Clouds are a familiar part of the UK’s weather, but their shapes, heights and structures can reveal a great deal about what is happening in the atmosphere.
The World Meteorological Organization classifies clouds into 10 basic genera, which can broadly be divided into low-, medium- and high-level clouds. Some remain confined to one layer of the atmosphere, while cumulonimbus clouds can extend from near the surface to the top of the troposphere.
This week’s Deep Dive focuses on two particularly striking features: lenticular clouds, which form in stable air around hills and mountains, and mammatus clouds, which are most commonly associated with thunderstorms and an unstable atmosphere.
Ten basic cloud types
Low-level clouds include stratus, stratocumulus and cumulus. Stratus often produces gloomy, overcast and sometimes drizzly conditions, while stratocumulus is one of the most common cloud types seen across the UK. Cumulus clouds are recognised by their familiar, cotton-wool appearance, although they can grow considerably when the atmosphere becomes unstable.
At medium levels are altostratus and altocumulus, along with nimbostratus, a cloud associated with widespread rain. High-level clouds include cirrus, cirrocumulus and cirrostratus, which form close to the top of the troposphere. Cumulonimbus completes the list, developing vertically through several atmospheric layers and potentially producing heavy rain, hail and thunderstorms.
READ MORE: Why has it been so humid recently?
Beyond these genera are different species, varieties and supplementary features. Lenticular clouds are commonly associated with altocumulus, while mammatus describes distinctive pouch-like formations that usually appear beneath a cumulonimbus cloud.
How lenticular clouds form
Altocumulus lenticularis clouds are smooth, lens-shaped clouds that can sometimes resemble flying saucers. They are particularly associated with hilly or mountainous areas, including Cumbria, Wales, Scotland and locations to the east of the Pennines.
Their formation requires stable, moist air and wind blowing across elevated terrain. As the airflow encounters a mountain, it is forced to rise. Because the atmosphere is stable, the air cannot continue rising indefinitely. Instead, it sinks again beyond the summit, creating an atmospheric wave downstream of the high ground.
As air rises within the wave, it cools. If enough moisture is present, water vapour condenses into droplets and a cloud forms. As that air descends and warms, the droplets evaporate. Further clouds can develop at subsequent peaks in the wave, producing a series of apparently stationary cloud formations.
This explains one of the most distinctive characteristics of lenticular clouds. Although strong winds may be flowing through them, the clouds can remain in much the same position because condensation is continually occurring at the wave’s crest and evaporation at its downstream edge. Several alternating bands of cloud and clear sky may sometimes be visible.
Mountain waves can also produce strong and gusty winds on the lee side of high ground, even if there is insufficient moisture for lenticular clouds to become visible.
Mammatus and an unstable atmosphere
Mammatus clouds form in very different atmospheric conditions. Rather than stable air passing across mountains, they are usually associated with unstable air and cumulonimbus clouds.
Atmospheric instability often develops when relatively warm air lies beneath much colder air. The warmer air rises and may initially produce small cumulus clouds. If the atmosphere is sufficiently unstable, these clouds can grow into towering cumulus and eventually cumulonimbus, extending towards the top of the troposphere.
READ MORE: Do changing seasons affect the jet stream?
When the rising cloud reaches the top of the troposphere, it encounters the more stable air above and can no longer rise freely. The cloud spreads out horizontally, creating the familiar anvil shape of a mature cumulonimbus cloud.
Mammatus appear as rounded pouches hanging from the underside of this anvil. The leading explanation involves cold, dense air beginning to descend from the cloud into drier air below. Energy is used as ice crystals melt and water droplets evaporate, helping the descending air remain colder than its surroundings. It can consequently sink below the main cloud base, forming the characteristic bulbous shapes. However, the precise science of mammatus formation is not yet considered completely settled.
Trough, ridge, trough, ridge
Clouds will remain a frequent feature of the UK’s weather this week as Atlantic systems move across the country. The broad pattern is being driven by a series of troughs and ridges within the jet stream, the fast-flowing ribbon of air near the top of the troposphere.
Troughs are generally associated with rising air, cloud, showers and rain. Ridges encourage sinking air and tend to bring drier and calmer interludes. With an active jet stream moving these features eastwards, the UK will alternate between unsettled spells and short-lived improvements.
A ridge brings an emphasis on drier weather and brighter spells on Wednesday, although a few showers remain possible. Another trough arrives on Wednesday night with a spell of rain, followed by a brief ridge on Thursday and another weather system during Friday.
Temperatures returning closer to average
Although the weather will change from day to day, temperatures are expected to remain relatively steady. Daytime values will generally be in the high teens, perhaps reaching 20°C, which is close to average for the time of year.
Nights will also feel fresher and less humid. Rural locations could fall into single figures, while many towns and cities remain around 10 to 12°C. There are tentative indications that warmer air could return to parts of southern Britain early next week if the jet stream moves farther north and high pressure builds across the south.
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