Mediterranean Climate: Why LA Has Dry Summers

A Mediterranean climate has dry, warm-to-hot summers and mild, wetter winters, driven by a subtropical high pressure system that shifts position through the year. Los Angeles has a Mediterranean climate because the eastern Pacific high strengthens and moves north in summer, blocking storms, then weakens and moves south in winter, letting Pacific storms in. That one mechanism explains why Downtown Los Angeles gets most of its yearly rain from November through March and almost none in July or August.

What Is a Mediterranean Climate?

A Mediterranean climate is a seasonal pattern, classified as Csa or Csb in the Köppen climate system, with a dry summer and a wet winter. Its defining feature is the timing of rain, not mild temperatures.

The warmest months are the driest, and the coolest months bring most of the annual rainfall. That is unusual: most rainy regions get their wettest weather when it is warmest, because warm air holds more moisture and fuels more storms. Los Angeles does the opposite, as the month-by-month numbers show.

The Mediterranean climate is named for the Mediterranean Basin, where it was first described, but it occurs on five continents wherever the same conditions meet. Those conditions are a west-facing coast at about 30 to 45 degrees latitude, a cold ocean current offshore and a subtropical high pressure belt that shifts with the seasons. Southern California has all three, which makes Los Angeles a textbook example.

The pattern holds in both hemispheres. Southern Hemisphere Mediterranean regions are dry from December through February, their summer. Los Angeles and the Mediterranean Basin are dry from June through August. The rule is the same everywhere: dry in the warm season, wet in the cool season.

Why Are Los Angeles Summers Dry?

Los Angeles summers are dry because a large, semi-permanent high pressure zone over the eastern Pacific, called the North Pacific High or eastern Pacific subtropical high, strengthens and moves north each summer, settling off the California coast. Sinking air in the high prevents rain clouds and pushes the Pacific storm track far north, into Canada and the Gulf of Alaska. Fronts and low pressure systems stay away from Southern California for months. Storms still pass to the north; the high steers them away.

The same high pressure system creates the marine layer, the low clouds and fog that often cover the coast on summer mornings. Sinking air in the high forms an inversion that caps cool, moist air near the ocean under warmer air a few thousand feet up. The inversion can keep the coast overcast while inland areas bake, but it rarely brings real rain. Coastal gray and inland heat on the same July day both come from the same high.

Why Does Winter Bring Most of the Rain?

Winter brings most of Los Angeles' rain because the subtropical high weakens and moves south each fall, letting the polar jet stream dip south and steer Pacific storms into Southern California. From about November through March, the reopened storm track brings atmospheric rivers, long plumes of tropical and subtropical moisture that cause the heaviest rain, and ordinary fronts that bring a day or two of steady rain. The NOAA Climate Prediction Center tracks this seasonal shift, because the jet stream's position each winter largely decides how wet the season is.

Big swings from year to year are normal for Los Angeles winters. One strong atmospheric river can supply a large share of a season's rain in a few days, so Los Angeles can swing between drought years and flood years depending on how many storms reach far enough south. The NWS Los Angeles/Oxnard office and the NWS San Diego office issue local forecasts, watches and warnings for these storms, including flood and debris flow warnings for burn scars when a wet winter follows a fire year.

Why Do LA Microclimates Still Differ?

Los Angeles microclimates differ within one Mediterranean climate because distance from the ocean, elevation, terrain and city surfaces change how the seasonal pattern plays out. The coast, basin, inland valleys and mountains share the same wet-winter, dry-summer calendar. Rainfall, daily temperature swings and fog frequency can still differ sharply within a few miles.

The coast stays cooler and foggier in summer because it sits closest to the cold California Current and gets the marine layer first. Farther inland, the ocean's cooling weakens. Downtown Los Angeles, about 15 miles inland, has warmer summer afternoons and a wider gap between daily highs and lows than beach towns. Farther inland or higher up, the ocean's influence fades more.

Woodland Hills, in the western San Fernando Valley, often records some of the county's hottest summer afternoons, because the Santa Monica Mountains block cool marine air from reaching it. Pasadena, at the base of the San Gabriel Mountains, shows a related pattern: foothill terrain can trap afternoon heat and, in the right pressure setup, bring downslope wind.

City surfaces add another layer. Pavement and buildings across the basin hold heat overnight, so built-up neighborhoods often stay several degrees warmer after dark than nearby open or green land at the same elevation and distance from the ocean.

Local microclimates do not change the regional climate type. Coastal fog, valley heat and foothill wind are all local versions of the same Mediterranean pattern, shaped by geography on top of the seasonal high pressure cycle.

How Does LA Compare With Other Mediterranean Regions?

Los Angeles shares its dry-summer, wet-winter pattern with four other major regions: the Mediterranean Basin, central Chile, South Africa's Western Cape and southwestern Australia around Perth. Each sits in a similar latitude band on a continent's west side, with a cold ocean current offshore and the same seasonal high pressure cycle, offset by six months between hemispheres.

Compare these regions by seasonal pattern, not annual average temperature, because averages can make different climates look alike or similar climates look different. Central Chile is dry from December through February, its summer, while Los Angeles is dry from June through August. Both are equally Mediterranean, because both are dry in local summer and wet in local winter.

Within California, the San Francisco area and points north share the same wet-winter, dry-summer calendar but have a stronger, more persistent summer marine layer along the coast. A colder stretch of the California Current and different terrain cause the difference.

Southern California's Mediterranean climate is on the warm, dry end of the range. Los Angeles sits closer to the desert Southwest than the cooler, wetter Mediterranean climates of Northern California or southern Europe. The region can swing from a wet Pacific storm track in winter to desert-like heat and, in some years, monsoon thunderstorms by late summer. For summer thunderstorms over the deserts east of Los Angeles, see what is the monsoon.

How Is the Climate Changing?

Climate change is intensifying Los Angeles' Mediterranean pattern at its extremes without erasing it, according to California's climate assessments. Hotter extremes, rainfall swinging between drought and heavy storms and a longer fire season are documented trends, while the wet-winter, dry-summer structure stays intact. NOAA Climate.gov, California's statewide climate assessments and the California Department of Water Resources describe rising temperatures, more winter rain arriving in fewer, stronger atmospheric rivers and droughts that stress water supply and vegetation between wet spells.

A longer, hotter dry season plus more concentrated winter rain feeds Southern California's fall fire risk, when vegetation dried by a rainless summer meets offshore Santa Ana winds. No credible assessment says Los Angeles will stop having a Mediterranean climate or that its dry-summer, wet-winter pattern will disappear. The intensity and variability are changing, so check current conditions, drought status and fire weather outlooks from official sources.

Los Angeles is dry in summer and wetter in winter because the eastern Pacific subtropical high rises and falls with the seasons and controls the storm track. Coastal fog, valley heat and foothill wind are local variations on that same pattern. Before planning a trip or hike around the season, compare a destination's monthly temperature and rainfall normals with the current forecast on WeatherLA's Los Angeles pages, because any single week can run wetter, drier, hotter or cooler than average.

The LA microclimates guide explains local variations, and California snowpack connects the wet season to water supply.