Why is San Francisco so cold in the summer?

In San Francisco, The Pacific waters are relatively cold all year long (water surface temperature will range from mid 50s to mid 60s). The colder water temperature will result in a cool and moist layer of air near the surface. When you have colder air near the surface and warmer air above it, you will have a stable environment. During the warm season, the air well above the surface will warm up significantly with the change in seasons. However, the ocean temperature will still remain relative cool. This will result in a the formation of a very stable layer near the surface also known as the coastal marine layer. Because this layer is so stable, the cool and moist air is stuck near the surface and can not mix with the drier and warmer air above.

It's beautiful outside all week this week - forecast is into next week, too. Sunny, warm but not hot. Just perfect.

Abigail---Your answer makes sense to me!