Weather

Summer weather at and near NASA Ames Research Center tends to be clear, sunny, and warm.  Rain is infrequent, except during February and March (though when it does rain, it often comes down like cats and dogs).  Absent irrigation and other human intervention, much of this area is a near-desert.

If you're planning on being outside for any length of time, bring water.  If you're going to be outside and away from anyplace you can get water, pack more water than you think you'll need.  Incidentally, the rule of thumb for many full-fledged deserts is, one quart/liter of water per hour of walking or light work in the sun.  That may be a bit extreme for these parts, but be prepared.

Also, at the risk of sounding like a graduation speaker, wear sunscreen.

It can get surprisingly cool after sundown, and the temperature can drop fast.  It is also often quite breezy.  If you're going to be out after dark, it's a good idea to have a light jacket or at least a long-sleeved shirt.  This is especially true if you're going to be close to the coast.  Mark Twain is often quoted as having said "The coldest winter I ever spent was a summer in San Francisco."  He didn't really say it, and anyone who believes it has never visited someplace with real winters, but you get the picture.

If you're going to be at ARC in the winter, it can cool and rainy, but not bitterly cold.  (Keep in mind that the author of that statement grew up in central Minnesota, where "cold" means "40 degrees below zero, with a stiff wind").  It rarely gets below freezing on ARC, or in the Silicon Valley generally.  However, if you go into the high country nearby (say, to visit Lick Observatory), you will occasionally encounter snow.