Under the Koppen climate classification system, the San Diego area straddles areas of Mediterranean climate (CSa) to the north and Semi-arid climate (BSh) to the south and east. As a result, it is often described as "arid Mediterranean" and "Semi-arid Steppe". San Diego's climate is characterized by warm, dry summers and mild winters with most of the annual precipitation falling between November and March. The city has mild, mostly dry weather, with an average of 201 days above 70 °F (21 °C) and low rainfall (9-13" annually). Summer temperatures are generally warm, with average highs of 70-78 °F (21-26 °C) and lows of 55-66 °F (13-19 °C). Temperatures exceed 90 °F (32 °C) only four days a year. Winter temperatures are mild, with average high temperatures of 66-70 °F (19-21 °C) and lows of 50-56 °F (10-13 °C). Average annual temperature of sea is 65 °F (18 °C), from 59 °F (15 °C) in January to 72 °F (22 °C) in August.
The climate in the San Diego area, like much of California, often varies significantly over short geographical distances resulting in microclimates. In San Diego's case this is mainly 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 approximately 5-10 miles (8.0-16 km) inland. This happens every year in May and June. Even in the absence of June gloom, inland areas tend to experience much more significant temperature variations than coastal areas, where the ocean serves as a moderating influence. Thus, for example, downtown San Diego averages January lows of 50°F and August highs of 78°F. The city of El Cajon, just 10 miles northeast of downtown San Diego, averages January lows of 42°F and August highs of 88°F. However sometimes the June gloom can last for several days even into July causing cloudy skies for San Diego for the entire day.
Rainfall along the coast averages about 10 inches (250 mm) of precipitation annually, which occurs mainly during the cooler months of December through April. Though there are few wet days per month during the rainy period, rainfall can be heavy when it does fall. Rainfall is usually greater in the higher elevations of San Diego; some of the higher elevation areas of San Diego can receive 11-15 inches (280-380 mm) of rain a year.