Climate of the United States

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For change, see Climate change in the United States.
Climate zones of the Contiguous United States.

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 U.S. is semi-arid to desert in the far southwestern U.S.. 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 is 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 landmasses 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 and the polar jet stream. In the northern hemisphere summer, the oceanic subtropical high pressure systems move north - and much of the central and southern U.S. see stable weather, sunshine, and warm to hot temperatures. In the Northern Hemispheric 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 warmer temperature. Areas in the extreme 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 often does not 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 U.S. 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.[1]

Contents

Regional Overview [edit]

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

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 much 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 is scanty all year, and many months average less than 0.7 inches (18 mm) of rain. The Southwest and the Great Basin are also affected by the monsoon from the Gulf of California from July–September, which brings localized thunderstorms to the region that can result in flash flooding, despite this, drought like conditions are frequent in the region, often lasting for periods of years or longer. Large Forest Fires across the Western United States (especially the southwest) occur annually.

The far southwest (California coast) has a mild 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.

Southeast

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 F, and combined with the moist tropical air, creates sultry summer weather conditions. In winter, daily temperatures in the southeast are much cooler with highs in the 50 °F (10 °C)’s and 60 °F (16 °C)’s and lows in the 30 °F (−1 °C) and 40 °F (4 °C)’s. Rainfall in plentiful in the southeastern U.S., and summer is normally the wettest time, especially in areas along the Gulf and south Atlantic coast.

The far southeast (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 Ft. Lauderdale, Miami, Naples, Palm Beach…etc. 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 U.S. mainland known to have never experience a freeze (32 °F (0 °C)) is the Florida Keys and some areas of coastal south Florida.

North-Central/Midwestern/New England

The Midwest, northern Plains, Great Lakes, and New England regions have a humid continental climate. Here there are four strong 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 powerful and life-threatening tornadic storms, 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 noreasters 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 far 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.[citation needed]

Southern Plains/Atlantic Coast (NYC to North Carolina)

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 long hot summers that are humid and moderately cold to cool winters with occasional snowfall. 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.

Pacific Northwest

The Pacific Northwest has a temperate oceanic climate. The climate is wet and cool in fall, winter, and early spring, and cool 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 northern and interior U.S.. Cool summers along the immediate coastline are also common. The Great Basin and Columbia Plateau (the Intermontane Plateaus) are arid or semiarid regions, with annual precipitation averages less than 15 inches (380 mm) as a result of the rain shadow of the Sierra Nevada and Cascades.

Precipitation [edit]

Average precipitation.
See also: United States rainfall climatology

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.[2] Nor'easters moving up the East coast bring cold season precipitation to the Mid-Atlantic and New England states.[3] Lake-effect snows add to precipitation potential downwind of the Great Lakes,[4] as well as Great Salt Lake and the Finger Lakes during the cold season. The 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.[5] The El Niño-Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics.[6][7][8][9]

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.[10] 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.[11] 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.[12]

Extremes [edit]

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).[13] On the other end of the spectrum, Death Valley, California once reached 134.78 °F (57.1 °C), the second-highest temperature ever recorded on Earth.[14]

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 U.S., with Quinault Ranger Station in Washington having an average of 137 inches (3,480 mm).[15] Hawaii receives even more, with 460 inches (11,684 mm) measured annually, on average, on Mount Waialeale, in Kauai.[16] The Mojave Desert in the southwest is home to the driest locale in the U.S. Yuma, Arizona, has an average of 2.63 inches (67 mm) of precipitation each year.[17]

Climate data for United States
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Record high °F (°C) 98
(37)		 105
(41)		 108
(42)		 118
(48)		 124
(51)		 129
(54)		 134
(57)		 127
(53)		 126
(52)		 116
(47)		 105
(41)		 100
(38)		 134
(57)
Record low °F (°C) −80
(−62)		 −75
(−59)		 −68
(−56)		 −50
(−46)		 −25
(−32)		 −1
(−18)		 10
(−12)		 5
(−15)		 −13
(−25)		 −48
(−44)		 −61
(−52)		 −72
(−58)		 −80
(−62)
Source: http://www.infoplease.com/ipa/A0762182.html

Overall average(s) [edit]

Climate data for Contiguous U.S. average([1])
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Daily mean °F (°C) 30.81
(−0.66)		 34.65
(1.47)		 42.56
(5.87)		 52.02
(11.12)		 61.04
(16.13)		 69.25
(20.69)		 74.29
(23.49)		 72.77
(22.65)		 65.42
(18.57)		 54.77
(12.65)		 42.51
(5.84)		 33.38
(0.77)		 52.79
(11.55)
Precipitation inches (mm) 2.2
(55.88)		 2.0102
(51.06)		 2.4
(60.96)		 2.4402
(61.98)		 2.8799
(73.15)		 2.9098
(73.91)		 2.7701
(70.36)		 2.6098
(66.29)		 2.4902
(63.25)		 2.1598
(54.86)		 2.1201
(53.85)		 2.2598
(57.40)		 29.2299
(742.44)
Source: NOAA (U.S.)[18]

Natural disasters and effects [edit]

See also: Floods in the United States, List of United States hurricanes, and Tornadoes in the United States
Total devastation in Gulfport, Mississippi following Hurricane Katrina in 2005.

