Cumulus Mediocris

Cumulus mediocris, Photo by Craig Johnson, Copyright July 2019

Cumulus mediocris - a medium size cumulus. This cloud, and several others of similar dimension, turned the sky into a three dimensional extravaganza. The dark base and subtle shading at its top caused the cloud to “pop” out of the deep blue sky. It’s almost as if we could touch the cloud or bounce up and down like on feather-soft pillow. This cumulus mediocris drifted across the sky and slowly evaporated as new cumulus formed.

Cumulus commonly form during the warmth of the day. They are most common in summer when heated air, warmer than surrounding air rises. To see this principle in action watch hot air balloons rise. As the balloon rises the air inside cools, eventually becoming cooler than the surrounding air. The balloon then begins to sink. The pilot must add more heat to cause the balloon to rise again or it will eventually sink to the ground.

A cumulus mediocris does not have the luxury of having a pilot add more heat. Instead, if the air in the cloud does not remain warmer than the surrounding air it sinks and the cloud evaporates.

So how do cumulus clouds keep rising? As the cloud rises water vapor in the air condenses, releasing heat into the cloud. As long as the heat released into the cloud keeps it warmer than the surrounding air the cloud grows. Sometimes a cumulus mediocris grows into a thunderstorm - called a cumulonimbus. The cloud in the picture above did not grow larger. It became cooler than the surround air and began to descend and evaporate.

It turns out that cloud formation, size, shape, and growth are determined by several factors. Something must lift the air. There must be enough water vapor in the air to condense into a cloud. The temperature difference between the growing cloud and its surroundings determines the height of the cloud. And the wind shapes the cloud. In the end all of those factors determine if rain, snow, or hail fall from the cloud.

On this day nothing fell from the cloud. It simply grew to the height you see in the photo and then evaporated - disappearing into thin air!