Climate and Rainfall



San Diego's climate and location play a role in finding lakes, reservoirs and ponds. For example, our 2019 rainfall total for San Diego was 15.28" -- the most rain in nine years, and a good time for me to go searching for bodies of fresh water!

Mediterranean climates like ours are found on several continents, at 30 to 45 degrees latitude north and south of the equator, on the western side of a land mass. On some continents a climate region may extend for hundreds of miles; however it does not in Southern California due to coastal mountain ranges which form a steep eastern boundary.  

San Diego county is almost entirely in the Mediterranean coastal region, although its eastern edge is desert. Looking to the west from Sunrise Highway at 5000 ft elevation, you see this green view and eastward-moving clouds...

Look to the east from the same highway and you see the Anza-Borrego desert:

Virtually all the atmospheric moisture is dropped on the west side of the mountain range, as the clouds are forced up and away from the earth by the slopes AND rising heat from the desert floor.

Seasonal changes ARE dramatic - though not like those in regions closer to the Poles. 

Our plant growth begins in Autumn, after summer's drought and dormancy. Most plants grow during the winter. That growth is ended in the summer by hot, dry "Santa Ana" winds and rising temperatures, which are tempered along the coastline by a heavy marine layer in the atmosphere.

In winter, periods of rain alternate with warm, sunny days. After three days of howling winds, mild sunny days arrive. In San Diego, the sunniest months of the year are November and December, while the grayest months are May and June (known locally as June Gloom).

The image below (and the one at the top of the page) shows you how a marine layer comes up to the coastline, on in a few miles, then stops.

This marine layer is not related to actual Marines along the San Diego County coastline. 


Irregular rainfall accentuates the severity of mediterranean climates. Rain varies considerably from year to year; thus ponds and lakes constantly change in size and depth. 

In researching this subject I learned many people PREDICT FUTURE RAINFALL, but very few people record and track RAINFALL HISTORY. John Stokes III is one of those few. The charts below were made by him.

In its entire recorded rainfall history (170 years) the city of San Diego has received less than 10 inches of rain in an average season. Click for more information.

There is very little snow pack in our local mountains, and there are no substantial underground aquifers. Our rivers are mostly seasonal, and not navigable. The following illustration shows the watersheds — areas that drain to the ocean via seasonal streams and a few rivers.  

[click any image to enlarge it]

Not only does the rain fall erratically from year to year, it falls very unevenly throughout the year -- as you can see from this monthly rainfall chart, November through March produce most of the rain. On average, only 44 days of 365 have measurable precipitation.

Because San Diego rainfall is recorded at the airport (at sea level on the coast) the numbers are lower than those in the coastal mountains. 

I've recently discovered that Camp Pendleton averages more than 20 inches per year. Its Case Springs weather station, in the Santa Margarita watershed, received more than 50 inches of rain in its wettest season! And less than 6 inches in its driest. Here is the data:

Because of these radical fluctuations in rainfall, we rely on dozens of reservoirs to catch and hold what we can -- which is about 60,000 acre feet a year, less than 10% of our total water usage. See the PAGE on Dams.