The climate of San Diego, California is classified as a hot-summer Mediterranean climate (Köppen climate classification Csa). The basic climate features hot, sunny, and dry summers, and cooler, wetter winters. However, San Diego is much more arid than typical Mediterranean climates, and winters are still dry compared with most other zones with this type of climate. The climate at San Diego International Airport, the location for official weather reports for San Diego, as well as the climate at most beach areas, straddles the border between BSh and BSk due to the mild winters and cool summers in these locations.
Temperatures
Average monthly temperatures range from in January to in August. On average, 344 days a year are hotter than , but only 25 days are hotter than .
Late summer and early autumn are typically the hottest times of the year with an average high of in August and in September. Temperatures occasionally reach or higher. Snow and ice are rare in the wintertime, typically occurring only inland from the coast when present.
San Diego experiences marine layer clouds, most often between May and August, which cause cool temperatures, cloudy weather and fog in the morning. Marine layer conditions linger until the heat of the sun becomes strong enough to evaporate the clouds. The local sayings "May gray" and "June gloom" refer to the way in which San Diego has the most trouble shaking off the early morning fog during those months, and cool, cloudy conditions often last into the afternoon or even all day.
Temperatures soar to very high readings only on rare occasions, chiefly when easterly winds (known as the "Santa Ana winds") bring hot, dry air from the inland deserts. Although confusing to tourists, newcomers, and out-of-the-region newscasters, this makes October the fire season for San Diego and the rest of Southern California due to low precipitation and sporadic heat waves that the average monthly temperatures do not immediately show. For example, the months with the highest record high temperatures are September at and October at .
The record high temperature at the National Weather Service office in San Diego of was on September 26, 1963. The record low temperature was on January 7, 1913. The record high temperature was tied only once and happened on September 27, 2010, 47 years and two days after the set record. Several cities near San Diego, including Los Angeles, broke their all-time records that day.
Precipitation
San Diego on average has 146 sunny days and 117 partly cloudy days a year. The average annual precipitation is less than , 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. While the summer months are virtually rainless, subtropical moisture from the North American Monsoon usually results in increased humidity and thunderstorms for at least a few days each summer. Rainfall is highly variable from year to year and from month to month, and San Diego is subject to both droughts and floods. While hurricanes are very rare, San Diego receives more tropical storms and remnants of tropical storms than anywhere else in California. Famous examples include the 1858 San Diego Hurricane and the 1939 California tropical storm, as well as the remnants of Hurricane Kathleen in 1976, all of which brought several inches of rain and high winds to San Diego. Inland areas like El Cajon can receive over per year on average; further inland at higher elevations in the Cleveland National Forest receive more yet, and some areas like Palomar Mountain average more than of rainfall per year.
At the National Weather Service office, there are an average of 41 days with measurable precipitation. The wettest year was 1941 with and the driest year was 1953 with . The most rainfall in one month was in January 1993. The most rainfall in 24 hours was on April 5, 1926.
Snow
Snow has been recorded falling on lowland San Diego communities only five times in over 125 years of record-keeping. Snow flurries were last seen in San Diego on February 14, 2008 around , and the last measurable snowfall to hit various neighborhoods and suburbs around the city fell on December 13, 1967. In winter, light snow is common in mountainous regions of east and north San Diego County (including Ramona and Julian) above .