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WildfiresWhy were California’s wine country fires so destructive?

By Jon Keeley

Published 27 October 2017

As of late October more than a dozen wildfires north of San Francisco had killed more than 40 people, burned approximately 160,000 acres and destroyed more than 7,000 structures. The path of the destructive 2017 Tubbs fire in Napa and Sonoma counties mirrors that of the Hanley fire of 1964. Strikingly, though, no lives were lost during the Hanley fire and only 29 structures were destroyed. Why did these two fires, 50 years apart, burn on the same general landscape, under similar extreme winds, with such different human impacts? Fire scientists will study these events intensively to parse out the relative importance of various factors. But it is clear that two factors probably were major contributors: wind and population growth. Drought and warmer climates have made wildfires a year-round hazard in California. Expanded urban development, in tandem with hot winds, seems to be the primary reason for the destruction this year.

Author Jon Keeley of the USGS // Source: yahoo.com

As of late October more than a dozen wildfires north of San Francisco had killed more than 40 people, burned approximately 160,000 acres and destroyed more than 7,000 structures.

This tragic loss of life and property is unprecedented in California. However, the fires are not anomalous events in terms of their size, intensity or the speed with which they spread. Indeed, the path of the destructive Tubbs fire in Napa and Sonoma counties mirrors that of the Hanley fire of 1964. This extreme wind-driven fire burned under similar conditions, across much of the same landscape and covered an area substantially greater than the recent Tubbs fire.

Strikingly, though, no lives were lost during the Hanley fire and only 29 structures were destroyed. Why did these two fires, 50 years apart, burn on the same general landscape, under similar extreme winds, with such different human impacts? Fire scientists will study these events intensively to parse out the relative importance of various factors. But it is clear that two factors probably were major contributors: wind and population growth.

Driven by Diablo winds
The Tubbs fire began on the night of Oct. 8 near Calistoga in Sonoma County under extreme fire weather conditions, with high winds and low relative humidity. Normally, winds in this region flow from the west, carrying cool, humid air from the ocean onshore. These winds reversed that pattern: They blew out of the northeast at 40 miles per hour, with gusts up to 75 miles per hour. Such winds are common in California during the autumn, and are known as Diablo, Mono or North winds in Northern California and Santa Ana winds in Southern California.

These hot, dry winds develop from an unusual pattern of high and low pressure cells, and are most prominent in autumn. They follow the normal summer and fall drought that occurs in this Mediterranean-type climate, leading to severe fire weather conditions. Such winds are associated with some of the most catastrophic fires in California’s history. In the San Francisco Bay area, they played a role in the 1991 Tunnel fire, where wind gusts of 60 miles per hour were responsible for 25 deaths, even though the fire measured only slightly over 1,000 acres. The speed of these fires is a major factor leading to the loss of human lives.