Weather: The United States
Entire shelves of books have been written about United States weather. In part, that's because the country is both sprawling and populous. The nation's agricultural roots and high mobility also play into its obsession with the atmosphere. But the weather itself deserves some credit. Europeans who settled America from east to west were progressively amazed by the spectrum of conditions they encountered. Not many nations can boast a temperature spread of -62°C/-80°F to 57°C/134°F, as well as a place where it rained more than 1000mm/39in in less than a day and a spot where it snowed more than 2500cm/1000in in a single winter.
The contiguous 48 US states are planted firmly in the temperate zone, so you could encounter pleasing temperatures – or uncomfortable cold or heat – almost any place. The people of Miami Beach watched snowflakes fly across the surf in January 1977, and temperatures have soared above 49°C/120°F in North Dakota, which shares a similar latitude to northern France. The Gulf of Mexico furnishes plenty of moisture across the eastern half of the nation, so that Minneapolis can be as humid as New Orleans in mid-summer. Things are drier and brisker in the Rocky Mountains, and California is a weather world unto itself. The Golden State manages to pair an inland zone full of brutal extremes – from the scorching Death Valley to the snow-packed Sierra Nevada – with a sunny coastal climate whose tranquillity is the stuff of legend.
New England and the Mid-Atlantic
A proto-typical land of four seasons, New England features warm summers and cold winters, a spring that's often sunny but chilly, and a crisp, cool autumn that brings some of the world's most dramatic autumn foliage. During any time of year – particularly the stretch from October to June – you can encounter a nor'easter (so called because the surface winds blow from the northeast as the storm approaches). As they steam up the coast, nordeasters typically bring a day or two of rain, wind and gloom. Just as often, however, a chunk of high pressure will move down from Canada to bring a spell of fine, clear weather. The great coastal cities of the mid-Atlantic share a climate roughly similar to New England's, modulated by increasing warmth as you go south. Washington DC, is about 5°C/9°F above Boston on average, enough to make its summers almost sub-tropical but its winters noticeably less bitter, with only about half as much snow as in Boston.
East Coasters often refer to typical summer weather with three Hs – hazy, hot and humid. The worst heat waves, with temperatures that can top 38°C/100°F, occur as drier west winds descend from the Appalachian mountains that parallel the coast from Maine to Alabama. Points at elevation are a few degrees cooler than those nearer the shore. Winter's cold can be brutal from upstate New York into northern New England, where lows can drop to -34°C/-30°F or worse. The major coastal cities rarely drop below -18°C/0°F, but in any winter they could experience a blizzard with gale-force winds and over 30cm/12in of snow that shuts down airports and highways for a day or more. Hurricanes and intense tornadoes are very infrequent this far northeast, but either can be devastating. Even near-misses from tropical storms can cause dangerous rip-tides and swells along the coast. All this travail aside, the big northeastern cities of the US get plenty of fine weather, especially in spring and autumn, when temperatures are often at their most moderate and the air at its clearest.
Only mid-winter completely escapes the heat and humidity that define Southern climate. Average highs from Louisiana to Georgia exceed 26°C/79°F from May into October and sometimes top 35°C/95°F in midsummer. Some relief can be found along the coast, where sea breezes kick in on most hot afternoons, or in the Appalachians, which extend as far south as northeast Alabama and north Georgia The long Southern summer is punctuated by frequent showers and thunderstorms and the occasional tropical storm or hurricane, which can dump enormously heavy rains. Winter brings more variety, with a good helping of bright, clear weather – often pleasingly mild, sometimes bitingly cold – and everything from thunderstorms to snow.
(Winter rains here tend to be heavier during El Nino years.) The belt from Arkansas to the Carolinas is prone to occasional ice storms. Tornadoes occur year-round across the South, but they are most likely in late winter and early spring, and middle-of-the-night twisters are a particular threat. North Florida is a climatic extension of the Deep South, while south of Orlando, the regime is tropical. Here, freezes are almost nonexistent, and the sun is intense even in winter. Florida's rainfall is focused on near-daily summer downpours with vivid lightning. From June through November, some of Florida's rain falls in tropical cyclones, which strike the state once per year on average (though the state endured a record-smashing seven hurricanes in 2004 and 2005).
