ASK A SCIENTIST
Question: What makes some clouds dark?
Answer: Everyone knows that towering slate-gray cumulonimbus clouds are a sure sign of violent weather. Just as they know that small, white puffy clouds forming in the afternoon indicate clear and pleasant weather for the rest of the day. Few people, however, understand why some clouds are light and some are dark.
The answer goes back to understanding what clouds are and what happens inside them as they grow. Clouds are composed of tiny, and I mean tiny, droplets of liquid water. These droplets form when cool temperatures high in the sky cause molecules of water vapor, an invisible gas, to condense on the dust, salt, or volcanic ash present in our atmosphere. Water droplets are so small that air currents, not gravity, control how they move. The updrafts generated when the sun heats the earth's surface can produce towering clouds.
As clouds grow, the water droplets inside them coalesce with other water droplets to increase in size. If these water drops grow heavy enough, they will fall as rain. In the strong updrafts of thunderstorms, water drops can even be blown to the top of the cloud where the frigid, high altitude temperatures quickly freeze the liquid water into ice that can eventually fall as hail.
Though water droplets are basically transparent, they do scatter sunlight to produce the pearly white cloud color we see most often in the sky. This scattering, for example, can easily be observed at night when you shine a flashlight into a thick fog. As clouds get thicker, it becomes harder for sunlight to pass through them. Typical thunderstorm clouds can be over 20,000 meters (60,000 ft.) high, twice as high as Mt. Everest. Most of the sunlight striking the top of these clouds cannot penetrate to the bottom. As a result the cloud appears grayish underneath.
Since thunderstorms can produce powerful winds, driving rain, lightning, and occasionally hail and tornadoes, everyone spending time outside should watch the sky for these billowing dark clouds that are a sure sign of dangerous weather. Meteorologists know that identifying clouds types are a key to predicting local weather for the next 6-12 hours. Many excellent cloud identification books are available at libraries. Check one out and become your own cloud expert!
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