The 2000 Fire Season: Lightning-Caused Fires

Miriam L. Rorig Pacific Northwest Research Station, USDA Forest Service, Seattle, Washington

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Sue A. Ferguson Pacific Northwest Research Station, USDA Forest Service, Seattle, Washington

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Abstract

A large number of lightning-caused fires burned across the western United States during the summer of 2000. In a previous study, the authors determined that a simple index of low-level moisture (85-kPa dewpoint depression) and instability (85–50-kPa temperature difference) from the Spokane, Washington, upper-air soundings was very useful for indicating the likelihood of “dry” lightning (occurring without significant concurrent rainfall) in the Pacific Northwest. This same method was applied to the summer-2000 fire season in the Pacific Northwest and northern Rockies. The mean 85-kPa dewpoint depression at Spokane from 1 May through 20 September was 17.7°C on days when lightning-caused fires occurred and was 12.3°C on days with no lightning-caused fires. Likewise, the mean temperature difference between 85 and 50 kPa was 31.3°C on lightning-fire days, as compared with 28.9°C on non-lightning-fire days. The number of lightning-caused fires corresponded more closely to high instability and high dewpoint depression than to the total number of lightning strikes in the region.

Corresponding author address: Miriam L. Rorig, Pacific Northwest Research Station, USDA Forest Service, 4043 Roosevelt Way NE, Seattle, WA 98105. mrorig@fs.fed.us

Abstract

A large number of lightning-caused fires burned across the western United States during the summer of 2000. In a previous study, the authors determined that a simple index of low-level moisture (85-kPa dewpoint depression) and instability (85–50-kPa temperature difference) from the Spokane, Washington, upper-air soundings was very useful for indicating the likelihood of “dry” lightning (occurring without significant concurrent rainfall) in the Pacific Northwest. This same method was applied to the summer-2000 fire season in the Pacific Northwest and northern Rockies. The mean 85-kPa dewpoint depression at Spokane from 1 May through 20 September was 17.7°C on days when lightning-caused fires occurred and was 12.3°C on days with no lightning-caused fires. Likewise, the mean temperature difference between 85 and 50 kPa was 31.3°C on lightning-fire days, as compared with 28.9°C on non-lightning-fire days. The number of lightning-caused fires corresponded more closely to high instability and high dewpoint depression than to the total number of lightning strikes in the region.

Corresponding author address: Miriam L. Rorig, Pacific Northwest Research Station, USDA Forest Service, 4043 Roosevelt Way NE, Seattle, WA 98105. mrorig@fs.fed.us

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