The climate in San Diego, like most of Southern California, often varies significantly over short geographical distances resulting in microclimates. In San Diego, this is mostly because of 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 (8.0–16.1 km) inland. Sometimes the June gloom can last into July, causing cloudy skies over most of San Diego for the entire day. 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 (10 °C) and August highs of 78 °F (26 °C). The city of El Cajon, just 10 miles (16 km) inland from downtown San Diego, averages January lows of 42 °F (6 °C) and August highs of 88 °F (31 °C).

What areas experience the most significant temperature variations?