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The climate of the United States varies due to differences in latitude, and a range of geographic features, including mountains and deserts. West of the 100th meridian, much of the US is semi-arid to desert in the far southwestern US, and Mediterranean along the California coast. East of the 100th meridian, the climate is humid continental in the northern areas east through New England, to humid subtropical in the Gulf and South Atlantic regions. Southern Florida is tropical, as is Hawaii and the US Virgin Islands. Higher-elevation areas of the Rocky Mountains, the Wasatch, Sierra Nevada, and Cascade Range are alpine. The West Coast areas in coastal Oregon and Washington are cool oceanic climate. The state of Alaska, on the northwestern corner of the North American continent, is largely subarctic climate, but with a cool oceanic climate in the southeast (Alaska Panhandle), southwestern peninsula and Aleutian Islands.
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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 warm, humid air over the eastern, southern and central United States - resulting in warm to hot temperatures, high humidity and occasional thunderstorm activity. In the Pacific Ocean high pressure builds toward the California coast resulting in a northwesterly airflow creating the typical sunny, dry, and stable weather conditions along the West Coast.
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 precipitation and periods of disturbed weather, as well as colder or mild air masses. Areas in the southern US (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 move 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 US 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 heavy 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 the Great Lakes 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 often bring rain, but on rare occasions can bring ice to areas of the interior southern states.
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. 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 sometimes approach or cross the Gulf and Atlantic states, bringing high winds, heavy rainfall, and storm surges (often topped with battering waves) to coastal areas.
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 60's. In winter, daily temperatures in the southwest are cooler with highs in the 50's and 60's, and lows in the 40's. Northern Arizona and New Mexico, central and northern Nevada and most of Utah (outside higher mountain areas) have a semi-desert climate, but with colder and snowier winters than points south due to higher elevations. As in other desert climates, the dry air results in large differences (sometimes over 40 F ) between daytime high and nighttime low temperatures.
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 40's F in winter. [citation needed]. 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, and coastal cities like Houston, New Orleans, Savannah and Charleston, have average summer highs in the lower 90's 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 typical found here. In the interior South, in cities like Raleigh, NC, Atlanta, Birmingham, AL, Nashville TN, San Antonio, TX, and Jackson, average summer highs and lows are similar to coastal areas, white some areas of interior eastern Texas having highs in the upper 90's 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 and southern 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.
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 the upper 80 °F (32 °C)s 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 US 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 (NYC/coastal Connecticut southward to Virginia) has a temperate climate, with cool to cold winters and long hot summers. Daytime highs range from 80 to 90 F in summer to 35 to 50 F in winter. Lows range from the 60's F in summer to 25 to 35 F in winter. Cities in this region include Wichita, Kansas, St. Louis, Springfield, Illinois, Indianapolis, IN, Columbus, Ohio, Pittsburgh, Philadelphia, Washington, D.C., New York City, 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 20 inches 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 the Southern Plains are well known for. Reflecting these air-mass conflicts, central Oklahoma, including the Oklahoma City and 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 (North Dakota/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, Chicago, Cleveland, Buffalo, Albany, Boston, Concord, Augusta, Maine. 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.[citation needed]
Pacific Northwest '
The Pacific Northwest has a 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 79 °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 49 °F (4 to 9 °C) and overnight lows from 30 to 39 °F (−1 to 4 °C).[citation needed]
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). In Seattle, WA and Portland, OR, winter-season snowfall varies greatly from one winter season to the next; in Seattle, the average winter-season snowfall is about 7 inches. In January 1950 (also the coldest January and winter month in Seattle history), Seattle received an unprecedented monthly snowfall of over 57 inches. 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.[citation needed]. 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 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).
Precipitation
The characteristics of precipitation across the United States differ 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.
In the central and upper eastern United States, precipitation is evenly distributed throughout the year, although summer rainfall increases as one moves southeastward, until a sharp wet summer and dry winter prevail in Florida. 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
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 Mountains, 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 US, with Quinault Ranger Station in Washington having an average of 137 inches (3,480 mm). Hawaii receives even more, with 460 inches (11,684 mm) measured annually, on average, on Mount Waialeale, in Kauai. 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.
Natural disasters and effects
The Great Plains, the Midwest and the southern United States - because of contrasting air masses - have frequent severe thunderstorms and tornado outbreaks during spring and summer. In central portions of the US, tornadoes are more common than anywhere else on Earth and touch down most commonly in the spring and summer. The strip of land from north Texas north to Nebraska and east into Southern Michigan is known as Tornado Alley, where many houses have tornado shelters and many towns have tornado sirens. Stretching across Mississippi and Alabama, Dixie Alley has experienced tornadoes and violent thunderstorms, with peak tornado season coming on as early as February and waning by May. Florida also reports many tornadoes but these are rarely very strong. The southern US has a second tornado season during the Fall. In general, the area at greatest risk for tornadoes migrates northward from February to June, peaking in the Gulf States in February and March, the Ohio Valley and lower Midwest in April, southern and central Plains and central Midwest in May, and Northern Plains and upper Midwest (Dakotas, Minnesota and Wisconsin) in June.
The Appalachian region and the Midwest experience the worst floods. Widespread severe flooding is rare. Some exceptions include the Great Mississippi Flood of 1927, the Great Flood of 1993, and widespread flooding and mudslides caused by the 1982-1983 El Niño event in the western United States. Localized flooding can, however, occur anywhere, and mudslides from heavy rain can cause problems in any mountainous area, particularly the Southwest. The narrow canyons of many mountain areas in the west and severe thunderstorm activity during the monsoon season in summer leads to sometimes devastating flash floods as well, while Nor'easter snowstorms can bring activity to a halt in the New England and Great Lakes regions.
In 2013, the US sustains $10 billion annually in damage from floods.
The Southwest has the worst droughts; one is thought to have lasted over 500 years and to have decimated the Anasazi people. Large stretches of desert shrub in the west can fuel the spread of wildfires. Although severe drought is rare, it has occasionally caused major problems, such as during the Dust Bowl (1931–1942), which coincided with the Great Depression. Farmland failed throughout the Plains, entire regions were virtually depopulated, and dust storms ravaged the land. More recently, the western US experienced widespread drought from 1999 to 2004.
In terms of deaths from heatwaves, 7,415 losses occurred from 1999 to 2010, a mean of 618 per year. A disproportionate amount of men, a full 68% of deaths, versus women have been affected. The highest yearly total of heat-related deaths in that period was 1999 while the lowest was 2004. In terms of deaths from waves of cold temperatures, the same gender inequality exists (66% of hypothermia-related deaths in 2002 were of males). From 1979 2002, 16,555 deaths occurred due to exposure to excessive cold temperatures, a mean of 689 per year.