Weather is probably a major factor when you plan your trip to San Bernardino, but bear 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 temperature of 28°C, while the coldest months are July and June, with an average of 18°C. The rainiest months in San Bernardino are October, November, December and April, with each month seeing an average of 190 mm of rainfall.