The last 30 days in Southern California's deserts and mountains were the hottest on record for that period, according to the National Weather Service in San Diego.
Average temperatures in Big Bear, Palm Springs, Palomar Mountain and Borrego Springs hit historic highs between June 18 and Monday, due to an unusually persistent span of heat and few changes in weather patterns — such as rain — meteorologist Brett Albright said.
"A pattern that is stagnant like that and is very persistent is going to give us a lot of heat," he said.
Big Bear's average temperature in the last month was 67.6 degrees, while Palm Springs' was 98, Palomar Mountain's was 77.6 and Borrego Springs' was 95.9.
By contrast, the second-hottest mid-June to mid-July period in Big Bear was recorded in 2002, at 67.1 degrees. Palm Springs' next hottest period was in 2006 at 96 degrees; Palomar Mountain's was in 1970 at 76.4 degrees; and Borrego Springs' was in 1990 at 95 degrees.
Over the last month, Palm Springs hit 122 degrees four times in the span of three weeks, edging close to its highest-ever recorded temperature of 123 degrees.
"That is really a pretty big number for them considering their all-time record high," Albright said.
Albright attributed the increase in mountain temperatures to warm nights. He also said that valleys were protected from the heat by onshore winds and sea breezes.