Welcome to the Cloud Index — a regular feature on Looking Up where we profile a type of cloud, explaining how they form, what they’re made of and how to use them to forecast the weather. You can find the full list of clouds we’ve covered in the Cloud Index index.

We’ve covered cumulus and stratus, so now it’s time to cover a third main type of cloud — cirrus — before we can start getting more specific.

‘Cirrus’ is a latin word for a curly lock of hair that refers to the characteristic shape of cirrus clouds, twisting gracefully high in the atmosphere. They’re white or very light grey in colour, and under the right conditions can stretch across entire continents.

There are many different kinds of cirrus, including cirrocumulus and cirrostratus, but all have one thing in common — their altitude. Cirrus are some of the highest clouds we can see, usually above five kilometres in temperate climates, lower at the poles (3km) and higher at the equator (6km).

At this height there is no liquid water — it freezes. That means that cirrus clouds are comprised entirely of ice crystals, which is what gives them their wispy appearance. Despite extending huge distances horizontally, they tend to be at most just a few kilometres deep.

How they form

There are several different ways in which cirrus clouds can form. The most common ‘natural’ way is when water vapour is pushed very high in the sky before it cools and condenses, which often happens at the leading edge of a warm front. Humidity is very low at these altitudes, so these types of cirrus clouds tend to be thin and insubstantial.

The other natural way that cirrus forms is in thunderstorms. The extreme convection during a storm can push water vapour very high in the sky, resulting in a wispy top of a thundercloud that spreads out several kilometres downwind, known as an “anvil”. These get enormous when they’re associated with the thunderstorms in a hurricane, typhoon or cyclone.

Finally, mankind creates thousands of cirrus clouds every day through aviation. The exhaust of a jet engine contains a lot of water vapour, and when this is ejected from the plane it condenses immediately around natural particles in the air, leaving a contrail. If the conditions are right, then these spread out and form large mats of cirrus clouds.

What they mean for the weather

As mentioned above, one of the ways that cirrus clouds form is often associated with the leading edge of a warm front. Unfortunately, frontal systems usually mean rain, so cirrus is often a bad sign for a barbecue.

In the tropics, meteorologists in the late 19th century used cirrus clouds to predict hurricanes. A veil of cirrus passes by about 36 hours before the eye of a hurricane will, so it’s a good indicator to batten down the hatches.

Cirrus clouds themselves don’t rain. You’ll sometimes see ice crystals dropping out of them, but they’re so high up that these “fall streaks” almost never reach the surface, evaporating before they get there.

Anything else?

Cirrus, unlike lower clouds, actually has a warming effect on the Earth’s climate because they’re bad at reflecting sunlight but very good at absorbing radiation heading out to space from the Earth. When combined with the huge increase in cirrus caused by aviation, that’s bad news when it comes to climate change.

Plus, thanks to the optics of ice crystals, cirrus clouds — and particularly cirrostratus clouds — often come with spectacular atmospheric phenomenon like haloes, glory and rainbow arcs.

Finally, cirrus has been observed on other planets. Mars, Jupiter, Saturn, Neptune and Uranus spacecraft have all recorded cirrus clouds — however it’s thought that they’re made of different things. On Jupiter, the clouds are comprised of ammonia, whereas on Neptune and Uranus, they’re more likely to be methane.

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