Pyrocumulus Clouds Over Los Angeles County, California

September 11, 2017


Photographer: Dave Lynch 
Summary Author: Dave Lynch 

When intense local fires produce strong updrafts, the rising air cools until water vapor condenses into droplets and forms a cumulus cloud. Such clouds are called pyrocumulus because fire generates them. Smoke particles from the fire act as condensation nuclei to promote the conversion of water vapor into water droplets. This process also releases heat that further enhances upward convection. Pyro is from Greek meaning fire, and cumulus is from Latin meaning heap or pile.

Smoke from the fire is composed of tiny, brownish ash particles. As they rise (by thermal convection) and cool, water vapor begins to condense on the ash particles, forming a droplet with a smoke particle in the center. As it rises higher, more and more water accumulates until the ash particle is just a speck compared to the water drop itself. At this point, the cloud simply looks like a white cumulus cloud, which of course it is. This explains why the entire structure is brownish near the bottom and white at the top.

The photo above shows two pyrocumulus clouds over the La Tuna fire in the Verdugo Hills of Los Angeles, California. Winds from the east blew the cloud away from the fire and over Burbank. The clouds’ formation was aided by moisture from tropical depression Lidia over Baja California (Mexico) that was pumping moisture into Southern California. Despite the hot, dry weather that had gripped the area for several days, Lidia’s moisture caused quite a bit of localized rain that helped reduce the spread of the fire.

Photo taken with an iPhone SE, at 10:48 a.m. Pacific Daylight Time, on September 3, 2017, from Burbank, California.