San Diego Weather Guide: Year-Round Climate & Microclimates

San Diego is defined by a hot-summer Mediterranean climate, characterized by mild, semi-arid conditions that many consider some of the most consistent weather in the United States. However, the region's complex topography—ranging from coastal bluffs to deep inland canyons—creates a mosaic of microclimates that behave quite differently throughout the day.

The Daily Rhythm

In coastal San Diego, the day often begins with a soft, gray blanket of clouds known as the marine layer. This cool, moist air mass is pushed inland overnight by the Pacific Ocean. As the sun rises and heats the ground, these clouds typically "burn off" by late morning, revealing bright blue skies. In the afternoons, a steady sea breeze (onshore flow) acts as a natural air conditioner, keeping coastal temperatures 10–20°F cooler than the inland valleys.

Inland areas like El Cajon or Escondido experience a more dramatic daily swing. These regions often lack the morning cloud cover and heat up rapidly by midday. In winter, they can see evening temperatures drop significantly faster than the coast, sometimes approaching freezing, while coastal neighborhoods remain buffered by the ocean's thermal mass.

Seasonal Trends

• Winter (December – March): This is the "rainy" season, though total annual rainfall is sparse, averaging only about 10 inches. February is typically the wettest month. Temperatures are cool but mild, with highs in the mid-60s°F and lows in the high 40s to low 50s.
• Spring (April – June): Known for the “May Gray” and “June Gloom,” this season is characterized by persistent overcast skies along the coast. While the sun may shine just a few miles inland, the beach can remain misty and cool for much of the day.
• Summer (July – September): August and September are the warmest months. This is also when the Pacific reaches its peak swimming temperature, though still brisk at around 68–70°F. Occasional humidity spikes occur if remnants of a tropical storm or the North American Monsoon push moisture up from Mexico.
• Fall (October – November): Many locals consider fall the best season. The marine layer dissipates, the crowds thin, and the weather remains summer-like. This is also the peak time for the Santa Ana winds.

Unique Weather Phenomena

• Santa Ana Winds: These warm, dry winds blow from the high-pressure Great Basin in the east toward the coast. They reverse the normal onshore flow, bringing ultra-dry air and soaring temperatures. A Santa Ana event can push October temperatures into the 90s°F and drop humidity to single digits, significantly increasing wildfire risk.
• The Catalina Eddy: This localized atmospheric rotation can strengthen the marine layer, causing clouds to persist all day and even push deeper into the inland valleys than usual.

Local Perspective and Adaptation

Locals live by the rule of the "Coastal-Inland Divide." Residents in neighborhoods like La Jolla or Ocean Beach rarely use air conditioning, relying instead on the cross-breeze. In contrast, those living 15–20 miles inland experience real summer heat and winter frosts.

Culturally, San Diegans are highly attuned to the marine layer; a "gloomy" morning isn't seen as a bad day but rather a precursor to a clear afternoon. Because the climate is so steady, even minor deviations—like a rare winter thunderstorm or a 90-degree day at the beach—become major talking points. Historically, snow at sea level is an almost non-existent event, occurring only a handful of times in the last century.