The continental US is subject to dramatically shifting weather patterns, most notably produced by westerly winds sweeping across the continent from the Pacific. The best time to visit depends on where you're going and what you're planning to see. The Northeast, from Maine down to Washington DC, experiences low precipitation as a rule, but temperatures can range from bitterly cold in winter to uncomfortably hot and humid in the summer. Florida’s temperatures are not dramatically high in summer, but humidity is a problem; in the winter, the state is warm and sunny enough to attract many visitors.
The Great Plains are alternately exposed to seasonal icy Arctic winds and humid tropical airflows from the Gulf of Mexico. Winters around the Great Lakes and Chicago can be abjectly cold, and it can freeze or even snow in winter as far south as Texas, though spring and autumn get progressively longer and milder further south through the Plains. Tornadoes (or “twisters”) are a frequent local phenomenon, tending to cut a narrow swath of destruction in the wake of violent spring or summer thunderstorms.
In the South, summer is the wettest season, with high humidity, and the time when thunderstorms are most likely to strike. One or two hurricanes each year rage across Florida and/or the Gulf of Mexico states between August and October. The winter is mild for the most part and the two shoulder seasons usually see warm days and fresher nights.
Temperatures in the Rockies correlate closely with altitude, so nights can be cold even in high summer. Beyond the mountains in the south lie the extensive arid deserts of the Southwest. In cities such as Las Vegas and Phoenix, the mercury regularly soars above 100ºF, though the atmosphere is not usually humid enough to be as enervating as that might sound and air conditioning is ubiquitous.
West of the Cascades, the Pacific Northwest is the only region where winter is the wettest season, and outside summer the climate is wet, mild and seldom hot. Further south, California’s weather more or less lives up to the popular idyllic image, though the climate is markedly hotter and drier in the south than in the north, where there’s enough snow to make the mountains a major skiing destination from November to April. San Francisco and the northern coast is kept milder and colder than the inland region by its propensity to attract sea fog.