Weather is probably a major factor when you plan your trip to San Bernardino, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually January and December with an average temp of 82°F, while the coldest months are July and June with an average of 65°F. The rainiest months in San Bernardino are October, November, December, and April, with each month seeing an average of 7 inches of rainfall.