The one thing about California weather.... You can be in Los Angeles for example, go 40 miles west, and be in a total blizzard. Same with San Francisco, you can travel across the bay to Oakland........
Also in real hot weather, just like cold weather, it's most important to keep your head temperature from the extremes. That's why in the desert, people wear hats or stuff to keep thier head from overheating.
With many people, you can't drink enough to keep yourself hydrated in really hot weather, especially if you aren't used to it. You will simply sweat more than what your body can absorbe to replace it.
The key is learn what heat-stroke and heat-exhaustion is and the signs/symtoms of it.. Headaches, nausea, weakness, etc... even a little bit means to stop and cool down. If you stop sweating in hot weather, you better get yourself cooled down real quick.
For most people, usually best to avoid riding in super hot conditions unless you **REALLY** know what you are doing and are used to it.... get your miles in the morning.. rest at library or other place for a few others... then get more miles at dusk.....