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Weather Reference - Clouds - Alto
Location: London (Default) Lat: 51.5N Sunrise: 06:03BST
  Lon: 0E Sunset: 19:59BST

Altostratus | Altostratus Undulatus | Altocumulus Mackerel Sky | Altocumulus Castellanus | Altocumulus Lenticularis | Disclaimer


These are featureless, white or bluish grey cloud layers and are found if the middle levels of the atmosphere and is always a sign of the presence of significant amounts of moisture in those layers.

They are the result of lifting and condensation of a large air mass, usually by an incoming frontal system and can extend over thousands of miles and can produce rain or snow over a wide area.

Altostratus Undulatus

This cloud has an undulating appearance due to the wave motion in the air mass. This motion is normally a result of the wind shear, which occurs when one layer of air slides over another layer moving at different speeds or in a different direction (or both). This creates waves of air between the layers, and if sufficient moisture is present, cloud will form where the wave rises and dissipates where it falls.

Dependant on the moisture content of the air mass and the degree of the wind shear, undulations may occur as fairly continuous waves across the sky with thin cloud connections at the base.

These clouds do not produce any significant weather and as sometimes regarded as a sign of local turbulence.

Altocumulus Mackerel Sky

These are small heaps of pale, white or bluish cloud less than 1 across without shading that look not dissimilar to the scales of a fish.

These clouds are produced by the lifting of a large, moist air mass, usually by an approaching cold front, combined with instability at cloud level. The exact cause of the fish scale pattern is not known but it is likely that a form of wind shear, similar to that which produces the much finer texture of the sky.

In folklore mackerel sky is seen to represent deteriorating weather.

Altocumulus Castellanus

Altocumulus Castellanus are turrets of cloud, rising from a common base, sometimes in lines. This formation indicates instability in the middle layers of the atmosphere and may indicate thunderstorms developing later in the day. They generally occur when a layer of colder air slides across an area of altocumulus cloud. This creates instability, and localized bubbles of air start to rise from the cloud deck. Condensation in these pockets of air create the Castellanus effect. Any further convection from the ground will be enhanced by this middle level instability.

Meteorologists are always alert to this type of cloud formation as it generally indicates thunderstorms later in the afternoon following further ground warming.

Altocumulus Lenticularis

Altocumulus Lenticularis are smooth, lens or almond shaped clouds that remain stationary in the sky and are found in the middle level.

They occur where a layer of stable air is forced upwards, usually by a hill or mountainside. Because the air is stable it descends on the other side and may sometimes set up a whole series of waves that stretch downwind and usually increase in size with distance. The cloud lies at the wave crests, where the air is carried up and above condensation level. The clouds may persist for hours if the wind and conditions remain stable.

There are generally no weather conditions associated with these clouds however if there is sufficient moisture in the surrounding atmosphere, these clouds can become thick enough to generate light rain, or snow in sub zero temperatures.


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