Due to its size, the United States of America experiences a wide range of weather, from the arid deserts of the Southwest to the hot and humid Southeast to the cold northern states. There's also room for the occasional tornado and hurricane to squeeze in there.
Most of the US' weather comes from the polar jet stream, which brings moves pressure systems through the country from the west to the east, with various curves and twists along the way. Thus, it's not uncommon for storms to move in from the Pacific Northwest and cross all the way to the Atlantic. This can lead to serial severe weather across the entire country, depending on the season.
Along the coasts, weather tends to be more static, while the center of the U.S. experiences a greater range of weather variables.
Note that, when reading measurements, the US uses American Customary Measurements rather than the metric system. See that page for translating between the two.
In general, the climates can be broken down by region.
Due to the geography, the Northeast generally experiences warm-to-hot summers, with average temperatures in the 80s and low 90s (though triple-digit temperatures usually only come a few days a year), and cold winters, with daily highs ranging from the high 20s to the low 40s and nighttime lows falling into the teens and single digits. Temperatures on the coast are moderated by the Gulf Stream, resulting in cooler summers and warmer winters in such areas, explaining why places like Cape Cod, the Hamptons and the Jersey Shore are such popular getaways. Heading further inland, the high altitude of the Appalachian Mountains produces cooler weather year-round, allowing them to support large winter resorts as well as a number of summer getaways (like the famous Borscht Belt resorts of the Catskills).
Precipitation averages about 46 inches year-round in the region, including both rain and snow. The latter tends to vary by area, largely due to the Great Lakes—the "lake effect" produces incredibly heavy snowfall of up to 200 inches per year, relegated to thin strips along the lakeside of upstate New York and northeastern Pennsylvania (including the cities of Erie, Buffalo and Rochester). Along the coast, there is the potential for ocean storms. Winter brings Nor'easters, powerful blizzards with tropical storm-force winds and precipitation (and even shape) to match—the terms "Snowmageddon", "Snowpocalypse" and "Snowzilla" are all known to people on the East Coast. Hurricanes during the summer are less common, and those that make it to the Northeast have usually downgraded to a Category 1 or tropical storm level due to the cooler waters. However, storms as big as Category 3 aren't unheard of (the last one hit Long Island and Connecticut in 1938), and New York City is considered to be at very high risk for such a storm—the aforementioned 1938 hurricane made landfall just sixty miles east of the city.
Also of note is Mount Washington in New Hampshire, the tallest peak in the eastern US at 6288 ft., which advertises itself as experiencing the worst weather in the world. While this is most definitely not the case, Mount Washington was the site of the highest directly measured wind speed at 231 mph.
The South experiences a generally subtropical climate, with hot, humid summers with daytime temperatures in the 90s and cool winters with highs typically running from the high 40s to the low 60s. During the summer, it's not uncommon for the daily highs to break into the triple digits. Though the Appalachian and Ozark Mountains get regular winter snowfall, as do Kentucky (which borders the Midwest), parts of Tennessee and the Chesapeake Bay area (which borders the Northeast), snow of greater than six inches isn't seen very often in the Deep South or the lowlands south of Virginia. This typically leads to a lot of ribbing from Northerners about how Southerners can't drive in the snow. It's also why Southern roadways are usually much smoother to drive on than their northern counterparts—it usually doesn't get cold enough for the water seeping into the ground to freeze and open cracks and potholes.
The average yearly rainfall is about 60 inches, increasing towards the Gulf of Mexico, though the further westward you travel, the hotter, drier, and more arid it becomes, particularly in the grasslands of western Texas and Oklahoma. On the other side, the further south you go, particularly in Florida, the weather becomes more tropical and wet.
This region is most at risk for both tornadoes (year-round) and hurricanes (late summer and fall), with about 40 deaths per year for the former, and about 90 deaths per year for the latter. There's also been recent concern with drought and excessive heat waves—the summer of 2011 seeing broken heat records across the entire region, with Savannah, GA recording 56 days of 100+ degrees in a row, Oklahoma City, OK recording 64 days, and San Angelo, TX recording 98 days. Not helping on the drought front is the rapid growth and development of the region's population, which has placed increasing pressure on existing reservoirs.
