Cloud Formation and Precipitation — Page 228, Lesson 187

I want to walk you through the two main theories that explain how precipitation actually forms inside a cloud, and then we’ll go through the different types of precipitation you’ll encounter as a professional pilot. Let’s start with the Bergeron theory. This theory presumes that at high levels in the cloud, some of the water droplets will turn to ice. Those ice crystals then grow in size by two processes: first, by sublimation of water vapour — that’s water vapour turning directly into ice without passing through the liquid phase — and second, by collision with supercooled water droplets. Supercooled water droplets are liquid water that remains unfrozen even though the temperature is below zero. The frozen droplets become much heavier than the existing water droplets, so they drop out at the bottom of the cloud. Depending on the temperature down there, they fall either as snow or as raindrops. Now, the Bergeron theory works well for many situations, but it’s difficult to see how it can account for summer precipitation, where the whole cloud is at a temperature above zero. That’s where the coalescence theory comes in. It may provide a better answer for those warm clouds. This theory assumes the presence of a range of droplet sizes inside the cloud. The larger droplets fall faster and unite with the smaller droplets as they descend. Eventually, the drop becomes overweight and falls out as drizzle or rain. Let’s move on to the precipitation types themselves. I’ll go through each one with its defining characteristics. First, drizzle. Its diameter is 0.2 to 0.5 mm. The visibility in drizzle is 500 to 3000 metres. It has an imperceptible impact — you barely feel it. Next, rain. The diameter ranges from 0.5 to 5.5 mm. Visibility in rain is 3000 metres down to 5.5 km — though in heavy rain it can drop to 1000 metres. Rain has a perceptible impact. Then we have snow. Snow comes in several forms. Grains or needles have a diameter less than 1 mm. Pellets are 2 to 5 mm in diameter. Flakes are a collection of crystals greater than 4 mm in diameter. An important point: the lower the temperature, the smaller the size of the flakes. For visibility: moderate snow gives about 1000 metres, heavy snow gives 50 to 200 metres. There’s also drifting snow — that’s snow lifted less than 2 metres above the surface — which will reduce the visibility figures I just gave. Blowing snow, also less than 2 metres above the surface, will GREATLY reduce those visibility figures. Next, hail. Hail diameter ranges from 5 mm to 50 mm and above. Weight can be up to 1 kg. Hail grows by collision with supercooled water droplets and by sublimation or deposition — that’s the same direct ice-from-vapour process we saw in the Bergeron theory. Then there’s soft hail or graupel. These are small rounded pellets, only a few millimetres in diameter. They fall from wintry, showery cloud. Soft hail represents an early stage of hail growth. Finally, ice pellets. These are transparent pellets, either spherical or irregular, with a diameter less than 5 mm. They fall from layered cloud. Now let’s look at the precipitation summary table, which organises precipitation by duration, intensity, and cloud type. For duration, we have three categories. Showers are always associated with convection or heap-type cloud — that’s cumuliform cloud. They are of short duration. Intermittent precipitation is associated with layer cloud. It falls from time to time, with no marked clearance. Continuous precipitation is also associated with layer cloud, and it has no breaks for 60 minutes or more. For intensity, we have three levels: slight, moderate, and heavy. For rain: slight is less than 0.5 mm per hour, moderate is 0.5 to 4 mm per hour, heavy is greater than 4 mm per hour. For snow: slight is less than 0.5 cm per hour, moderate is 0.5 to 4 cm per hour, heavy is greater than 4 cm per hour. For showers: slight is less than 2 mm per hour, moderate is 2 to 10 mm per hour, heavy is 10 to 50 mm per hour. Now, the cloud type table ties it all together. Heap cloud — that’s Cu for cumulus, associated with instability — gives rain or snow showers. Cb, cumulonimbus, gives rain, snow, or hail showers. These range from light to moderate for Cu, and moderate to heavy for Cb. Layer cloud — associated with stability — includes Cc (cirrocumulus), Cs (cirrostratus), As (altostratus), St (stratus), Ac (altocumulus), Sc (stratocumulus), and Ns (nimbostratus). For layer cloud, Cc and Cs produce nil precipitation. As, St, Ac, Sc produce slight rain or snow. Ns produces moderate to heavy rain or snow. Finally, let’s talk about how we measure this precipitation. Rainfall recorders are used at some Meteorological Offices. They indicate the rate of fall — that is, the intensity — of precipitation. You can see this in Figure 13.14. Rain gauges, on the other hand, merely measure the amount of precipitation falling at the station. They don’t give you intensity directly. The intensity would have to be estimated, and where visibility is measured, a table may be used. That covers the Bergeron and coalescence theories, all the precipitation types with their diameters and visibility effects, the duration and intensity classifications, the cloud types that produce each kind of precipitation, and the difference between rainfall recorders and rain gauges.