Clouds of the Day: From Clear to Cumulus

The mid-morning sky was a blue as blue can be. Northerly breezes brought in air from the large rural areas of the Upper Midwest and Canada. This is what the sky looks like without haze or high humidity. It was a day with pleasantly cool air after .70 inches of rain yesterday.

By 10:30 AM CDT one cloud patch appeared in the distance moving to the southeast (right to left). This altostratus cloud was the first sign of change.

Between 10:45 and 11:30 the area of clouds had expanded as more moisture aloft and upward motion created these altocumulus.

A close-up a few minutes later reveals the altocumulus, including altocumulus floccus near the bottom of the photo with virga (also called fallstreaks) of water droplets and ice crystals evaporating/sublimating) into water vapor long before they could have reached the ground.

Here is a close-up of a cumulus. This photo is looking northwest and the cloud is moving from right to left. It looks like a cotton ball. This cloud changed its shape very quickly and evolved into one of the clouds in the photo below.

Before Noon, looking south, we can see the remnants of the altocumulus in the background while cumulus formed in the foreground. While the altocumulus formed because of upward motion generated by winds aloft, the cumulus are thermodynamically driven: the heating of the earth’s surface is heating the air above it creating upward which is stronger than the motion that formed the altocumulus.

More cumulus. These cumulus are named cumulus humilus: humble cumulus. They are also called fair weather cumulus because the atmosphere is not set up to allow further growth of these clouds. They will dissipate around sunset as the heating of the day escapes to space and air temperatures cool. Cooler low level air is stable which stops the rising motion that created the cumulus.

Nearing the end of the day we see these beautiful cirrus cloud formations. The cumulus have evaporated in the dry lower level air when the upward motion ceased.

At 6:20 PM CDT, as temperatures cooled, the cumulus dissipated leaving only thin fibrous cirrus in patches overhead. The clouds contain ice crystals which creates the fibrous edges. In the cold drier air aloft the ice sublimates: it does not evaporate. Evaporation occurs when water changes directly to water vapor. Sublimation occurs when ice crystals change directly to water vapor.

Cirrus can travel for hundreds of miles in the cold air aloft because sublimation is a slow process. Notice the streamers trailing from the clouds. Winds aloft change direction and speed with altitude. The changes are not usually dramatic but are sufficient to create the beautiful formations we associate with the family of cirrus clouds.

All photos are Copyrighted in 2022 by Craig Johnson, Weather Briefing, LC.