San Diego has a temperate Mediterranean climate (Koppen climate classification CSb).
It enjoys mild, Dry-Summer subtropical, sunny weather throughout the year. Average monthly temperatures range from about 57 °Fahrenheit (14°C) in January to 72 °Fahrenheit (22°C) in July, although late summer and early autumn are typically the hottest times of the year.
The average annual daily temperature is 62.6° Fahrenheit (17°C). Snow and ice are virtually nonexistent in the wintertime, typically occurring inland from the coast when present. "May gray and June gloom", a local saying, refers to the way in which San Diego sometimes has trouble shaking off the marine layer, a cloudy layer typically higher in the atmosphere than fog, that comes in during those months. Temperatures soar to very high readings on rare occasions, chiefly when easterly winds bring hot, dry air from the inland deserts (these winds are called "Santa Anas").
The record highest temperature at the airport is 111°F (44°C) on September 26, 1963, and the record lowest temperature is 29°F (-2°C) on January 4, 1949. The National Weather Service reports that San Diego's all-time lowest temperature was 25°F (-4°C) on January 7, 1913.
The National Climatic Center says that temperatures exceed 90°F (32°C) on an average of 3.5 days annually and 100°F on .2 day annually.
The average annual precipitation is less than 12 inches (300 mm), resulting in a borderline arid climate. Rainfall is strongly concentrated in the cooler half of the year, particularly the months December through March, although precipitation is lower than any other part of the U.S. west coast. The summer months are virtually rainless. Rainfall is highly variable from year to year and from month to month, and San Diego is subject to both droughts and floods.
Thunderstorms are very rare. National Weather Service research has found documentation of a hurricane which hit San Diego on October 2, 1858, the only hurricane in history to reach California.
There is even greater documentation of a tropical storm which affected much of the southern California coast in September 1939. The greatest 24-hour rainfall at the airport was 2.71 inches on February 6, 1937, and the greatest monthly rainfall was 9.09 inches in January 1993.
Measurable snowfall has never occurred in downtown San Diego; snow flurries were reported at the airport in January 1949 and January 1979.
Light amounts of snow have occurred in higher locations within the city limits, according to the National Weather Service's climatological summary.
Climate in the San Diego area often varies dramatically over short geographical distances, due to the city's topography (the Bay, and the numerous hills, mountains, and canyons): frequently, particularly during the "May gray / June gloom" period, a thick "marine layer" cloud cover will keep the air cool and damp within a few miles of the coast, but will yield to bright cloudless sunshine between about 5 and 15 miles (24 km) inland—the cities of El Cajon and Santee for example, rarely experience the cloud cover.
This phenomenon is known as microclimate.
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