cumulonimbus photo
Photo by Marcos Minutti

Definition: (noun) An extremely dense, vertically developed cumulus with a relatively hazy outline and a glaciated top extending to great heights, usually producing heavy rains, thunderstorms, or hailstorms.
Synonyms: thundercloud.
Usage: The cumulonimbus looming above our heads did not bode well for the future of our barbecue.
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In a Flash

As storm clouds develop, the temperature at the top of the clouds becomes much cooler than that at the bottom. For reasons that scientists still do not understand, this temperature difference results in the accumulation of negatively charged particles near the base and positively charged particles near the top of the storm cloud. This buildup of electrical charge causes a high-voltage discharge in the form of lightning.


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