San José State University
Thayer Watkins
Silicon Valley
& Tornado Alley

The Flatness of the Underside of Clouds

The undersides of clouds are remarkably flat; much flatter than their top sides. There may be multiple factors contributing to this flatness.

Air masses which are warmer and/or moister than the surrounding air rise. They rise to a level where the temperature and pressure of the atmosphere is such that their net bouyancy balances their weight. It is a remarkable fact that air containing more water vapor is lighter than air containing less water vapor. This is counter to intuition but the explaination is simple. The average molecular weight of dry air is 29 units. This is a weighted average of the molecular weights of the nitrogen N2, oxygen O2 and other constitutents of dry air. The molecular weight of water H2 is 18, so adding water vapor decreases the molecular weight of air. So moister air, even at the same temperature of surrounding air, is lighter than that air and therefore rises.

One explanation of the flatness of the undersides of clouds is that a rising mass of moist air cools and at a particular temperature some of the water vapor condenses forming the mist of clouds. That temperature occurs at a particular altitude and so the air mass above that altitude is visible due to the condensation. The portion of the air mass below that altitude is invisible. This is depicted in the following diagram.

Air masses located in the vicinity will encounter the critical temperature for condensation at about the same altitude so all the air masses in a location will display the lower limit of condensation at about the same altitude.

Another explanation for the flatness of the undersides of clouds is that clouds are like bodies resting on an ocean of air. This effect is more easily visualized in terms of oil and water. Oil is less dense than water so a blob of oil will rise to the surface of the water. Surface tension in the oil will give the blob under water an approximately spherical shape. At the surface however that sphere flattens out generally into an ellipsoidal shape but specifically on the bottom it would be strictly flat. This effect for clouds is depicted in the following diagram.

The two explanations for the flatness of the undersides of clouds are not contradictory. Both may be operating. A test of the two effects would be to observe what happens when the temperature of the surrounding air is lower. If the condensing temperature is achieved at a lower altitude the clouds will appear to have grown thicker. On the other hand if only the second effect is operating the clouds will not grow thicker but only change their altitude.

For material on the discreteness of clouds see Discreteness of Clouds.

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