Dear Tom,
This past Memorial weekend near Flagstaff, Ariz., smoke rising off a forest fire formed a mushroom-shaped cumulus cloud which was gray and smoke-colored on the bottom, yet normal white on the top. How did this happen in such a dry atmosphere?
Mike Bell
Dear Mike,
Even with low relative humidities, upper air soundings across the Southwest frequently reveal 0.3 to 0.4″ of moisture in the air. The violent heating produced by any forest fire generates a swiftly rising column of air able to break through atmospheric “caps” like the ones so often found in Arizona during the warm season. Caps are layers of warm air that discourage vertical air movement, thereby inhibiting the cooling required for cloud formation. The extreme heat generated by the fire you observed was able to create a column of air so hot and buoyant it was able to break the day’s cap, then cool sufficiently to reach condensation and produce a thunderstorm.
The thunderstorm’s smoky base indicates a temperature inversion remained in place away from the fire. This appears to have trapped smoky particulates in the lower atmosphere, obscuring the cloud base while the clearer, smoke-free air above fostered the brilliant white appearance so often associated with cumulus clouds.
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Tom Skilling is chief meteorologist at WGN-TV. His weather forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
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