Climate of the United States
The climate of the United States varies due to large differences in latitude, and a wide range of geographic features, including mountains and deserts. West of the 100th meridian, much of the US is semi-arid to arid, even desert in the far southwestern US. 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, but with a cool oceanic climate in the southeast (Alaska Panhandle), southwestern peninsula and Aleutian Islands, and a polar climate in the north. The archipelago state of Hawaii, in the middle of the Pacific Ocean, is tropical, with rainfall concentrated in the cooler season (November to March).
Like most land masses located in the middle and lower middle latitudes, 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 "Bermuda High" over the subtropical Atlantic Ocean typically sends warm, humid air over the eastern, southern and central United States - resulting in southerly airflow, warm to hot temperatures, high humidity and occasional thunderstorm activity. In summer, high pressure over the central Pacific typically results in northwesterly airflow, stable conditions and cool to mild conditions along most of the immediate Pacific coast, from Washington state to San Diego, CA. In the Northern Hemisphere winter, the subtropical highs retreat southward. The polar jet stream (and associated conflict zone between cold air masses from Canada and warmer air masses from the Southwest or, more commonly, Gulf of Mexico) moves further south into the United States - bringing much greater weather variation, including major rain and snow events, and much more variable, and sometimes dramatically colder, temperatures. 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.
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 in the United States. The Gulf of Alaska is where most of the storms that enter the United States begin. These "North Pacific lows" enter the US through the Pacific Northwest, then move eastward across the northern Rocky Mountains into the northern Great Plains. From there, they often take several tracks - toward the Great Lakes, northeast through the St. Lawrence Valley into Quebec, or off the upper East Coast. Across the central states in winter and 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 severe thunderstorms south and east of the storm track - often simultaneously. Across the northern states in winter (usually Montana/Dakotas eastward), "Alberta clipper" storms can be frequent, usually bringing light to moderate snowfalls, but often, windy and severe Arctic outbreaks behind them.
In the summer months, storms are much more localized (short-duration thunderstorms are 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. Sometimes, often-overnight mesocscale-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 during the summer months. From mid-summer to mid-fall (generally July to October), tropical cyclones sometimes approach or cross 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 southern and southeastern areas. The United States has more tornadoes than the rest of the countries of the world combined.
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. during the summer months. In winter, daily temperatures in the southwest are cooler with highs in the 60s and lows in the 30s and 40s.
In Phoenix, Las Vegas and similar Southwestern desert areas, June is often the driest month, after Pacific-originating winter storms have concluded and before the summer "monsoon" begins. The Southwest and the Great Basin are affected by said 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.
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. 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 far southwest (central and southern California coast) has a Mediterranean climate. Daily high temperatures range from the 70s to low 80s in the summer to the 50s to low 60s in winter...with low temperatures from the 60s in summer to the 40s in winter.. Like most Mediterranean climates, much of 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 over 100 F.
- Gulf Coast/Tennessee Valley/South Atlantic states
The Gulf and South Atlantic states have a subtropical climate with mostly mild winters and hot, humid summers. Coastal cities like Houston, New Orleans, Orlando, and Charleston, have average July highs in the lower 90s, and combined with moist tropical air, this creates sultry summer weather conditions. In winter, humidity and dew points fall, and cooler, more variable weather conditions prevail. The direction of prevailing winds change from southerly (tropical) in summer to northerly (continental) in fall and winter. Daily high temperatures range from the upper 40s to 60s, and lows from upper 20s to the mid 40s. Much of the interior South (Tennessee, Kentucky and the northern Gulf states) has evenly distributed precipitation across the year, but with a slight winter or spring maximum, with March commonly the wettest month and August to October the driest months. Part of the Mid-South includes "Dixie Alley," an area of relatively high tornado risk, comprising much of Tennessee, Kentucky and northern Alabama, Mississippi and Georgia. Here tornado risk begins rising in February, peaks in March and April and diminishes in May and June. A secondary period of higher tornado risk in Dixie Alley is late fall (November). In the coastal south Atlantic states (roughly Norfolk, VA area southward), the wettest month is usually July or August, transitioning toward a four-month (June-September) "wet season" in peninsular Florida due to frequent summer thunderstorms. In central and southern Florida the dry season (November through April) becomes pronounced, with brush fires and water-use restrictions frequent from January to April.
Southern Florida has a tropical savanna climate, with all months having a mean temperature of higher than 65 °F (18 °C), and a wet season from May through October (due to frequent summer thunderstorms and in some years, torrential rain from hurricanes and tropical storms), and dry season from November through April. In cities like Fort Lauderdale, Miami, Naples, and Palm Beach average daily highs range from the 70s in winter to the lower 90s in summer. Average overnight lows range from the upper 50s in winter to the mid and upper 70s in summer. The only areas of the US mainland known to never have experienced a freeze (32 °F (0 °C)) are the Florida Keys and the coastal areas of Miami.
While tornadoes in the Midwest are more severe, a higher rate of deaths are experienced in Florida, due to a higher population density and quantity of manufactured homes.
- Southern Plains/lower Midwest/Middle East Coast
The region from the southern Plains, to the lower/eastern Midwest, eastward to the central East Coast (NYC/coastal Connecticut southward to southern Virginia) has a temperate humid climate. Cities in this region include Wichita, KS, St. Louis, MO, Peoria and Springfield, IL, Indianapolis, IN, Columbus, OH, Pittsburgh and Philadelphia, PA, Washington, DC, and New York City. This climate features hot summers and cool to cold winters with occasional snowfall. 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. Average high temperatures range from near 30 to the low 40s in winter to the 80s to 90s (in Oklahoma) in summer, while lows range from the 10s and 20s in winter to 60s in summer (70s in Oklahoma).
- Western Great Plains/North-Central/Great Lakes/New England
The northern half of the Western 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 10s to 30s in winter to 70s and 80s in summer, while overnight lows range from below 0 in winter (in much of Minnesota and North Dakota) to 50s and 60s 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. From late fall to spring, the New England states, especially coastal and island areas (i.e. Nantucket, Martha's Vineyard), are subject to "nor'easters" - intense storms that can bring very high winds, heavy rains, prodigious snowfalls and damaging coastal storm surges topped by battering waves. 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). From northern Indiana and Michigan westward to and including Iowa, Minnesota, Nebraska and both Dakotas, average precipitation is lowest in winter (typically February) and highest in June. Here, the May-August period is relatively wet on average.
In the Midwest (and 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, even 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.
- Pacific Northwest
The Pacific Northwest has a temperate 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. The wettest month is typically December; the driest, July. In the summer months normal 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).
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. ( i.e. Minnesota/North Dakota). 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 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).
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 (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.
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)|
Natural disasters and effects
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 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 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 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.
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