<P> The cool California Current offshore, enhanced by upwelling of cold sub-surface waters, often creates summer fog near the coast, creating a warm - summer Mediterranean climate (Köppen climate classification Csb). Further inland, the climate becomes more continental, with some areas turning semi-arid (Köppen BSk), with colder winters and markedly hotter summers . Low - lying inland valleys, especially the Central Valley, have a hot - summer Mediterranean climate (Köppen Csa), with subtropical temperatures but a well - defined summer dry season and a foggy, rainy season in winter . </P> <P> The temperature gradient between immediate coast and low - lying inland valleys in the south is about 7 ° F (4 ° C) in winter, the coast being warmer, and in summer roughly 25 ° F (14 ° C), the interior being warmer . For example, the average daily high in San Francisco in July and August is between 62 and 68 ° F (17 and 20 ° C), and in Walnut Creek, some 20 miles (32 km) inland, the average daily high in July and August is 90 ° F (32 ° C): a temperature gain of more than one degree (Fahrenheit) per mile . In the south, the figures are approximately 4 ° F and 23 ° F (2 ° C and 13 ° C), respectively . At the coast in Santa Monica, the average high in August is 75 ° F (24 ° C), while in Burbank, approximately 10 miles (16 km) inland, the average high in August is 95 ° F (35 ° C): a temperature gain of about two degrees Fahrenheit per mile . </P> <P> During the cooler winter months (October--March), the Coachella Valley regularly has the warmest winter temperatures out of any place west of the Rocky Mountains . East Los Angeles, the Gateway Cities, and parts of the San Gabriel Valley average the warmest winter high temps (72 ° F, 22 ° C) in all of the western U.S., and Santa Monica averages the warmest winter lows (52 ° F, 11 ° C) in all of the western U.S. Palm Springs, a city in the Coachella Valley, averages high / low / mean temperatures of 75 ° F / 50 ° F / 63 ° F, (24 ° C / 10 ° C / 17 ° C) respectively during the period of cooler weather from November to April . </P> <P> The extreme southwest, around San Diego, has a subtropical semi-arid or steppe climate (Koppen BSh) as winters are drier there . </P>

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