Clouds of the Day - Sunday, December 26, 2021

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Altocumulus and Altostratus

A low pressure system is organizing over western South Dakota today. The forecast map below shows the expected position for fronts and precipitation at 6:00 p.m. this evening. Clouds have been overspreading Iowa today and ceilings are expected to continue to lower this afternoon. Three photos below show the mid-level cloud types that have been visible so far as the storm system expands upward motion eastward.

Forecast Map from NOAA/Weather Prediction Center valid 6:pm CST 12-26-2021

Wave Clouds - Altocumulus with altostratus above

Wave Clouds - Altocumulus with altostratus above

Left above - Upward air motion is causing water vapor to condense into mid-level clouds. Waves in the air flow are rippling through the cloud layer to form these wave clouds - altocumulus. Altostratus are visible above the altocumulus layer.

Right above is another view of the altocumulus looking to the southeast. The flow at the cloud level is from the northwest so this photo looks down stream with the flow. Altstratus is visible above the altocumulus.

The left photo above looks to the north and shows altocumulus lined up in rows from southwest to northeast and as individual cloud elements.

The right photo is looking west in the early afternoon and shows mostly altostratus clouds. However, there are still ripples of altocumulus waves apparent in the bottom and lower right corner. These photos show the broad scale of the upward motion and the mid-level clouds that have formed in the area of upward motion. The clouds cover northeast Iowa will continue to lower as moisture increases within the area of broad upward vertical motions. The expected sequence is for the altostratus to invade the entire sky followed by precipitation aloft obscuring the cloud ceiling. Once the precipitation reaches the ground the cloud will be named nimbostratus. The precipitation will be rain. The temperature as I post this text is 41 degrees how the precipitation will begin as rain and snow aloft, but the snow will melt before reaching the ground.

Clouds of the Day - Thursday, December 23, 2021

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Summer Sky vs Winter Sky

Summer skies are dynamic - with exciting action. Winter skies are more placid. But don’t be fooled. Compare the photos below. The first one was taken in June of 2021. The second was taken today, December 23, 2021.

In June the sky was roiling with turbulence. Warm moist air was rising rapidly producing bulging clouds called cumulus congestus (large, congested cumulus) and cumulonimbus (thunderstorm). Today the clouds looked serene with a layer of altostratus (high stratus) and another layer of weak (almost flat) cumulus. Upward motions in cumulonimbus sometimes exceed 60 mph while altostratus form where air is rising a few inches per second. Thunderstorms are intense localized storms while altostratus or altocumulus form in vast cloud sheets covering many 10s or 100s of square miles.

The clouds shown below show the contrast between summer and winter skies.

Cumulus congestus and cumulonimbus in background.

Altostratus in background and altocumulus in the foreground. The Sun is visible through the clouds.

Clouds of the Day - Wednesday, December 22, 2021

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CONTRAIL

CONTRAIL is short for CONdensation TRAIL. It is the condensation of moisture in the exhaust of jet engines. The first photo below also shows a small patch of cirrocumulus clouds, possibly created from a previous CONTRAIL. The second photo is a closer view of the CONTRAIL and the aircraft. It takes an eagle eye but if you look closely at the second photo you can see the aircraft located ahead of the condensation trail. Condensation occurs a short distance behind the aircraft when the hot gases of the exhaust have cooled to the condensation temperature of the water vapor.

CONTRAIL with cirrocumulus patch

CONTRAIL

Clouds of the Day - Tuesday, December 21, 2021

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Cumulus

Cumulus is one of 10 principal cloud types. Its name refers to the puffy or heaped shape caused by upward motion as air warmer than the surrounding air rises. As it rises it expands because the surrounding air at higher levels is under less pressure. As it expands it cools and the invisible water vapor in the air condenses. The level where it condenses is called the condensation level.

The two photos below show cumulus clouds with different shapes and sizes in a group. The clouds were moving from the northwest to southeast in a cold air mass. The afternoon sunshine was warming the earth creating masses of warmer air rising to the condensation level where the moisture condensed into puffy clouds.

Cumulus

Cumulus

Clouds of the Day: Monday, December 20, 2021

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Altocumulus: Today dawned with cirrus spreading overhead from the west. During the day, the cirrus was joined by a lower layer of altostratus. The first photo below shows wisps of cirrus mixed with thicker altostratus in the lower half and a sheet of darker altostratus in the upper half. The second photo shows thick altocumulus.

Altostratus

Altocumulus (cloud bases higher than 6,000 feet)

Surface temperatures warmed from the 20s into the mid-30s during the afternoon. Winds at the cloud level were blowing at 40 to 50 mph. By early afternoon the cirrus thickened with cloud bases dropping. By the time second photo was taken vertical tufts of altocumulus were visible. The clouds were still high, near the top of the level where middle clouds form and just below the cirrus level. The clouds in the second photo below are altocumulus. Look closely and you will see puffy clouds building out of the flat cloud bases - the telltale sign of cumulus.