Clouds of the Day & There’s a Storm Brewing - Wednesday, October 11, 2023

Map plotted using Digital Atmosphere available from Weathergraphics.com.

CLOUDS OF THE DAY

After frost yesterday morning with a low of 33 degrees today started with temperatures in the upper 30s and Altocumulus in the eastern sky. Altocumulus are middle level clouds between 6,500 and 20,000 feet. They indicate a thin layer of unstable air. The clouds are plumes of upward motion. The upward motion moves air higher where its moisture is sufficient to condense as cloud droplets. These clouds are usually made up liquid rather than ice when temperatures are below freezing, or a mixture of liquid droplets and ice crystals. In between the visible plumes is open sky where the air is sinking. As it sinks it is compressed because of higher air pressure as lower levels which warms the air and does not allow condensation to occur.

Photo copyright by Craig Johnson, Weather Briefing LC October 2023

The photo below is very interesting because of the multiple cloud types that are visible at the same time. Starting at the top, the first row of clouds are Altocumulus stratiformis. The clouds have the cellular structure that are association with the cumulus cloud type but they are in a continuous layer reminiscent of stratus. The smaller cells down the photograph are also Altocumulus but are a little higher and smaller.

The middle cloud band is a Stratus formation with small ripples or waves. The layer has some cumulus characteristics, especially in the left end of the cloud band. Overall this is a Stratus type formation so we will call it an Altostratus undulatus (undulating) cloud type.

Finally, the layer at the bottom of the photo is classic Altostratus even though there are hints of small wave action within the layer.

Photo copyright by Craig Johnson, Weather Briefing LC October 2023

STORM BREWING

The clouds in the photos above were forming in advance of a low pressure system that is organizing along the eastern slopes of the Rocky Mountains. Upward motion in advance of the storm is raising the air mass over Iowa to saturation. The first photo above shows a layer of Altocumulus floccus (looks like flocks of wool) which reveals instability in the layer. However, overall the air mass above the cloud layer is stable so the layer of condensation is thin.

The next photo shows several different cloud types also forming in the middle layer of the atmosphere. These clouds have stratus characteristics but showed how broad the rising air mass is. The clouds should continue to gradually thicken and lower as rising motion continues causing clouds to thicken and lower as precipitation develops.

Light rain began here at 2:25 p.m. and produced .02 inches of precipitation. The rainfall rate was .12 inches/hour which is in the moderate rain rate category. It was brief so the total rainfall was light. It is just the beginning. During the next two days much more moisture is due to arrive from the Gulf of Mexico and produce showers and thunderstorms as the low pressure gets better organized and drifts eastward. Looking at the Surface Map above it is raining along the Gulf Coast. Dew points will continue to increase as the moist air reaches the cooler and drier air mass over the Central Plains. Dew points are already in the lower 60s in northern Missouri and in the 40s north of a warm front along the Iowa - Missouri border. The lifting of the air mass along the boundary will continue to thicken clouds in the cooler air setting the stage for widespread rain.

If you know how to decode the station model plots on the map you can see the weather conditions at the plotted cities. If you do not know how to decode the plots back to our home page and scroll down to the tutorial.

The next photo shows Nimbostratus. While it is difficult to see the light rain falling from this cloud if you look closely in the lower left it is visible at the bottom of the cloud where the sky lightens up. In that transition from the darker cloud base and the lighter sky is a very thin curtain of gray that is the rain. It is also barely visible in the lower right between the trees. If there was no rain falling this would be classified as a Stratus or Altostratus. The cloud ceiling at the nearby airport was 6,500 feet which is the border of Altostratus and Stratus. My personal preference would be to name it Altostratus in that case because the clouds have been lowering from the middle levels above. Regardless of the name before the rain started, it is Nimbostratus now because it is a stratus type cloud and rain is falling.

Photo copyright by Craig Johnson, Weather Briefing LC October 2023