Are There Any Elephants in Your Clouds?

Forrest M. Mims III

Those familiar puffy, cottony clouds that grace Texas skies on most summer days are called cumulus clouds. Like plants and animals, scientists use Latin to assign names to clouds. Cumulus comes from the Latin word meaning “heap,” which well describes a cloud that looks like a pile of wool or cotton balls.

Some cumulus clouds more closely resemble popcorn than anything else. When they become larger, you might see faces, cartoon characters, furniture and all kinds of objects seemingly embedded in them.

Cumulus clouds are the most familiar clouds of summer in regions where the air is warm and moist. They are a product of thermals, the rising columns of warm air that allow vultures and hawks to soar high in the sky without flapping their wings.

Figure 1. This rapidly growing sea breeze thunderstorm contains many hundreds of tons of water. Photograph by Forrest M. Mims III.

Thermals occur when the sun heats soil, rocks and plants, which then warm the adjacent air. Warm air expands and is less dense than cool air. So the warm air rises, carrying with it abundant water vapor found near the ground.

As the air rises, it eventually encounters cool air above. Cool air can hold much less water vapor than warm air. As the rising air cools, its water vapor condenses into tiny droplets of liquid water, and a cloud is born.

Besides being fun to watch, cumulus clouds provide important clues about the sky and the weather. Small to medium cumulus are signs of fair weather. Their flat bottom indicates the elevation in the sky where the temperature of the air is cool enough for condensation to occur.

Larger cumulus clouds signal instability in the atmosphere that can lead to rapid cloud growth. When a cumulus cloud suddenly begins rapidly expanding in size and growing straight up, it becomes a cumulus congestus cloud. By comparison with the relatively gentle uplift of a thermal, cumulus congestus clouds indicate very powerful and turbulent up and drown drafts. Rain may fall from them, and pilots avoid them.

If a cumulus congestus grows to the point that its top acquires an anvil shape, flashes of lightning are seen, and rain is falling, it becomes known as a cumulus nimbus or simply a thunderstorm.

How much water do clouds contain? Dr. Peggy LeMone, a scientist at the National Center for Atmospheric Research in Boulder, Colorado, did the math and found that a typical cumulus cloud contains 500 or more tons of water! She converted tons to elephants and concluded that a typical cumulus clouds weighs as much as 100 elephants.

She then calculated that a large thunderstorm equals about 200,000 elephants, and a hurricane is around 40 million elephants.

So the next time you think you see an elephant outlined in a cumulus cloud, you just might be right. Ignore those who make fun of you and keep on looking. You just might see a few hundred more.

"World of Science" columns are selected and sometimes revised from columns published in the San Antonio Express-News or the Seguin Gazette-Enterprise. The columns are intended for a general audience. Forrest M. Mims III and his science are featured online at www.forrestmims.org and www.sunandsky.org .