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The Climate of the United States varies due to its positioning of states in latitude, and range of geographic features. West of the 100th meridian, much of the US is semi-arid to desert in the far southwestern US. To the east of the 100th meridian, the climate is humid continental in the northern areas (locations above 40 north latitude), to humid temperate in the central and Atlantic coast regions, to humid subtropical in the Gulf and south Atlantic regions. The southern tip of Florida is tropical. Much of the Rocky Mountains, the Sierra Nevada, and the Cascade Range are alpine. The climate along the coast of California is Mediterranean, while the upper West coast areas in coastal Oregon and Washington are cool temperate oceanic. The state of Alaska—on the northwestern corner of the North American continent—is largely subarctic, with an oceanic climate in its southern edge and a polar climate in the north. The archipelago state of Hawaii, in the middle of the Pacific Ocean, is tropical.
Like most land masses located in the middle and lower middle latitudes, the primary driver of weather in the contiguous United States is 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 oceanic subtropical high pressure systems move north[clarification needed] - and much of the central and southern US see stable weather, and warm to hot temperatures. In the Northern Hemisphere winter, the subtropical highs retreat southward and the polar front jet stream moves further south into the United States - bringing much greater weather variation and much colder temperature. Areas in the extreme 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.
Mobile weather systems (cyclones/fronts/jets) are more active in the winter/colder months than in the summer/warmer months in the United States. In the winter months, storms come from the Pacific Ocean and enter the US through the Pacific Northwest, then move out across the Great Plains, then move eastward off the central and northern Atlantic seaboard. In the summer months, storms are much more localized (short duration thunderstorms are common in many areas east of the 100th meridian) and large scale storms are much less frequent and the duration is much shorter. In late summer and fall, tropical cyclones infrequently move toward the Gulf and south Atlantic states, bringing high winds, heavy rainfall, and tidal surges to the coastal plain.
Extreme weather is not uncommon—tornadoes regularly occur in the area of the Midwest referred to as Tornado Alley, heavy snowstorms can impact the far northern areas, and tropical cyclones can strike the far southern areas. The United States has more tornadoes than the rest of the countries of the world combined.
The Southwest is a hot desert climate, cities like Phoenix, Las Vegas, Yuma, and Palm Springs have average highs over 100 °F (38 °C) during the summer months. In winter, daily temperatures in the southwest are cooler with highs in the 60 °F (16 °C)’s and lows in the 30 °F (−1 °C) and 40 °F (4 °C)s. Rainfall in the southwest average less than 0.7 inches (18 mm) of rain monthly. The Southwest and the Great Basin are affected by the monsoon from the Gulf of California from July–September. This brings localized thunderstorms to the region that can result in flash flooding. Despite this, 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 annually.
The far southwest (California coast) has a Mediterranean climate. Like most Mediterranean climates, coastal California has a wet winter and dry summer. Early summers can often bring cooler and overcast weather (low stratus clouds) to coastal California. As such the warmest summer weather is delayed until August and September in many areas of the California coast. Upwelling of cold Pacific waters also contributes to the frequent chilly weather in coastal California, especially coastal areas in northern California. Daily high temperatures range from the mid and upper 70’s in the summer to the low to mid 60 °F (16 °C)’s in winter...with low temperatures from the 60 °F (16 °C)’s in summer to the 40 °F (4 °C)’s in winter.
The southeast has a warm to hot, humid, subtropical climate. Cities like Houston, New Orleans, Atlanta, and Charleston have average highs in the lower 90 °F (32 °C)'s, and combined with the moist tropical air, creates sultry summer weather conditions. In winter, daily temperatures in the southeast reach highs in the 50 °F (10 °C)’s and 60 °F (16 °C)’s and lows in the 30 °F (−1 °C). 40 °F (4 °C)’s with icy conditions possible for brief periods. Rainfall is plentiful in the southeastern US, and summer is normally the wettest time, especially in areas along the Gulf and south Atlantic coast.
