What causes the powerful, dry winds that are fanning the flames in Napa and Sonoma counties

Friday, 13 October 2017, 04:27:57 PM. A look inside the weather conditions that have helped spread the fast-moving wildfires ravaging Northern California.

Source of the winds

Circulation around high pressure in the West generates strong northeasterly winds at the surface, which accelerate and warm as they blow downslope from the Sierra Nevada into Napa and Sonoma counties in California’s wine country.

what-causes-the-powerful-dry-winds-that-are-fanning-the-flames-in-napa-and-sonoma-counties photo 1

Why mountain and canyon winds are stronger

Air from areas of high pressure over elevated ground inland flows down toward sea level in California. The sinking air heats up, loses moisture and speeds up, especially as it squeezes through canyons.

what-causes-the-powerful-dry-winds-that-are-fanning-the-flames-in-napa-and-sonoma-counties photo 2

Venturi effect

Mountain ranges block winds in some areas while winds howl elsewhere. Winds seek openings through the mountains, much like water would. Canyons and passes provide narrow openings that accelerate the winds like a nozzle on a garden hose does to water.

what-causes-the-powerful-dry-winds-that-are-fanning-the-flames-in-napa-and-sonoma-counties photo 3

Terrain’s role with the winds during wildfires

Terrain can play a part in a fire’s progress. On hillsides with gullies or ravines, a chimney effect can cause hot gases to rise up these channels. The fire creates a draft, pulling oxygen in, and blazes roar up rapidly as if in a chimney.

what-causes-the-powerful-dry-winds-that-are-fanning-the-flames-in-napa-and-sonoma-counties photo 4

Embers spread new fires

As fires rage and winds blow, hot embers may be carried far away from burning areas. They can touch off smaller spot fires well ahead of or behind a main fire, and these may turn into new wildfires.

what-causes-the-powerful-dry-winds-that-are-fanning-the-flames-in-napa-and-sonoma-counties photo 5

Sources: National Weather Service, U.S. Geological Survey, Times reporting

Support our journalism

Please consider subscribing today to support stories like this one. Already a subscriber? Your support makes our work possible. Thank you. Get full access to our signature journalism for just 99 cents for the first four weeks.

Copyright © 2017, Los Angeles Times

Share this
Article What causes the powerful, dry winds that are fanning the flames in Napa and Sonoma counties compiled by www.latimes.com