Our current method of naming and classifying clouds was developed in 1803 by Luke Howard, an English naturalist. His system employed Latin words to describe the clouds as they appear from the ground. Clouds that appeared to look like sheets were called stratus, which is Latin for layer. Puffy clouds were called cumulus, which is Latin for heap. Wispy clouds were called cirrus, which is Latin for curl of hair, and finally, rain clouds were called nimbus, meaning rain in Latin.
So in essence, Howard’s system had four different cloud types. These types could then be combined to cover other types of clouds. For example, stratus clouds that have rain falling from them would be called nimbostratus, cirrus clouds that form a layer would be called cirrostratus and so on.
In 1887, Abercromby and Hilde-brandsson expanded Howard’s system by creating four different groups. These would be high clouds, middle clouds, low clouds and a fourth group that spans more than one region — vertically developed clouds. The table shows the four major cloud groups and their types.
This addition to the naming and classifying of clouds does at first seem simple and straightforward, but there are a couple of things that make it a little confusing. First, we have cloud types of stratus (layer) and cirrus (wispy). To add in the height of the cloud we could use the term cirrus for all high clouds, alto for middle-level clouds and stratus for low-level clouds. So they are kind of using the same word for two different things. When we take a closer look, it does make some sense.
Wispy cirrus clouds only occur at high altitudes so anything high in the atmosphere will either be cirrus or have cirro attached to the front. Mid-level clouds are easy as we simply add the term alto to the cloud type. Naming low-level clouds works like the high-level clouds, except this time, we use the term stratus to describe the cloud. For example, low-level cumulus clouds would be called stratocumulus.
Starting at the top, let’s take a closer look at high clouds. As we have learned, these 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 are 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 the strong upper-level winds.
Cirrus clouds generally travel in a west to east direction as they are blown along with the prevailing westerly winds. They are usually 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 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 entire 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, especially if they are followed by lower cloud types.
Middle- and low-level clouds
Dropping down in the atmosphere we come to our middle-level clouds. Here we only have two main types: altostratus and altocumulus. I will look at cumulus clouds in a different article so let’s take a look at the altostratus clouds. This cloud type 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. 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’s drop down to our low clouds or our stratus clouds. These, 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 status clouds. 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 difficult to see due to the falling precipitation, the fact that precipitation is falling pretty much tips us off as to the cloud type.
In the next article, it’s time to look back at how this summer has been shaping up and what the latest long-range forecasts are calling for.