What weather conditions are associated with clouds?

By keeping an eye on the skies above you, and knowing a little about how clouds form, walkers can forecast the weather and predict whether rain is on the way, perfect for keeping dry when out on a guided walking holiday or setting out alone on a self-guided walk. See what the characteristics of each cloud are below, get to know your Cirrus from your Cumulus and discover what weather is associated with them too, to know what you're in for when out.

The classification of clouds into types was first proposed by Luke Howard in 1802 and we largely use the same system today. Most of our names for clouds come from Latin and are usually a combination of the following prefixes and suffixes:

Stratus/strato = flat/layered and smooth
Cumulus/cumulo = heaped up/puffy, like cauliflower
Cirrus/cirro = high up/wispy
Alto = medium level
Nimbus/Nimbo = rain-bearing cloud

Where these names are combined, we can often build up an idea of that cloud's character. For example, if we combine nimbus and stratus we get 'nimbostratus' - a cloud which is flat and layered and has the potential for rain.

How do clouds form?

Clouds form when air cools to the dew point, the temperature at which the air can no longer hold all its water vapour. At this temperature, water vapour condenses to form droplets of liquid water, which we observe as a cloud. For this process to happen, we require air to be forced to rise in the atmosphere, or for moist air to come into contact with a cold surface.

Types of clouds and weather conditions associated with them

Image

High clouds (base above 20,000ft)

Cirrus
Cirrus Cloud

Cirrus form very high in the atmosphere. They are wispy, being composed entirely of ice crystals falling through the atmosphere – if carried horizontally by winds they take on a characteristic hooked shape. Cirrus clouds often indicate a change in the weather is coming and form in advance of a warm front, where the air masses meet at high levels. If you notice that cirrus begins to cover more of the sky, and gets lower and thicker, this is a good indication that the front is drawing near. Although they release precipitation, the drops re-evaporate before reaching the ground.

Cirrocumulus
Cirrocumulus Cloud

Cirrocumulus is a fairly rare cloud formation, forming ripples that may resemble honeycomb. Cirrocumulus clouds are usually associated with fair weather, as precipitation from them never reaches the ground, although their appearance can often be before stormy weather, meaning you should make the most of the sun while you still can.

Cirrostratus
Cirrostratus Cloud

The thin, layered cirrostratus cloud is composed of ice crystals. Cirrostratus form a veil or thin sheet of cloud that covers all or part of the sky. Although they don’t produce precipitation, they usually indicate a warm front is approaching and a change of weather is expected soon, most likely with rain.

Medium clouds (base 6,500ft–20,000ft)

Altocumulus
Altocumulus Cloud

Altocumulus clouds normally appear white or grey with shading. They are usually seen in settled weather. They are usually composed of droplets, but may also contain ice crystals. Precipitation is rare, but if it does fall, it never reaches the ground, re-evaporating before it reaches the surface.

Altostratus
Altostratus Cloud

Altostratus evolves as a thin layer from a gradually thickening veil of cirrostratus. They appear as almost featureless sheets of cloud and are usually grey or blue. Their appearance precedes a warm front and usually indicates a change in the weather is close. As the front passes, the altostratus deepens and bulks out to become nimbostratus.

Nimbostratus
Nimbostratus Cloud

These dark, grey, featureless clouds are usually accompanied by moderate rain or snow, lasting several hours. Nimbostratus clouds are dark, grey, featureless layers of cloud. They are thick enough to block out the sun and often produce persistent rain or snow that will last until the front has passed. If there is hail, thunder or lightning present as well as rain, it is a cumulonimbus cloud rather than nimbostratus.

Low clouds (base below 6,500ft)

Stratocumulus
Stratocumulus Clouds

Stratocumulus cloud consists of large, rounded, clumpy masses of cloud that form groups, lines or waves. They can be seen in all weather conditions from dry settled to wetter weather. Colours range from white to dark grey. Although sometimes seen with rainy conditions, they themselves usually only produce light drizzle, if anything in the way of rain.

Stratus
Stratus Clouds

Stratus clouds tend to be featureless, low altitude clouds that cover the sky in a continuous blanket of white or grey. Stratus cloud is thin, so while conditions may feel gloomy, rain is unlikely, and at most, even when at its thickest, will be a light drizzle. If it is cold enough this can manifest itself as light snow. In fact, Stratus is identical to fog, so if you’ve ever been walking in the mountains on a foggy day, you’ve been walking in the clouds.

Cumulus
Cumulus Cloud

These fluffy, cauliflower-shaped, cotton wool clouds are one of the most common and distinctive types of cloud. They form on bright sunny days as a result of convection and usually indicate good weather. Cumulus clouds do not generally rain – you’re in for fine weather but if conditions allow, cumulus can grow into towering cumulus congestus or cumulonimbus clouds, which can produce showers.

Cumulonimbus
Cumulonimbus Cloud

Cumulonimbus clouds exist through the entire height of the troposphere. With their distinctive, flat-topped anvil-shape, these mighty clouds are linked with extreme weather. While small cumulus do not rain, if you notice cumulus getting larger and extending higher into the atmosphere, it’s a sign that intense rain is on the way. This is common in the summer, with morning cumulus developing into deep cumulonimbus (thunderstorm) clouds in the afternoon. Look out too for hail and thunderstorms, lightning and even tornadoes.

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