Cumulus clouds

The fluffy, cauliflower-shaped cumulus is one of the most common and distinctive types of cloud. All cumulus clouds develop as a result of convection.

• Height of base: 1,200 - 6,500 ft
• Shape: cauliflower of fluffy
• Latin: cumulus - heap
• Precipitation: occasional rain or snow showers

What are cumulus clouds?

Cumulus clouds are detached, individual, cauliflower-shaped clouds usually spotted in fair weather conditions. The tops of these clouds are mostly brilliant white tufts when lit by the Sun, although their base is usually relatively dark.

How do cumulus clouds form?

All cumulus clouds develop because of convection. As air heated at the surface is lifted, it cools and water vapour condenses to produce the cloud. Throughout the day, if conditions allow, these can grow in height and size and can eventually form into cumulonimbus clouds.

Along coastlines, cumulus may form over land during daylight hours as a sea breeze brings in moist air, which is then warmed by the surface. This effect reverses overnight as the sea becomes warmer than the land and cumulus form over the sea.

What weather is associated with cumulus clouds?

Mostly, cumulus indicates fair weather, often popping up on bright sunny days. Though if conditions allow, cumulus can grow into towering cumulus congestus or cumulonimbus clouds, which can produce showers.

A cloud is a mass of water drops or ice crystals suspended in the atmosphere. Clouds form when water condenses in the sky. The condensation lets us see the water vapor. There are many different types of clouds.13 

The range of ways in which clouds can be formed and the variable nature of the atmosphere results in an enormous variety of shapes, sizes and textures of clouds.

Many people believe that clouds are just made of water vapour (a gas). However, this is not strictly true. Water vapour is invisible, and it is around us all the time in the air. Sometimes there is more water vapour in the air and it feels humid or muggy. Other times, the air has less water vapour and it feels drier and fresher.

Clouds appear when there is too much water vapour for the air to hold. The water vapour (gas) then condenses to form tiny water droplets (liquid), and it is the water that makes the cloud visible. These droplets are so small that they stay suspended in the air.

What causes the air to rise?

1. The sun – The sun heats the ground, which then heats the air just above it, causing it to rise upwards in the sky (warm air rises). This tends to produce cumulus clouds.

2. Hills and mountains - When air is travelling towards a mountain or hill, it cannot go into the hill and so it rises upwards along the terrain. Stratus clouds are often produced this way.

3. Weather fronts - A weather 'front' is where warm air meets cold air. The warm air rises up and over the cold air (warm air rises). This produces nimbostratus clouds, amongst others.

4. Convergence - Streams of air flowing towards each other from different directions are forced to rise when they meet, or converge. This can cause cumulus cloud and showery conditions.

5. Turbulence - A sudden change in wind speed high up can create circulations in the air which can bring the air at the surface high up into the sky.

Nacreous clouds

Nacreous clouds form in the lower stratosphere over polar regions when the Sun is just below the horizon. The clouds are illuminated from below and often glow in vivid colours.

Height of base: 68,500 - 100,000 ft

Shape: large thin discs, often reflecting vivid colours

Latin: no direct Latin translation, but the old English word “Nacre” means “mother of Pearl”

Precipitation: none

Nacreous clouds

Nacreous clouds form in the lower stratosphere over polar regions when the Sun is just below the horizon. The clouds are illuminated from below and often glow in vivid colours.

Height of base: 68,500 - 100,000 ft

Shape: large thin discs, often reflecting vivid colours

Latin: no direct Latin translation, but the old English word “Nacre” means “mother of Pearl”

Precipitation: none

What are Nacreous clouds?

Nacreous clouds are rare and very high clouds, known mainly for the coloured light they reflect after sunset and before sunrise. The colours are reminiscent of the colours which reflect from a thin layer of oil on top of water, an effect known as iridescence.

How do Nacreous clouds form?

Nacreous clouds form in the lower stratosphere over polar regions when the Sun is just below the horizon. The ice particles that form nacreous clouds are much smaller than those that form more common clouds. These smaller particles scatter light in a different way, which is what creates the distinctive luminescent appearance.

Due to their high altitude and the curvature of the Earth’s surface, these clouds are lit up by sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn and after dusk. They are most likely to be viewed when the Sun is between 1º and 6º below the horizon and in places with higher latitudes, such as Scandinavia and northern Canada. For this reason, they are sometimes known as polar stratospheric clouds. Nacreous clouds only form below -78 °C so are most likely to occur during the polar winter.

Because of the very low temperatures required, nacreous clouds are usually only visible from the UK when the cold air which circulates around polar regions in the stratosphere (known as the stratospheric polar vortex) is displaced and hovers temporarily over the UK.

What weather is associated with Nacreous clouds?

Because they form in a polar vortex, they are associated mostly with very cold and dry weather.

