Relative humidity plummets to 3% in burning Ventura County as Santa Ana winds rage
It's just about the worst fire conditions you can have -- strong winds on top of warm temperatures and super dry air, but that's the situation in southern California this week as the infamous Santa Ana winds make for continued terrible fire conditions in the area.
Several homes were burning in Ventura County north of Los Angeles on Friday, aided by unbelievably dry winds.
In nearby Burbank, on midday Thursday just before the Santa Ana winds arrived, the temperatures were in the mid-upper 60s with dew points in the mid 50s and a comfortable humidity in the 50-65% range -- a typical nice California autumn day.
But then the northeast Santa Ana winds arrived, courtesy of much higher pressure over the inland deserts and lower pressure offshore, courtesy of a thermal trough. Those winds coming from the arid desert are already hot and dry to begin with, but as they climb over and down the Sierra Nevadas and, more locally to Ventura County, the San Gabriel Mountains, the air sinks and undergoes an additional warming and drying process due to compressional heating.
Temperatures zoomed into the upper 70s and low 80s, but even more importantly, the dew point and relative humidity plummeted. Six hours after lunch Thursday, the dew point had dropped to a super-arid -7 degrees -- yes, *minus* 7, meaning the air would have to cool to 7 below zero for fog to form. That made for a relative humidity of 4%.
It was even worse late Friday morning with a dew point of -9 and a humidity of 3%. The region is expected to get a brief break in the winds Saturday, but dangerous conditions return Sunday into early next week. "There is a near imminent chance of widespread critical fire weather conditions across Ventura and Los Angeles Counties," according to a Red Flag Warning posted by the National Weather Service.
For now, gusts to 55 mph with relative humidity between 2 and 10 percent are expected to continue until Friday evening.