Southern Florida has a tropical climate, with all months having a mean temperature of higher than 65 °F (18 °C). Like most tropical climates, southern Florida has a wet summer and relatively dry and sunny winter. In cities like Fort Lauderdale, Miami, Naples, and Palm Beach average daily highs range from the upper 70 °F (21 °C)’s in winter to the upper 80 °F (27 °C)’s and low 90 °F (32 °C)’s in summer. Overnight lows range from the 70 °F (21 °C)’s in summer to the upper 50 °F (10 °C)’s and low 60 °F (16 °C)’s in winter. The only area of the US mainland known to have never experience a freeze (32 °F (0 °C)) is the Florida Keys and some areas of coastal south Florida.
The Midwest, northern Plains, Great Lakes, and New England regions have a humid continental climate. Here there are four seasons with warm to hot summers, cold and snowy winters, and rain in all seasons. Temperatures can rise or drop rapidly; winds can be extreme; and the flow of dry or moist hot air from the subtropics clashing with incoming air from the subarctic can spawn tornadoes, particularly in the Spring and in the Midwest and Plains. In winter, snowstorms can bring heavy lake effect snows to the areas from Wisconsin, Michigan, Ohio, and New York, and northeasters can bring heavy snow/rain/and coastal flooding to the Boston/New England region. Temperatures can plunge to −20 °F (−29 °C) to −30 °F (−34 °C) in the winter at night in the northern areas of the region. In summer, temperatures surpass 90 °F (32 °C)s on the hotter days and can reach 100 °F (38 °C) for a short time.
The central plains and East Coast (NYC/Connecticut southward) has a temperate humid climate. Cities in this region like St. Louis, Cincinnati, and NYC, have hot summers that are humid and moderately cold to cool winters with occasional snowfall, sometimes heavy. Rainfall is spread fairly evenly throughout the year, though there is a reversal of wind between summer and winter along the East Coast. Most areas from NYC south to North Carolina have more rainfall in the six warmer months (May through October) when the winds are often from a southerly direction (onshore), than in the six cooler months (November through April), when winds are often from a northerly direction and offshore.
The Pacific Northwest has a temperate oceanic climate. The climate is wet and cool in fall, winter, and early spring, stable and drier in the summer months. In winter rain, upwards of 100 inches (2,500 mm) annually[clarification needed] in some areas, create an overcast and cool climate, but without severe cold like the interior northern US. Cool summers along the immediate coastline are also common. The Great Basin and Columbia Plateau (the Intermontane Plateaus) are arid or semiarid regions with high summer temperatures surpassing 100 °F (38 °C)'s at low elevations, with annual precipitation averages less than 15 inches (380 mm) as a result of the rain shadow of the Sierra Nevada and Cascades.
The characteristics of rainfall 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. Nor'easters moving up the East coast bring cold season precipitation to the Mid-Atlantic and New England states. 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 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.
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 134.78 °F (57.1 °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 Mojave 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 data for United States|
|Record high °F (°C)||98|
|Record low °F (°C)||−80|
|Climate data for Contiguous US average()|
|Daily mean °F (°C)||30.81|
|Precipitation inches (mm)||2.2|
|Source: NOAA (US)|
The United States is affected by a large variety of weather related natural disasters. Deadly and destructive hurricanes occur almost every year along the Atlantic seaboard and the Gulf of Mexico. Hurricanes can also strike Hawaii in the Pacific Ocean. Particularly at risk are the central and southern Texas coasts, the area from southeastern Louisiana east to the Florida Panhandle, the east coast of Florida, and the Outer Banks of North Carolina. Hurricane season runs from June 1 to November 30, with a peak from mid-August through early October. Some of the more devastating hurricanes have included the Galveston Hurricane of 1900, Hurricane Andrew in 1992, and Hurricane Katrina in 2005. The remnants of tropical cyclones from the Eastern Pacific also occasionally impact the southwestern United States, bringing sometimes heavy rainfall.
The Great Plains, the Midwest and the southern United States - because of the 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. Florida also reports many tornadoes but these are rarely very strong. The southern US has a second tornado season during the Fall.
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 throughout the Northeast (although heavy snowstorms can occur almost anywhere).
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 1999-2004.
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.