San Diego is blessed with a mild climate, low humidity, and good air quality. In fact, Pleasant Weather Rankings, published by Consumer Travel, ranked San Diego's weather no. 2 in the world. It's worth keeping in mind, though, that San Diego County covers more than 4,500 square miles and rises in elevation from sea level to 6,500 feet. It can be a pleasant day on the coast but blisteringly hot on the inland mesas; or it can be a foggy day at the beach but gloriously sunny just minutes away downtown.
With its coastal setting, the city of San Diego maintains a moderate climate. Although the temperature can change 20° to 30°F between day and evening, it rarely reaches a point of extreme heat or cold; daytime highs above 100°F (38°C) are unusual, and the mercury dropping below freezing can be counted in mere hours once or twice each year. San Diego receives very little precipitation (just 10 in. of rainfall in an average year); what rain does fall comes primarily between November and April, and by July, the hillsides start to look brown and parched. It's not unusual for the city to go without measurable precipitation for as long as 6 months in the summer and fall.
Perhaps the best time of year in San Diego is the fall. The days are still warm (even hot), and the cool nights remind you that yes, even in Southern California, there is a change of seasons. February and March are also beautiful periods when the landscapes are greenest and blooming flowers at their peak, although it's still too cold for all but the heartiest people to go into the ocean without a wet suit. Beachgoers should note that late spring and early summer tanning sessions are often compromised by a local phenomenon called May Gray and June Gloom -- a layer of low-lying clouds or fog along the coast that doesn't burn off until noon (if at all) and returns before sunset. Use days like these to explore inland San Diego, where places such as the Zoo Safari Park are probably warm and clear.
A more unpredictable Southern California phenomenon is the hot, dry winds known as Santa Anas. They usually hit a couple times a year, typically between September and December, and can last for several days. These desiccating winds heighten wildfire danger and can be a backcountry firefighter's worst nightmare, but Santa Anas invariably bring warm temperatures and crystal-clear skies. Occurring irregularly every 2 to 7 years, the El Niño weather pattern -- storms created by a warming of Pacific Ocean waters -- can cause unusually heavy winter rains. A 1988 El Niño storm even toppled a research platform off Mission Beach (it can now be explored by divers as part of San Diego's Wreck Alley).
San Diego is busiest between Memorial Day and Labor Day. The kids are out of school and everyone wants to be by the seashore; if you visit in summer, expect fully booked beachfront hotels and crowded parking lots. The week of the July 4th holiday is a zoo at Mission Beach and Pacific Beach -- you'll either love it or hate it. But San Diego's popularity as a convention destination and its temperate year-round weather keep the tourism business steady the rest of the year, as well. The only slow season is from Thanksgiving to early February. Hotels are less full, and the beaches are peaceful and uncrowded; the big family attractions are still busy on weekends, though, with residents taking advantage of holiday breaks. A local secret: Although they're in the coolest, rainiest season (relatively speaking, anyway), November through February are also the sunniest months of the year.
Banks, government offices, post offices, and many stores, restaurants, and museums are closed on the following legal national holidays: January 1 (New Year's Day), the third Monday in January (Martin Luther King, Jr., Day), the third Monday in February (Presidents' Day), the last Monday in May (Memorial Day), July 4 (Independence Day), the first Monday in September (Labor Day), the second Monday in October (Columbus Day), November 11 (Veterans' Day/Armistice Day), the fourth Thursday in November (Thanksgiving Day), and December 25 (Christmas). The Tuesday after the first Monday in November is Election Day, a federal government holiday in presidential-election years (held every 4 years, and next in 2012).