The Great Lakes and Midwest
The heartland of America has a continental climate that imports elements from every direction. September through October offer the best chance of comfortable weather: summer's worst heat is gone, but serious chill has yet to arrive. Winters here can get as cold or colder than in the Northeast (often with severe wind chill) but the snowfall tends to fall in quick shots rather than in huge dumps. The exception is along the south and east shores of the Great Lakes, where cold air passing over the relatively warm lakes in autumn and early winter can trigger enormous lake-effect snow squalls. These can generate more than 100cm/39in in a day or two along the coast and almost nothing just an hour's drive inland. Warm spells in winter tend to be a bit drier and sunnier here than in the Northeast Afternoons in late spring and summer can bring intense thunderstorms, especially along the tier just south of the Great Lakes from Iowa and southern Minnesota to Ohio. On the shores of the lakes themselves, summer temperatures are moderated, leading to strings of sunny, warm days broken only on occasion when a heat wave builds in. Mid-summer humidity can be surprisingly high across the agricultural Midwest, as the croplands trap and evaporate moisture. Combined with the heat radiating off buildings and pavements, the humidity can keep temperatures in cities like St Louis and Chicago above 26°C/79°F for several nights on end.
The Great Plains
It was once dubbed the Great American Desert by European setders, but the land from North Dakota to Texas is actually a steppe climate – a transition zone between the moist regime further east and the mountains and true deserts to the west. Summers can be scorchingly hot across all of the plains, but the humidity is generally lower as you head northwest. Autumn is the most pleasant season here. Sharp wintertime cold fronts – called “blue northers” in Texas – can push sub-freezing weather from the Canadian border to the Gulf Coast in less than two days. Equally fast warm-ups often sweep in, especially just east of the Rockies, where modified Pacific air can replace Arctic air in a flash: temperatures have been known to jump more than 22°C/40°F in minutes. The northern Plains are markedly colder than their counterparts to the south, but the difference in rainfall from east to west is just as important in Great Plains climate. In southeast Texas, Houston averages 1200mm/47in of rain a year, while the Rio Grande Valley town of Del Rio gets only about half that amount With no major mountain ranges to steer the air flow, the plains are a playground for low-pressure centres that spin up just east of the Rocky Mountains and draw in air from all quarters. Winds often gust to more than 64kph/40mph across the heart of the plains, and its a rare day when there is no noticeable breeze at all. In a dry spring, dust storms can tinge the sky copper and even drop a thin layer of grit, an echo of the disastrous Dust Bowl storms of the 1930s. When there's moisture and a front approaching, spectacular thunderstorms and deadly tornadoes may dance across the landscape.
The Rocky Mountains
Temperature extremes – season-to-season and hour-to-hour – are the defining feature of the vast Rocky Mountains. Thanks to the altitude and the relative lack of moisture, a day's temperature spread can easily span 20°C/36°F. Much of the interior West falls within the Great Basin – an irregular plateau with winter snowfall that's usually fairly light and brief summer storms that can trigger flash floods. Arctic air rarely flows into the basin, but once trapped there, it can produce weeks of frigid smog in cities like Boise and Salt Lake City. Summers in the Great Basin are gloriously bright and warm by day and cool by night. In the vast deserts and sprawling cities of Arizona, southern Nevada and southeast California, summers are blisteringly hot and dry, except for the few weeks (usually in July and August) when the summer monsoon – a weaker version of India's – brings down temperatures slightly but adds humidity. Cities astride the east slopes of the Rockies, such as Denver, have a more prairie-like climate, with thunderstorms in the summer, frequent cold blasts in the winter and occasional rounds of strong wind. Without much moisture to temper the transition seasons, autumn and spring can run hot or cold in the Rockies.
