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Monitor the evolving appearance of cottonball-shaped cumulus clouds, especially those taller than they are broad. If the emerging head of a towering cumulus starts looking hazy compared to the bright, sharp-edged cloud beneath, a full-blown thunderstorm threatens—sometimes within minutes.
• Beware of lightning as soon as you see precipitation falling from the cloud base. Gauge the storm’s proximity by counting the seconds separating a bolt from its thunderclap: Five seconds equals about 1 mile, and lightning can strike your location from a storm 10 miles away. Shortening intervals between flash and boom indicate the storm is getting closer.
• Evaluate the storm’s direction by observing the location of successive lightning flashes. Also note shifting winds: Cool, moist, ozone-scented gusts that may be 30 mph or stronger signal downdrafts sweeping outward from the thunderhead’s base. These bursts suggest the storm may be upon you momentarily.
• When storms threaten, descend from ridgelines and high points to lower, less lightning-prone terrain.
• If the fuzzy appearance of the cloud top spreads throughout its crown, the storm may be decaying. Air-mass thunderstorms in the mountains tend to be brief—often 30 minutes or less.