San Diego has a hot-summer Mediterranean climate, though it is notably more arid than most regions sharing this designation. Existing at the edge of a semi-arid environment, the city experiences a scarce rainfall pattern that is predictable. Rainfall is infrequent and heavily concentrated within a short annual window, which dictates the region’s ecology and water management.
Quantifying San Diego’s Rainfall Totals
The average annual rainfall for San Diego, measured near the coast, is typically under 10.5 inches of precipitation. This low figure is comparable to a desert environment. On average, San Diego experiences measurable precipitation (0.01 inches or more) on approximately 41 days per year.
This low frequency contributes to the region’s reputation for having a high number of sunny days. The 10.5-inch annual total is an average that smooths out significant year-to-year variability; some years receive less than five inches, while others exceed 20 inches. This fluctuation means the city’s water supply often depends on a few major storm events.
The Extreme Seasonal Distribution of Precipitation
Rainfall is not evenly distributed across the calendar; it is heavily concentrated in a distinct wet season. The vast majority of annual precipitation (85% to 90% of the total) falls during the cooler months, generally spanning from November through March. This concentrated period contrasts sharply with a prolonged, nearly rainless dry season.
The dry season typically begins in May and extends through October, during which measurable rain is a rare occurrence. Monthly averages illustrate this contrast: February is often the wettest month, accumulating approximately 2.2 inches of rain. Conversely, the months of June, July, and August are practically devoid of precipitation, frequently recording less than one-tenth of an inch combined. This distribution means that the summer months rely entirely on stored water reserves.
The transition between seasons is generally abrupt, with storm activity sharply declining after April. This extreme seasonality is a defining characteristic of the Mediterranean climate type. The lack of summer rainfall is a major factor in the prevalence of native chaparral and scrubland vegetation.
Meteorological Factors Governing Local Rainfall
The scarcity and seasonal timing of rainfall are governed by large-scale atmospheric patterns and local topography. The most significant factor is the semi-permanent North Pacific High-Pressure System, a massive area of high pressure dominating the eastern Pacific Ocean. During summer, this system shifts northward, effectively blocking storm systems from reaching Southern California and leading to the long, dry season.
In winter, the Pacific High weakens and shifts southward, allowing the North Pacific storm track to affect the region. These frontal systems are the primary source of the annual rainfall total. The location of San Diego near the sinking branch of the Hadley Cell also contributes to dry conditions, as sinking air suppresses cloud formation and precipitation.
The topography of the region also plays a crucial role in precipitation totals. Mountain ranges immediately east of the city, such as the Peninsular Ranges, create a partial rain shadow effect. As moist air moves inland and is forced upward, it releases most moisture on the western slopes, meaning the coastal plain receives less total rainfall.
Year-to-year variability is often linked to the El Niño–Southern Oscillation (ENSO) cycle and the influence of Atmospheric Rivers. El Niño conditions are associated with warmer Pacific waters and a southern storm track, which can lead to significantly wetter-than-average winters. Atmospheric Rivers, which are narrow corridors of concentrated moisture, can deliver a substantial portion of the annual rain in just a few extreme events.