Cumulonimbus clouds bring thunderstorms. This was a particularly nasty storm that Ken and I watched from the car on a small hill above the Civic Center Park in Oak Ridge in April 2006. The ominous dark color foretells the wrath that the storm was about to unleash upon us! This cloud is like a supersaturated sponge that can no longer hold the water. We had less than a mile to drive home before the storm hit --- we didn't make it!
Cumulonimbus means "piled up rain cloud". These clouds are the "bad boys" of the cloud types, towering up to 7 miles high, capable of delivering pounding rain, downdraft winds, hail, and even tornadoes. The vertical winds inside these storms can reach 100 miles per hour. Hail forms when small ice crystals are lifted up inside the cloud and begin to grow, much like the layers of an onion. When the hailstone gets heavy enough, it will fall to the ground at up to 120 mph! Airline pilots will avoid these types of clouds at all cost. We once were flown miles out of our normal flight path and then under the cloud to avoid being in the worst of the storm. It was still a very bumpy ride, the passengers applauded when we touched down in Knoxville!