The Midwest can be broken into two parts. The eastern half, huddled around the Great Lakes, enjoys a continental climate with similar conditions to the Northeast, including problems with lake effect snow (Michigan, surrounded by lakes on three sides, is especially hard-hit from this). Rainfall averages about 35 inches, slightly less than the Northeast due to its distance from the ocean.
The western half of the region, on the Great Plains, is much more arid, getting an average of just 15 inches of precipitation per year, with generally warmer summers and colder winters than its eastern counterpart due to the flatter terrain. This also allows for strong blizzards in the winter. Back in the 19th century, explorers called the Great Plains the "Great American Desert", seeing it as too dry and barren to support much habitation, and even today, much agriculture is reliant on aquifers and rivers.
The entire region is also regularly hit by tornadoes in the summer, though not quite to the extent of the Southeast.
The Southwest is an overall dry region, with hot, dry summers and cold winters. The cold winters is particularly true in the Rocky Mountain region, home to many of America's premier winter resorts, although New Mexico and Arizona are also known to experience very cold winters up in the mountains. (Flagstaff, Arizona goes over two hundred days a year with low temperatures below freezing, the highest number of days in the nation outside Alaska.) The hot summers, meanwhile, hold fast in the deserts of Arizona, New Mexico, Nevada, southern Utah and southern California. In these areas, it's fairly common for summer temperatures to break into the triple digits, with Death Valley, CA holding the second hottest recorded temperature in the world at 134 °F.
Foehn winds, strong mountain winds that blow over the mountain tops and warm the leeward side to startlingly high temperatures (records of raising the temperature nearly 100 °F is not unheard of), are found in the more mountainous areas, particularly in Colorado. High snowfall and blizzards are also a problem in the mountains.
The coastal areas of central-to-southern California have a climate that is unique in the nation. Called a "Mediterranean" climate after the largest area in the world to possess it, it is characterized by dry summers, wet winters, and mild temperatures year-round. Outside California and the Mediterranean, it is only found in a few other places worldwide, such as Santiago, Cape Town, |Adelaide and Perth. Of course, since Hollywood happens to be located in this tiny region, filmmakers often assume that this is what it's like in most parts of the world, leading to the trope It's Always Spring.
This entire region is particularly susceptible to drought, leading to water rights issues, something that has sparked political fights between and within states in the past. In addition, southern California experiences Santa Ana winds, which can cause widespread damage as well as increased wildfire danger.
The Northwest has two main climates, divided by the Cascade Mountains running through central Washington and Oregon and the Sierra Nevada range in northeastern California. To the east, summers ranging from warm to incredibly hot (occasionally in the triple digits in some areas) are paired with bitterly cold winters, and temperatures in the negatives are common. Rainfall is very light—Spokane, Washington and Cheyenne, Wyoming, two of the "wetter" cities in the area, receive only about 15 inches of precipitation per year, and much of eastern Oregon and southern Idaho are either arid or semi-arid. This area has problems with blizzards and foehn winds, though water rights tend to overshadow these in the warmer seasons.
The western portion is much rainier, thanks to the mountains keeping the moist air from moving eastward (and leading to the water rights problems to the east). Both summers and winters are mild, with summer temperatures often hovering in the 70s and winter temperatures rarely falling below 40. Snow isn't unheard of, but not particularly common, excluding the more mountainous areas. Whereas the areas east of the mountains are dry, the areas to the west are almost stereotypically wet; Seattle and Portland are always depicted as exceptionally rainy, while San Francisco's fog is almost as much a part of the city's image as cable cars, the Golden Gate Bridge, tie-dye Volkswagens and rainbow flags.
Alaska and Hawai'i
The non-continental states enjoy fairly static weather patterns.
