La Jolla enjoys a Mediterranean climate and has few very hot days throughout the year and only a handful of very cold days. In addition, the humidity is usually low and summer nights can be enjoyed outdoors because mosquitoes and other pests are rare. Most locals here do not have air conditioning in their homes and some older beach cottages may have only fireplaces for heat. Daytime and nighttime temps can vary by 30 degrees, so it's important to bring a jacket or sweater as the evenings can be very cool, even in the summer.

The best months to visit are July through October, when it is generally in the 70s or low 80s during the days and cools to the  low 60s at night - and very rarely rains.  November through March are more likely to be rainy, but can also be wonderful, sunny, and warm.  April through June are typically the foggy months - "May gray" and "June gloom" are what the natives call it.  Often the fog will clear to a glorious day, sometimes before noon and sometimes not at all. 

 The weather is nearly perfect, but avoiding the rainy season will make for a much better beach vacation.