Layer, even if you think you won't need to. Forget what you've read about sunny California: San Francisco in summer is subject to finger-numbing fogs, and even Los Angeles has its chilly days. Also, malls, museums, cinemas, and airlines cabins are often air-conditioned to a wintry degree. The best bet is a lightweight, waterproof jacket, stuffable into your carry-on. A shawl-size scarf--virtually weightless and warm--is a woman's best friend, and can act as a beach cover-up or wrap.
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