Relax at North Pacific Beach
San Diego's excellent climate is often referred to as being 'Mediterranean.' That's no surprise, really, considering it is situated on the same latitude as Italy, Greece, and Spain and is, therefore, the same distance from the equator.
The year-round mild temperatures, even in the height of summer, can be attributed to the Maritime Effect from the Pacific Ocean, the effect the ocean and coastline has on the weather.
In the summer, ocean currents from thousands of miles away are cool and have the effect of lowering the temperatures on the nearby areas of San Diego and keeping the weather dry.
In the winter, the currents are warmer, which means that the air passing over them isn't cooled down as much and prevents the land from becoming too cold. They also bring welcoming rain. The result is mild winters and relatively cool summers and a cool sea breeze along the coast for much of the year.