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 farther south one travels, and drier the farther west, until one reaches the West Coast.
West of 100°W, much of the U.S. has a cold semi-arid climate in the interior upper western states (Idaho to the Dakotas), to warm to hot desert and semi-arid climates in the southwestern U.S. East of 100°W, the climate is humid continental in northern areas (locations roughly above 40°N, 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.https://www.princeton.edu/~ota/disk3/1984/8426/842609.PDF Princeton.edu. Chapter 6 – U.S. Tropical Forests: Caribbean and Western Pacific. Retrieved January 8, 2019. 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 100°W and south of 40°N.
In the warm season, storm systems affecting a large area are less frequent, and weather conditions are more solar 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.
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 °F. 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, Arizona eastward toward El Paso, Texas), winter-season precipitation decreases, while the summer monsoon increasingly provides a summer precipitation maximum. For example, El Paso and Albuquerque, New Mexico 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 in many years, and can be severe to extreme in especially hot, dry summer seasons.
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 °F, but low temperatures drop into the low 60s °F and even 50s °F. As in other temperate desert climates, the dry air results in large differences (sometimes over 40 degrees °F) 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 areas in 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. 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, Fresno, Sacramento areas), the wet-winter, dry-summer pattern remains, but winters are cooler, subject to periods (several days to possibly several weeks) of Tule fog, and more prone to occasional frost or freeze. Here, summers are much hotter (July being the hottest month), 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, Charleston and Wilmington 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.
In the interior South, in cities like Raleigh, Atlanta, Birmingham, Nashville, Lexington, 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 around 95 to 97 °F., and lows around 75 to 77 °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, both regions bordering the tropical climate zone.