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.[19] Hurricanes can also strike Hawaii in the Pacific Ocean.[20] 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.[21] Some of the more devastating hurricanes have included the Galveston Hurricane of 1900,[22] Hurricane Andrew in 1992,[23] and Hurricane Katrina in 2005.[24] The remnants of tropical cyclones from the Eastern Pacific also occasionally impact the southwestern United States, bringing sometimes heavy rainfall.[25]

A powerful tornado in Texas.

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 U.S., tornadoes are more common than anywhere else on Earth[26] 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 U.S. 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).

The Southwest has the worst droughts; one is thought to have lasted over 500 years and to have decimated the Anasazi people.[27] 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 U.S. 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.[28] 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 dude to exposure to excessive cold temperatures, a mean of 689 per year.[29]

See also [edit]

Portal icon United States portal

References [edit]

  1. ^ Charles A. Doswell III. Severe Storms. Retrieved on 2008-03-22.
  2. ^ Norman W. Junker. West Coast Cold Season Heavy Rainfall Events. Retrieved on 2008-03-01.
  3. ^ George J. Maglaras, Jeff S. Waldstreicher, Paul J. Kocin, Anthony F. Gigi, and Robert A. Marine. Winter Weather Forecasting throughout the Eastern United States. Part 1: An Overview. Retrieved on 2008-03-01.
  4. ^ Thomas W. Schmidlin. Climatic Summary of Snowfall and Snow Depth in the Ohio Snowbelt at Chardron. Retrieved on 2008-03-01.
  5. ^ Martin A. Baxter, Charles E. Graves, and James T. Moore. A Climatology of Snow-to-Liquid Ratio for the Contiguous United States. Retrieved on 2008-03-21.
  6. ^ John Monteverdi and Jan Null. WESTERN REGION TECHNICAL ATTACHMENT NO. 97-37 NOVEMBER 21, 1997: El Niño and California Precipitation. Retrieved on 2008-02-28.
  7. ^ Nathan Mantua. La Niña Impacts in the Pacific Northwest. Retrieved on 2008-02-29.
  8. ^ Southeast Climate Consortium. SECC Winter Climate Outlook. Retrieved on 2008-02-29.
  9. ^ Reuters. La Nina could mean dry summer in Midwest and Plains. Retrieved on 2008-02-29.
  10. ^ National Weather Service Forecast Office Flagstaff, Arizona. The Monsoon. Retrieved on 2008-02-28.
  11. ^ Roth, David M; Weather Prediction Center (January 7, 2013). "Maximum Rainfall caused by Tropical Cyclones and their Remnants Per State (1950–2012)". Tropical Cyclone Point Maxima. United States National Oceanic and Atmospheric Administration's National Weather Service. Retrieved March 15, 2013. 
  12. ^ Walker S. Ashley, Thomas L. Mote, P. Grady Dixon, Sharon L. Trotter, Emily J. Powell, Joshua D. Durkee, and Andrew J. Grundstein. Distribution of Mesoscale Convective Complex Rainfall in the United States. Retrieved on 2008-03-02.
  13. ^ Williams, Jack Each state's low temperature record, USA Today, URL accessed 13 June 2006.
  14. ^ "Weather and Climate" (PDF). Official website for Death Valley National Park. National Park Service U. S. Department of the Interior. January 2002. pp. 1–2. Retrieved October 5, 2006. 
  15. ^ National Atlas. Average Annual Precipitation, 1961-1990. Retrieved on 2006-06-15.
  16. ^ Diana Leone. Rain supreme. Retrieved on 2008-03-19.
  17. ^ Hereford, Richard, et al., Precipitation History of the Mojave Desert Region, 1893–2001, U.S. Geological Survey, Fact Sheet 117-03, URL accessed 13 June 2006.
  18. ^ "CONTIGUOUS UNITED STATES Climate Summary". 
  19. ^ Hurricane Research Division. Chronological List of All Hurricanes which Affected the Continental United States: 1851-2005. Retrieved on 2008-03-02.
  20. ^ Central Pacific Hurricane Center. Previous Tropical Systems in the Central Pacific. Retrieved on 2008-03-02.
  21. ^ National Hurricane Center. Peak of Season Graphic. Retrieved on 2008-03-02.
  22. ^ The Galveston storm of 1900—The deadliest disaster in American history. National Oceanic and Atmospheric Administration. Retrieved on 2007-12-18.
  23. ^ Edward Rappaport; National Hurricane Center (1993-12-10). Hurricane Andrew (Preliminary Report). United States National Oceanic and Atmospheric Administration's National Weather Service. http://www.nhc.noaa.gov/1992andrew.html. Retrieved 2012-06-21.
  24. ^ Knabb, Richard D; Rhome, Jamie R; Brown, Daniel P; National Hurricane Center (December 20, 2005; updated August 10, 2006) (PDF). Hurricane Katrina (Tropical Cyclone Report). United States National Oceanic and Atmospheric Administration's National Weather Service. http://www.nhc.noaa.gov/pdf/TCR-AL122005_Katrina.pdf. Retrieved 2006-05-30.
  25. ^ David M. Roth. Tropical Cyclones in the West. Retrieved on 2008-03-02.
  26. ^ NOVA, Tornado Heaven, Hunt for the Supertwister, URL accessed 15 June 2006.
  27. ^ O'Connor, Jim E. and John E. Costa, Large Floods in the United States: Where They Happen and Why, U.S. Geological Survey Circular 1245, URL accessed 13 June 2006.
  28. ^ http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6136a6.htm
  29. ^ http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5407a4.htm

External links [edit]

Climate of the United States
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Climate of North America
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