Cirrostratus clouds

The thin, layered cirrostratus cloud is composed of ice crystals and forms a veil that covers all or part of the sky.

• Height of base: 20,000 - 40,000 ft
• Shape: Layered
• Latin: cirrus - lock or tuft of hair; stratus - flattened or spread out
• Precipitation: None

Cirrostratus cloudsThe thin, layered cirrostratus cloud is composed of ice crystals and forms a veil that covers all or part of the sky.Height of base: 20,000 - 40,000 ftShape: LayeredLatin: cirrus - lock or tuft of hair; stratus - flattened or spread outPrecipitation: NoneWhat are cirrostratus clouds?Cirrostratus are transparent high clouds, which cover large areas of the sky. They sometimes produce white or coloured rings, spots or arcs of light around the Sun or Moon, that are known as halo phenomena. Sometimes they are so thin that the halo is the only indication that a cirrostratus cloud is in the sky.Cirrostratus clouds can span thousands of miles, can be smooth or fibrous and are often fringed with cirrus clouds. Shadows will normally still be cast by the Sun when shining through cirrostratus clouds, which can help distinguish them from similar altostratus clouds.How do cirrostratus clouds form?As a result of slowly rising air, cirrostratus cloud can form. Usually generated at the forefront of frontal weather systems, the movements of cirrostratus can be used to predict what the weather will do in the next 24 hours.Cirrostratus clouds can also form through contrails, the vapour trails left by planes as they fly through a dry upper troposphere. These streaks can spread out and become cirrus, cirrostratus and cirrocumulus.What weather is associated with cirrostratus clouds?Though cirrostratus itself does not produce precipitation, it can indicate whether or not precipitation is likely. If cirrostratus nebulosus exists in the sky it is likely that an incoming warm front will bring persistent rain within a day. If cirrostratus fibratus is spotted, stratus may proceed it, bringing only light drizzle.How do we categorise cirrostratus clouds?There are two main 'species' of cirrostratus:Cirrostratus fibratus - similar to cirrus, but with more consistency. Wispy strands all tightly knitted together, making for more of an animal fur lookCirrostratus nebulosus - a uniform veil-like layer covering the sky. Sometimes almost unnoticeable, cirrostratus nebulosus is featureless, composed entirely of ice

Stratus cloudsStratus clouds tend to be featureless, low altitude clouds that cover the sky in a blanket of white or grey.Height of base: 0 - 1,200 ftShape: layeredLatin: stratus - flattened or spread outPrecipitation: lightWhat are stratus clouds?Stratus clouds are low-level layers with a fairly uniform grey or white colour. Often the scene of dull, overcast days in its 'nebulosus' form, they can persist for long periods of time. They are the lowest-lying cloud type and sometimes appear at the surface in the form of mist or fog.How do stratus clouds form?Stratus clouds form in calm, stable conditions when gentle breezes raise cool, moist air over colder land or ocean surfaces. These clouds can exist in a variety of thicknesses and are sometimes opaque enough to darken days, allowing for little light to pass through.What weather is associated with stratus clouds?Stratus is usually accompanied by little to no rainfall but if it is thick enough, it can produce light drizzle. This drizzle can also fall in the form of light snow if cold enough.How do we categorise stratus clouds?Due to the nature of stratus it only has two defined species:Stratus nebulosus - a featureless, dark layer which is capable of producing drizzle.Stratus fractus - a stratus layer which is starting to break up or 'dissipate,' leading to breaks in the cloud.

Altostratus clouds

Altostratus evolves as a thin layer from a gradually thickening veil of cirrostratus and is usually grey or blue, with very few features.

• Height of base: 6,500 - 20,000 ft
• Shape: Layered and featureless
• Latin: altum - height; stratus - flattened or spread out
• Precipitation: None

What are altostratus clouds?

Altostratus are large mid-level sheets of thin cloud. Usually composed of a mixture of water droplets and ice crystals, they are thin enough in parts to allow you to see the Sun weakly through the cloud. They are often spread over a very large area and are typically featureless.

How do altostratus clouds form?

Altostratus layers are often composed of both water and ice and usually form when a layer of cirrostratus descends from a higher level. The Sun often cannot cast shadows when shining through altostratus clouds. These layers can sometimes contribute to the formation of optical effects such as coronas and iridescence.

What weather is associated with altostratus clouds?

Altostratus clouds often form ahead of a warm or occluded front. As the front passes, the altostratus layer deepens and bulks out to become nimbostratus, which produces rain or snow. As a result, sighting it can usually indicate a change in the weather is on the way.

How do we categorise altostratus clouds?

Altostratus clouds are featureless with little character, so are not classified into 'species' like other cloud types. They are similar to nimbostratus in this way. They do have many pattern-based varieties though, such as undulatus, radiatus and duplicatus, and thickness based varieties; translucidus and opacus.