Cloud Formation and Precipitation — Page 217, Lesson 186

Let’s start with inversions. An inversion in the atmosphere is a layer where temperature rises with an increase in height. That’s the opposite of the normal decrease we usually see as we climb. Because warm air sits above cooler air, the atmosphere becomes extremely stable — there’s very little vertical motion. That stability must inhibit the formation of cloud, because cloud development needs rising air to cool and condense. An inversion always exists above turbulence cloud — that’s the layer that caps the top of cumulus-type cloud that forms from mechanical mixing. And inversions have a similar effect at ANY altitude: wherever you find one, it suppresses vertical movement and therefore cloud growth. Now, precipitation. Clouds consist of water droplets averaging 0.02 mm in diameter — that’s about twenty microns. At that size, their rate of fall is negligible; they essentially float. But by colliding with other droplets, they may increase in size. They keep growing until they are too heavy to be supported by the upcurrents inside the cloud, and then they drop out as precipitation. So the key process is droplet collision and growth overcoming the updraft. There are currently two theories governing the formation of these precipitation drops. We’ll look at those theories in detail as we move forward. The book also references two figures here: Figure 13.13 shows precipitation, and Figure 13.12 shows the effect of inversions.