The Alaskan panhandle enjoys a climate not unlike the western part of the Northwest, though with more snow and about ten degrees cooler due to the higher latitude. However, the further north and further inland you go, the colder it gets. A majority of the state experiences sub-arctic conditions, with cold summers and freezing winters. Temperatures in the negatives are the norm, with summer temperatures barely warming above freezing in the northern regions of the state. Even the city of Anchorage, located along the southern coast and having a mild urban heat island effect, has experienced temperatures below freezing in every month other than July, and the average winter high temperature is in the low 20s. Overall, Alaska holds the coldest temperature records for all of the months (excluding July and August), as well as the lowest temperature recorded in North America at -80 °F. It is very much the American Siberia, right down to conspiracy theories claiming that the shadow government is building a gulag-esque concentration camp just outside of Fairbanks.
Cold is the worst aspect of Alaska's climate, with strong winter storms in the Bering Sea bring in white-out conditions and large ocean waves.
In contrast, Hawaii enjoys a tropical climate with warm summers and warm winters, though it's less humid than many other tropical regions. Hawai'i is the only state in the U.S. to not record a sub-freezing temperature, though snow isn't unheard of on the slopes of Mauna Loa and Mauna Kea. At the same time, temperatures above 100 °F are also unheard of. The state has the second highest rainfall average at 460 inches per year, though due to the mountainous features, dry portions on the islands are also commonplace.
Hurricanes aren't common, but there have been instances, with the most recent being Hurricane Iniki in 1992. Tornadoes are also not commonplace, but still have happened, with 40 total instances recorded since 1950. In contrast, flash flooding and mudslides are a much larger problem for the state.
Most prominent is America's propensity for severe weather, with tornadoes being the most unique.
The U.S. has the highest amount of tornadoes in the world, typically around 1,300 per year. They are most common in the eastern half of the U.S., though every state has had at least 7 tornadoes (Alaska is the least at risk) since 1950. Texas has the highest concentration of tornadoes overall, though the state with the most tornadoes per square mile is Florida. When it comes to strong tornadoes (F/EF-3 and higher), Oklahoma tops the list. However, due to less measures taken in tornado safety, the Southeast is typically the worst hit in terms of damage costs and deaths.
Tornadoes are worst in the spring and early summer, though it is important to note that there isn't a "tornado season," as they happen year round, just in different geographical regions. During the winter months, tornadoes are most frequent in the Southeast, particularly in February and March, before shifting to the Plains in spring, and then moving into the Midwest and Great Lakes region during the summer months.
During the late summer and fall, hurricanes are the biggest threat, affecting all of the Gulf states and the Eastern shoreboard. Florida gets the short end of the stick, getting hit on both sides by 40% of all hurricanes. Texas isn't far behind, with a total of 83% of category 4+ hurricanes hitting either Texas or Florida. In total, only about 2 hurricanes hit the U.S. per year, but tend to cause more cost in damages and death than tornadoes.
Other severe weather events include Nor'easters, heat/cold waves, and flooding. While the former is region-specific, affecting the Atlantic side of the U.S., the latter two are much less choosy. Because of this, the U.S. has a much greater deathtoll from these (960 and 84, respectively). However, in general, Nor'easters have a greater reach, causing vast damage from the Gulf states to eastern Canada.
- Connecticut, Delaware, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Vermont
- Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Virginia, and West Virginia
- Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, Wisconsin
- Arizona, (Southern) California, Colorado, Nevada, New Mexico, and Utah
- (Northern) California, Idaho, Montana, Oregon, Washington, Wyoming
- Seattle itself actually receives only 37 inches per year, less than most cities on the East Coast, as it has dry summers and is itself in the rain shadow of the Olympic Mountains. Ditto for Portland, which sits in the rain shadow of the Oregon Coast Range. However, it is exceptionally cloudy and drizzly outside the summer -- between October and May, it's overcast six out of every seven days in Seattle.