Altocumulus, which we abbreviate as Ac or AC — Page 207, Lesson 173

I want to walk you through the next group of cloud types in the high and medium levels. We've already covered the low-level clouds, so now we're moving up into the middle and high cloud families. Let's start with Altocumulus, which we abbreviate as Ac or AC. Altocumulus appears as speckled white or grey cloud. The patches look like rounded masses that can be either fibrous or diffuse in appearance — so sometimes you see distinct edges, sometimes the edges are softer and less defined. Now, there are two specific forms of altocumulus that are particularly important for us as pilots. The first is Altocumulus lenticularis, which you'll often hear called simply lenticular cloud. This cloud forms downwind of mountainous or hilly areas, and its presence tells us something critical: it indicates mountain wave activity is occurring. Because it sits downwind of high ground, moderate or even severe turbulence may be associated with the presence of altocumulus lenticularis. But here's a very important distinction — the air inside the lenticular clouds themselves is always smooth. So the lenticular cloud shape marks where the wave is, but the turbulence is found outside the cloud, in the surrounding wave system. The second significant form is Altocumulus castellanus. This is a "bubbly" form of normal altocumulus. The name comes from the "towers" that form in it — they look like battlements on castles, hence castellanus. These clouds are significant because they often herald a change to showery, thundery weather. They are a feature of summer weather in temperate latitudes. And here's a key operational point: cumulonimbus clouds sometimes develop from altocumulus castellanus when instability is present at medium levels of the troposphere. So if you see altocumulus castellanus on a summer day, you need to watch for potential thunderstorm development. Now let's move to Altostratus, abbreviated As or AS. Altostratus is a grey or bluish sheet, or layer, of cloud. It can be fibrous or uniform in appearance. Sometimes altostratus covers the whole sky, and when it does, it gives a "ground glass" effect around the sun or moon — meaning the sun or moon is visible but its outline is blurred, like looking through frosted glass. Altostratus can be from around 2000 feet to 8000 feet thick. But despite that thickness, altostratus is not a dense cloud, and the sun is usually perceptible through the cloud layer. Next we go higher to Cirrus, abbreviated Ci or CI. The name comes from Latin cirrus, meaning curl. Cirrus is the highest of all the cloud types and is composed entirely of ice crystals. It takes the form of white delicate filaments, in patches or narrow bands. You might also hear it described as fibrous or hair-like. Cirrus often heralds the approach of a warm front — so when you see cirrus advancing, you know a warm front is likely on its way. Then we have Cirrostratus, abbreviated Cs or CS. Cirrostratus is a transparent, whitish cloud-veil of fibrous or smooth appearance, totally or partially covering the sky. It is made up of ice crystals. Like cirrus, the presence of cirrostratus usually indicates the approach of a warm front. In tropical regions, cirrostratus is often associated with the presence of tropical revolving storms — so that's a significant warning sign in those latitudes. Cirrostratus often produces a halo around the sun or the moon — that's a ring of light caused by the ice crystals refracting sunlight or moonlight. Finally, Cirrocumulus, abbreviated Cc or CC. This is probably the cloud which is least often seen in the sky. Cirrocumulus is a thin, white and patchy layer of cloud, with ripples that are more or less regularly arranged. Like cirrus and cirrostratus, cirrocumulus consists of ice crystals. So to summarise the high-level picture: altocumulus and altostratus are medium-level clouds, while cirrus, cirrostratus, and cirrocumulus are high-level ice-crystal clouds. Each gives us different clues about what the atmosphere is doing and what weather is approaching.