Clouds of the Day for Friday, November 4, 2022 - Nimbostratus

Today was a rainy Friday. As of 8:45 p.m. the rain gauge recorded 1.23 inches of rain with light rain still falling. The photo below was taken this afternoon. It shows wet streets and if you look closely you might even see a few splashes of rain drops hitting the concrete.

The ground is wet from continuous rain. Looking skyward we see the dull gray sky of Nimbostratus clouds. Nimbus means rain and a Nimbostratus is a cloud with rain. Rain must be present without thunder or lightning for the cloud to be Nimbostratus. If thunder and lightning are present with rain the cloud is Cumulonimbus.

Nimbostratus usually have an indistinct cloud base below 6,000 feet which is obscured by rain. Nimbostratus may form in the middle levels as altostratus and then lower and evolve into Nimbostratus as rain begins to fall and moistens the lower level of the atmosphere. The cloud base lowers and gradually becomes obscured.

Finally let’s look at two charts. The first chart shows the thermograph of the past 48 hours. T his chart is plotting temperature, dew point, and relative humidity from left to right. We can see when the leading edge of the cold front passed just before 4:00 a.m today. Notice how the temperature trace (solid red line) suddenly begins dropping from near 60 degrees to the lower 40s by Noon. The dew point also began dropping with the passage of the front as drier low level air moved in from the west. The relative humidity began to rise (blue dashed line) as the temperature cooled and rain began around 8:00 a.m. The relative humidity increases when the temperature cools and the dew point remains about the same and it also increases when rain evaporates more moisture into the air as water vapor.

Finally, here is the trace from the rain gauge. The blue link shows the total rainfall with time and the purple line shows the rainfall rate with time in inchs/hour. At the end of the trace the total was 1.25 inches.