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Weather — it’s all in the cloud(s)

A look at the various cloud types and how they got their names

I’m currently off exploring the mountains of southeastern B.C. and southwestern Alberta and due to my travels, will have go with a more generic weather article this issue. I decided to take a look at something that ties into pretty much all of our weather — clouds.

If we were to define a cloud, we could say that it is a visible collection or aggregate of tiny water droplets or ice crystals floating or suspended in the air. Stealing a line from Meteorology Today, “Some clouds are found only at high elevations, while others nearly touch the ground. Clouds can be thick or thin, big or little — they exist in a seemingly endless variety.”

Identifying all of the different types of clouds can be a little tough, but with some practice, you can become reasonably proficient.

Our current method of naming and classifying clouds was developed in 1803 by Luke Howard, an English naturalist. His system employed the use of 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 violent rain.

So in essence, Howard’s system had four different cloud types. These 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 Hildebrandsson expanded Howard’s system by creating four different groups that clouds can be divided into. These would be high clouds, middle clouds, low clouds and a fourth group for clouds that span more than one region — vertically developed clouds.

This is the system that we use today. Clouds are classified by the height at which they occur and then by their appearance. The table below shows the four major cloud groups and their types.

four major cloud types

This addition to the naming and classification of clouds does at first seem fairly simple and straightforward, but there are a couple of things that make it a little confusing.

First of all, 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.

Cloud formation

Before we go onto examine each type of cloud, we should first take a look at the bigger picture — cloud formation.

For clouds to form we need to cool the air so that condensation will occur. Condensation is the process by which the tiny molecules of water in the air come together to form larger droplets or ice crystals. Without going into topics such as vapour pressure, mixing ratios, and absolute and specific humidity, condensation occurs because, as the air cools, the ratio of evaporation to condensation tilts in favour of condensation.

Evaporation and condensation of water vapour are continuous and ongoing processes in the atmosphere. Usually, more evaporation is taking place than condensation, so, as the microscopic water droplets or ice crystals collide and grow, they quickly evaporate back to their original microscopic size. When we cool a parcel of air enough, the amount of evaporation that takes place decreases because evaporation takes energy that comes from heat.

As the microscopic water droplets or ice crystals collide and grow, they do not evaporate as quickly. As more and more collisions occur they eventually grow big enough to become visible — they have grown large enough to be called a cloud droplet.

Two processes

There are two main processes that allow for clouds, rain, and snow to form — the “collision-coalescence process” and the “Bergeron process.” The first is fairly self-explanatory. The microscopic water droplets collide and combine (coalesce). With each collision the droplets get larger, eventually becoming visible and possibly even becoming a raindrop.

The Bergeron process happens with ice crystals. They grow because they absorb water vapour more readily than water droplets. That is, in a cold cloud, water droplets can be supercooled below the freezing point and still remain liquid. If an ice crystal is near one of these supercooled water droplets the ice crystal will absorb the water from the water droplet by causing the water droplet to evaporate while the ice crystal uses this newly evaporated water to grow.

Daniel Bezte is the weather columnist for the Manitoba Co-operator. His article appeared in the July 14, 2016 issue.

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