The West Coast
In 1981, author Joel Garreau proposed forming a separate state called Ecotopia – the sliver of coastline from near Los Angeles to British Columbia. From a meteorological point of view, it makes perfect sense to split off this green, moist strip from the more arid eastern parts of California, Oregon and Washington. The exception is the skyscraping Sierra Nevada mountain range, which captures enough Pacific moisture for legendary snowfalls. On the Pacific coast is a far different land: a Mediterranean climate zone that seldom gets muggy or frigid Summers are bone-dry here – it virtually never rains along the California coast from late June into September – but the air is softened by marine moisture, and fog is a frequent companion on spring and summer mornings. The world-famous fogs of San Francisco result largely from Pacific air flowing through the Golden Gate (the narrow strait for which the famous bridge is named). California's Central Valley is a former inland sea with more seasonality than the coast Across California, winter is the wet season, when hills turn green and days of downpour can be interspersed with spells of sunshine (even in a typical mid-winter, only about one out of three days in San Francisco sees rain). Further north, the winter rains are typically more persistent: after two weeks of solid overcast, weathercasters may announce when “sun breaks” are expected. Summers here are as reliably bright as in LA, although the latitude keeps this area a few degrees cooler. Most winter storms along the West Coast are rather gentle by eastern standards, but there are exceptions. Extended wet periods can cause flooding, high winds may knock out power, and when Arctic air sneaks over the Continental Divide, western Washington can get pummelled by ice or heavy snow all the way to the coast. During El NiAo winters, the rains usually increase in California while the Pacific Northwest is prone to drought La Nifia reverses this picture: Seattle and Portland often get even more winter rain than usual, while southern California may miss out.
Alaska's climate is far more similar to that of its next-door neighbour, Canada, than to that of the lower 48 US states. Summers bring the fabled midnight sun to Alaska, along with daytime temperatures that often exceed 21°C/70°F in the interior. Even in summer, however, most of Alaska dips below 10°C/50°F almost every night, and showers are frequent, with a few thunderstorms thrown in In winter, cold air masses often build up over a stretch of days in eastern Alaska and northwest Canada, as if resting before their plunge south. Fairbanks, near the centre of the state, can stay below -40°C/-40°F for days, and ice fog (exacerbated by smoke and car exhaust fumes) makes matters worse. The south coast – an enormous arc that runs from the Aleutians to the Panhandle – tends to be dry and cool in summer and much milder than the rest of the state in winter. Pacific storms that slam into the Panhandle can bring near-biblical amounts of rain or snow. The bogs and tundra of northwest Alaska are unrelentingly cold in the winter and often cloudy and foggy in summer, especially along the coastlines. The air is too cold for much moisture, so it seldom rains or snows heavily. Barrow, the states northernmost settlement, actually qualifies as a desert, receiving only about 1000mm/4in of rain and melted snow a year.
The 50th US state delivers on its tropical promise: trade winds, sunshine and reliable temperatures. There's only a slight warm-up from winter to summer, and the typical range between day and night throughout the year is only about 6°C/10°F. What makes the difference in Hawaiian weather is location. Because of the persistent northeast trades, the eastern sides of the islands experience heavy rainfall – Hilo, on the Big Island, averages 3300mm/130in a year – while the west shores, such as the Big Island's Kona Coast, are perpetually parched. Even in Hilo, it doesn't rain constantly: showers are actually focused during the evening and overnight hours. (Emissions from the Big Island's two volcanoes sometimes mix with fog to produce a hazy compound that locals call “vog”) Honolulu, on the southeast coast of Oahu, has a nearly rain-free summer and a winter with just enough rain to keep things green. El Nirto tends to make winter and spring in Hawaii drier than usual. Although the waters surrounding Hawaii are a little too cool for hurricanes to thrive, one manages to straggle through every few years.