The last really bad frost in Southern California was in January 2007, Hodel said. Most weather reports will note if frost is expected, especially in wind-protected areas of the valleys. “I always look at the weather. If it’s a clear, dry day with no wind, and the high temperature stays in the 50s, there’s a good potential for frost that night. But it has to be dry,” he said, adding: “Dry air loses heat but water holds a lot of heat, so if it rains and stays in the 50s, it will probably stay in the 40s that night, because the air is so moist.”
Source: latimes.com – Los Angeles Times