Climate Zone Map, Created by SurfTheTurf.com, courtesy of Sandag.org GIS data.

The western part of San Diego County has four zones:

Zone	Coverage	Climate
1. Marine Zone	The marine zone extends to six or so miles from the coast, but is characterized by being less than 400 feet in elevation.	This area is dominated by the ocean, and the ocean conditions pretty much dictate what the Marine Zone is experiencing. Temperature changes are far less day to night than the other zones, there is less seasonal change. Much higher humidity and summer fog.
2. Coastal Zone	The coastal zone extends from the maritime zone for another eight to thirty miles, and sits at higher land elevations.	This area is far away (and at high enough elevation) that it is no longer dominated by the ocean like the marine zone; however, it is certainly influenced by it and the cool breezes that come off the ocean. Though the temperatures fluctuate daily more than the marine zone, they are definitely buffered by marine climate.
3. Transitional Zone	This area is found in valleys, foothills and low mountain ranges between the coastal zone and interior zone.	Depending upon local weather events (e.g., direction of the wind), the transitional zone can seem more like the coastal zone one day and the interior zone the next. If affected by weather from the interior zone, residents can see both cold and hot temperatures.
4. Interior Zone	The primary factor is the distance from the ocean rather than particular topography.	Because the interior zone is so far removed from the marine zone, there is little influence from the ocean. The interior zone experiences large daily and seasonal temperature change, and is air is typically dry/warm in the summer. Nightime temperatures will fall, causing dew.
5. Desert Zone	Past the interior zone and mostly in the East part of the County is the desert zone.	The air is very dry and residents see large temperature changes. This zone is not shown on the map as it is too far inland.

As you will see from the above table, the zones are largely defined by how much the ocean affects or doesn't affect the change in temperature. Rainfall levels in the marine to transitional zones are low (typically less than 10 inches/year), rising to over 40 inches when the moist ocean air hits the inland mountains in the interior zone.

WHAT DOES IT ALL MEAN?
Here are some practical tips about what all this climate talk means. If you like to see the ocean close-up, then be prepared for a much cooler, damper San Diego experience because you'll be living in the marine zone. If you have good circulation, you'll enjoy the coolness, but if you have bad circulation, you may feel like you're going to freeze half of the year. Because of the low elevation and the fog, homes near the ocean get less sunshine than the coastal zone. The high humidity takes a toll on wood structures, and if you are really close to the ocean, the high salt levels will have a major impact on metal (e.g., metal windows).

If you are willing to trade the ocean view for glorious sunshine, then the coastal zone is a great place to be. Because the coastal zone is higher in elevation, it doesn't get the fog seen in the marine zone. There may be no whitewater views, but one will wake up to spectacular blue skies almost every day. This extra sunshine tends to warm things up quickly in the morning, but the coastal breezes keep the temperature from going very high.

If you really prefer to have variety, then the interior zone is probably best. The coastal and marine zone climate beauty rarely missteps, so it can seem monotonous at times. If you are someone who likes seasonal changes and warmer days and cooler nights, then head for the transitional zone. However, while people in the coastal zone may be able to live without air conditioning, folks in the transitional zone will probably want it for those days when it gets really hot. Total energy expense (heating/cooling) is probably higher than for those closer to the ocean.

A Note about Climate and Fire: The transitional and interior zones are far more prone to experience the devastating wildfires seen in Southern California. If you decide to live in a more rural area, think carefully about taking steps to minimize the wildfire threat by clearing brush away from the house and making your home more fireproof. Be fire-wise.

Additional Reading
For more information about San Diego's climate, you may want to take a look at The Weather of Southwest California by the National Weather Service. There are some other reasources at their home page, including information about the local San Diego office.