From Summer to Fall

There is a definitely a difference between “summer” clouds and “autumn” clouds. Summer clouds show more vertical development while autumn clouds begin the transition to a winter sky. Autumn brings a mix of summer and winter clouds - with more summer clouds at the beginning of autumn giving way to winter type clouds by the first day of winter. This past week the Upper Midwest turned a corner.

The following sequence of clouds occurred during an 8 day period from September 17th to September 24th. Each photo is identified by date and cloud type. The transition occurred as a cold front crossed Iowa bringing very heavy rain, river flooding, and a switch to much cooler temperatures. In the beginning, high temperatures reached 90 degrees, by the end readings topped out in the low 60s.

Cumulus Congestus. September 17, 2018 at Cedar Falls, Iowa

Cumulus congestus are one stage away from cumulonimbus (thunderstorm) clouds.

The congestus stage is part of the natural progression from the cumulus stage to the cumulonimbus stage. Sometimes the progression from smallest to the tallest clouds requires much of the day. However, on some days it can take less than an hour. The progression looks like this: cumulus, cumulus mediocris, cumulus congestus, cumulonimbus. As the clouds grow in size their tops increase in height penetrating into colder air aloft. The tops of summer thunderstorms reach heights where temperatures fall to -60 degrees F.

Precipitation may be seen falling near the end of the cumulus congestus stage, just before the cloud becomes a cumulonimbus. On the 17th thunderstorms with heavy rain developed.

Cumulus mediocris. September 19, 2018 at Cedar Falls, Iowa

As time passed the atmosphere became more stable over Iowa but not before another round of storms occurred on the 19th when the cold front crossed the state. These cumulus mediocris clouds are part of the cumulus to cumulonimbus progression. They represent the second stage in development before cumulus congestus. This cloud type does not produce precipitation. Thunderstorms with heavy rain also developed on the 19th.

Stratocumulus. September 21, 2018 at Cedar Falls, Iowa

Stratocumulus form in a more stable atmosphere. While the cumulus progression to thunderstorms occurs when the atmosphere is very unstable, stratocumulus occur when rising motion occurs only in the low levels. The clouds do not develop to great heights.

Stratocumulus clouds look a little like stratus (layered clouds) and a little like cumulus (unstable puffy clouds) formations. The clouds in the above photo formed behind the cold front. Stratocumulus do not produce precipitation, however in an unstable atmosphere stratocumulus may progress to a deeper cumulus formation that does produce precipitation - especially during the warmest part of the afternoon on cool days. On this day no precipitation fell.

Cirrostratus. September 24, 2018 at Cedar Falls, Iowa

Cirrostratus are a high thin layer of cloud found above 18,000 feet and is often seen at altitudes above 30,000 feet. If you have a window seat on an airplane you may see cirrostratus clouds. They will be a sheet-like formation of wispy clouds made of ice crystals. From the ground the disk of the Sun shines through cirrostratus.

These cirrostratus are heralding the next storm system. After the cold front passage we had two days of stable weather. Then it was time for the next storm system, moving from the High Plains to the Midwest, to approach. The photo is looking west. When cirrostratus form in a continuous sheet they are often the first sign of the next storm. These clouds race far in advance of the main storm which is many hundreds of miles to the west or northwest. Astute early settlers and native Americans would watch for thickening cirrostratus to warn of a possible approaching storm.

Cirrostratus with fall streaks. September 24, 2018 at Cedar Falls, Iowa

Finally, this photo shows the same cirrostratus except it is looking south instead of west. Near the center bottom of the photo thin fall streaks are visible against the background of clear blue sky. These streaks are falling ice crystals. Looking toward the middle and top of the photo, narrow streaks of denser clouds are visible. These bands are the fall streaks as they look from the bottom up.

Cirrus formations are some of the most beautiful clouds in the sky. They are delicate and often paint wonderful streak formations that must be seen to be appreciated. One of my earlier blogs shows some nature’s handiwork. Wind speeds may range from a few miles per hour to more than 150 mph, depending on the season and type of weather systems moving overhead. Look up on a day when cirrus dominate the sky and you may be treated to quite a show.