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If you spot one of these, you better take shelter!

The 12 types of clouds you meet in the spring and summer


Lori Knowles

Tuesday, May 5, 2015, 11:53 AM - Have you ever looked up at the clouds in the summer sky and wondered what exactly you were seeing? If so, here's a primer on the 12 types of clouds you're most likely to see in the spring and summer months -- and what they mean for the weather.

First, let's go over some basics. In meteorology, a cloud is a visible mass of liquid droplets or frozen crystals of water and various chemicals suspended in the atmosphere. Clouds form in the troposphere, which is the layer of the atmosphere that is closest to Earth's surface. In the troposphere, the air contains with tiny gas particles called water vapour. There are also tiny particles of dust and salt called aerosols that constantly collide with the water vapour. When the air is cooled, the water vapour collects on the aerosols -- a process called condensation. Eventually, water droplets form around the the aerosols and stick to other droplets, forming clouds. 

There are many ways air can become cooled to form clouds; this is why there is such an incredible amount of variety in the types of clouds we see in the sky and the types of weather they can produce. 

Clouds can be sorted into five basic categories: cirriform, stratiform, stratocumuliform, cumuliform, cumulonimbiform. That's quite the mouthful of Latin to remember, so here's the the least you need to know:

- The prefixes alto and cirro denote the height at which each cloud type forms 

- Cumulus clouds are puffy while stratus clouds are flat and layered

- Nimbus clouds produce precipitation

Now, let's meet the clouds!


1. Cirrus clouds

These wispy, ethereal clouds form at the highest levels of the troposphere and are made mostly of ice crystals. Their classic trailing shape has earned them the nickname "mares' tails."

2. Cirrostratus clouds

These very high, icy clouds can be difficult to see because they are so thin and widely spread. When visible light interacts with the ice crystals in cirrostratus clouds, it can create a number of optical phenoma such as sun dogs and halos, as in the photo above.

3. Altostratus clouds

Along with their thicker, wetter cousins the stratus and nimbostratus clouds, altostratus clouds can be blamed for dull grey days. They form a flat, uniform sheet across the sky and are just thin enough to let some sunlight leak through. If they thicken enough to produce precipitation, they become nimbostratus clouds.

4. Stratus clouds

Stratus clouds are the low, flat, fog-like, featureless clouds no one likes to see on a summer day. They block out most sunlight, keep temperatures cool and often produce drizzle or light rain.

5. Nimbostratus clouds

These dreary clouds are flat, opaque and featureless. You are most likely to "see" nimbostratus clouds on days of steady, moderate rainfall.

6. Cirrocumulus clouds

These high-altitude "popcorn" cloudlets can be a harbinger of unsettled weather. If they occur with cirrus and cirrostratus clouds spreading broadly across the sky, they may mean rain is on the way within 8-12 hours. If they occur in small patches with only a few wisps of cirrus, they can signify that conditions are ripe for thunderstorms.

7. Altocumulus clouds

Often called "mackerel sky" because of their resemblance to fish scales, these puffy clouds herald a change in the weather. Altocumulus castellanus (towering altocumulus) indicate instability in the middle layers of the troposphere and are a good sign that thunderstorms could develop later in the day. For this reason, they are one of three types of warning clouds recorded by the aviation industry, with the other types being towering cumulus and cumulonimbus clouds.


There are several subtypes of altocumulus cloud. One of these is the lenticular cloud, which resembles a flying saucer and typically forms in the lee of mountains:

There's also the rare, mysterious undulatus asperatus, which was proposed as an entirely new cloud type by the Cloud Appreciation Society in 2009.

8. Cumulus clouds

If the sky looks like the opening sequence of "The Simpsons," you're looking at classic cumulus cloud formations. Normally, cumulus clouds by themselves don't produce precipitation, but given enough instability and moisture, they can develop into the juggernaut of the atmosphere: the mighty cumulonimbus cloud.

9. Cumulonimbus clouds

Towering into the highest reaches of the troposphere, these imposing storm clouds are unmistakeable. They form when surface heating and atmospheric instability send water vapour rocketing upward and they can quickly become capable of producing heavy downpours, damaging winds, hail, and tornadoes. As the tops of cumulonimbus clouds reach the tropopause (the boundary between the troposphere and the stratosphere), intense atmospheric winds spread the cloud top for many miles, creating the classic anvil shape of summer thunderstorms:

A mature thunderstorm with a strong, rotating updraft is a supercell and may have a structure resembling a stack of plates:

Supercells can last for hours and typically produce the most dangerous weather, including large hail, frequent lightning and strong, long-track tornadoes. Supercells are most common in the Great Plains of the United States ("Tornado Alley") during the spring and attract storm chasers from all over the world to witness their awesome power.

10. Arcus/roll cloud

When it comes to summer storms, the cumulonimbus cloud brings the power, but the arcus (shelf) cloud brings the drama.

Shelf clouds form when cool air from a storm's downdraft cuts under warm air and pushes it upward, creating a cloud that rolls ahead of the storm. Many people mistake shelf clouds for wall clouds (which can produce tornadoes), but this is not the case as shelf clouds appear at the front of an approaching storm, while a wall cloud will form at the area where the storm's updraft is strongest.

Not all arcus clouds are associated with thunderstorms; the roll cloud is the shelf cloud's rarer, gentler cousin, often forming along outflows of cold air from sea breezes. This image shows an arcus cloud not associated with a thunderstorm over Lake Ontario:

11. Wall cloud

The formation of a wall cloud is the best indication that a storm is gearing up to produce a tornado. A wall cloud appears as a large, often rapid lowering of cloud at the rear of a supercell, near the main updraft. It may or may not be rotating, but if you see one, you should immediately seek shelter in a safe location.

12. Fallstreak hole

Some cloud formations defy classification but are important to mention, such as the elusive fallstreak hole or "hole-punch cloud."

Have you ever left a water bottle in your freezer overnight only to find it miraculously still unfrozen in the morning? That's because the water has been supercooled, meaning its temperature is below freezing but it remains in a liquid state because there was nothing in the water to trigger the formation of ice crystals. All it takes is the introduction of a single ice crystal to set off a chain reaction that turns your liquid water to solid ice. The exact same process occurs with fallstreak holes, but on a much larger scale. When something disturbs the supercooled water vapour in alto- or cirrocumulus clouds -- such as a plane passing through -- the sudden introduction of ice crystals creates a domino effect. As the crystals grow, they fall through the center of the hole, creating wispy trails of ice.

Now that you know the basics of cloudspotting, it's time to show off. Upload your best cloud photos to our website here; you could see them on TV or in a future Weather Network article!

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