts are more common than tornadoes produced by supercells, which produce stronger, longer lasting tornadoes, especially with respect to inland areas, and also produce the largest hail, such as the 2012 Hawaii hailstorm. An annual average of approximately one tornado, either emanating from supercells or by other processes, occurs.

Kona storms are features of the winter season. The name comes from winds out of the "kona" or usually leeward direction. Rainfall in a well-developed Kona storm is widespread and more prolonged than in the usual cold-front storm. Kona storm rains are usually most intense in an arc, extending from south to east of the storm and well in advance of its center. Kona rains last from several hours to several days. The rains may continue steadily, but the longer lasting ones are characteristically interrupted by intervals of lighter rain or partial clearing, as well as by intense showers superimposed on the more moderate continuous, steady rain. An entire winter may pass without a single well-developed Kona storm. More often there are one or two such storms a year; sometimes four or five.

Hurricanes

The hurricane season in the Hawaiian Islands is roughly from June through November, when hurricanes and tropical storms are most probable in the North Pacific. These storms tend to originate off the coast of Mexico (particularly the Baja California peninsula) and track west or northwest towards the islands. As storms cross the Pacific, they tend to lose strength if they bear northward and encounter cooler water.

True hurricanes are rare in Hawai'i, thanks in part to the comparatively cool waters around the islands as well as unfavorable atmospheric conditions, such as enhanced wind shear; only four have affected the islands during 63 years. Tropical storms are more frequent. These have more modest winds, below 74 mph (119 km/h). Because tropical storms resemble Kona storms, and because early records do not distinguish clearly between them, it has been difficult to estimate the average frequency of tropical storms. Every year or two a tropical storm will affect the weather in some part of the islands. Unlike cold fronts and Kona storms, hurricanes and tropical storms are most likely to occur during the last half of the year, from July through December. Three strong and destructive hurricanes are known to have made landfall on the islands, an unnamed storm in 1871, Hurricane Dot in 1959, and Hurricane Iniki in 1992. Another hurricane, Iwa, caused significant damage in 1982 but its center passed nearby and did not directly make landfall. The rarity of hurricanes making landfall on the Islands is subject to change as the climate warms. In the Pliocene era, where CO2 levels were comparable to those we see today, the waters around Hawai'i were much warmer, resulting in frequent hurricane strikes in computer simulations.

Effect on trade winds

A true-color satellite view of Hawaii shows that most of the flora on the islands grow on the north-east sides, which face the trade winds. The texture change around the calmer south-west of the islands is the result of the shelter provided from the islands.

 

The top image above shows the winds around the Hawaiian Islands measured by the Seawinds instrument aboard QuikSCAT during August 1999. Trade winds blow from right to left in the image. The bottom image shows the ocean current formed by the islands’ wake. Arrows indicate current direction and speed, while white contours show ocean temperatures. The warm water of the current generates winds that sustain the current for thousands of miles.

 

Despite being small islands within the vast Pacific Ocean, the Hawaiian Islands have a surprising effect on ocean currents and circulation patterns over much of the Pacific. In the Northern Hemisphere, trade winds blow from northeast to southwest, from North and South America toward Asia, between the equator and 30 degrees north latitude. Typically, the trade winds continue across the Pacific — unless something gets in their way, like an island.

Hawai‘i's high mountains present a substantial obstacle to the trade winds. The elevated topography blocks the airflow, effectively splitting the trade winds in two. This split causes a zone of weak winds, called a "wind wake", on the leeward side of the islands.

Aerodynamic theory indicates that an island wind wake effect should dissipate within a few hundred kilometers and not be felt in the western Pacific. However, the wind wake caused by the Hawaiian Islands extends 1,860 miles (3,000 km), roughly 10 times longer than any other wake. The long wake testifies to the strong interaction between the atmosphere and ocean, which has strong implications for global climate research. It is also important for understanding natural climate variations, like El Niño.

There are number of reasons why this has been observed only in Hawai‘i. First, the ocean reacts slowly to fast-changing winds; winds must be steady to exert force on the ocean, such as the trade winds. Second, the high mountain topography provides a significant disturbance to the winds. Third, the Hawaiian Islands are large in horizontal (east-west) scale, extending over four degrees in longitude. It is this active interaction between wind, ocean current, and temperature that creates this uniquely long wake west of Hawaii.

The wind wake drives an eastward "counter current" that brings warm water 5,000 miles (8,000 km) from the Asian coast. This warm water drives further changes in wind, allowing the island effect to extend far into the western Pacific. The counter current had been observed by oceanographers near the Hawaiian Islands years before the long wake was discovered, but they did not know what caused it.