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Weather school: High, low, and middle clouds

Precipitation rarely falls out of stratus clouds, other than occasional drizzle

I know, I know — some of the long-range winter forecasts are starting to come out, so it must be time for me to take a look ahead to see what this fall and winter might have in store for us. Unfortunately, my review of all the different long-range forecasts is going to have to wait until next issue, as some of the long-range models will be coming out with new outlooks in the next week, so just hold on a little bit longer and we will be able to see the whole picture.

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In this issue we are going to continue our look at clouds. I got a little ahead of myself in the first article when I jumped right in looking at basic cloud naming and classification. Last issue I took a step back and talked about cloud formation. So now, as promised earlier, we are going to move on to examine in more detail our high, middle and low cloud types.

Starting at the top, let us take a closer look at high clouds. These clouds, as we have learned, are known as cirrus clouds. The most common type of cirrus cloud is simply called cirrus. These are those high, wispy clouds that often look like they have been stretched or blown out into long streamers — what’s often referred to as “mares’ tails.” The reason for this appearance is that cirrus clouds are made up of tiny ice particles that are easily blown about by strong upper-level winds.

Cirrus clouds will generally travel in a west-to-east direction as they are blown along with the prevailing westerly winds. Generally, they are associated with fair weather, but they can also signal the approach of stormy weather. Cirrus clouds can be blown off the tops of thunderstorms and stretch out for several hundred kilometres ahead of the storm. Approaching areas of low pressure can also be preceded by cirrus clouds. In both cases, cirrus clouds slowly thicken and are replaced with lower clouds, so when this happens, there is a good chance that wet weather may be moving in.

Along with everyday cirrus clouds we also have cirrostratus clouds. These are high-level clouds that cover the whole sky like a sheet. These thin clouds are also made up of ice crystals and are usually thin enough that you can see the sun and moon through them. With these clouds we will often see halos around the sun and the moon. Since these clouds are a thicker form of cirrus clouds, the arrival of the cirrostratus clouds usually means a storm system is moving in or is nearby — especially if they are followed by lower cloud types. Cirrocumulus clouds are usually made up of a bunch of smaller-looking clouds that group together forming ripples or kind of a scaly appearance. Therefore, this cloud type is often referred to as a “mackerel sky.”

Dropping down in the atmosphere, we come to our middle-level clouds. Here we really only have two main cloud types: altostratus and altocumulus. Altocumulus clouds are similar in appearance to cirrocumulus, but they will appear a little larger and will usually have some grey or shading to them. They occur when the atmosphere is unsettled, but precipitation is rare from these clouds. Altostratus, meanwhile, is grey or sometimes blue grey and will usually cover the entire sky. With these clouds, the sun or the moon can sometimes be dimly seen, but unlike cirrostratus clouds they will not produce a halo. Another way to determine if the clouds are altostratus or cirrostratus will be their colour, as cirrostratus tends to be white while altostratus will always be grey or blue grey. Altostratus clouds are often found just ahead of storm systems, so seeing this type of cloud move in will usually be the precursor to some kind of precipitation event.

Finally, let us drop down to our low clouds or our stratus clouds. Stratus clouds, like the alto and cirrostratus clouds, will usually cover the entire sky with a uniform-looking cloud. Stratus clouds tend to be a uniform grey colour and are often compared to fog that does not touch the ground. In fact, when fog rises and is no longer covering the ground, it becomes a layer of stratus clouds. Precipitation rarely falls out of stratus clouds, but we can see occasional drizzle with these cloud types. When stratus clouds thicken and become very heavy and wet looking, they are referred to as nimbostratus, which simply means stratus clouds that are producing rain. These cloud types are associated with continuous precipitation, whether it be rain or snow. While these types of clouds can be hard to see due to the falling precipitation, the fact that precipitation is falling pretty much tips us off as to the cloud type. As for the type of weather associated with nimbostratus, well… rainy or snowy!

In the next article we will take a break from our look at clouds and look at the fall and winter forecasts. We will then follow that up by looking at some of the additional terms we can use to describe clouds; then we will finish our cloud discussion by looking at some rare cloud types and clouds with vertical development.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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