|Weather Reference - Clouds - Cirrus
Cirrus Clouds | Cirrus Uncinus | Cirrostratus | Cirrus Kelvin-Helmholtz | Cirrocumulus | Disclaimer
Cirrus clouds are wispy, thread like cloud, often drawn out across the sky by high winds.
These formations indicate the presence of moisture at high levels of the atmosphere. At these levels the temperature is normally below freezing, and any air mass that cools to saturation will produce ice rather then water, therefore these clouds consist of ice crystals which are blown about by upper levels winds, producing the characteristic white streaks.
Cirrus clouds may form in isolated patches or cover a wide area of sky, depending on the distribution of moisture.
Cirrus may be the result of local thunderstorm activity.
Cirrus Uncinus are the same as normal cirrus clouds but with a distinctive hooked tip.
They are formed in the same way as cirrus clouds however its distinctive pattern of filaments is the result of a high speed wind below the level at which the crystals form. As the crystals descend under the influence of gravity, this wind rapidly smears them across the sky forming the distinctive hooked shape.
Like other cirrus clouds they are a result of high level moisture and it is therefore associated with the approach of a frontal system.
They do not produce any significant weather on the ground although snow showers may be visible immediately above cloud level. These usually evaporate well before reaching the ground.
These are even layers of cirrus that covers a wide area of the sky and as with other cirrus formations they are formed when a moist air mass is lifted to a level where it cools to saturation and forms ice crystals. In the case of Cirrostratus this lifting occurs on a large scale. Meteorologists distinguish many types of Cirrostratus:
Fibratus consists of long thin filaments known as striations that spread evenly across the sky. The texture of this formation results from the ice crystals being blown by strong, steady, high level winds.
Nebulosis – the uplift that gives rise to the cloud is very gentle, and the resulting ice crystal layer is extremely thin, with vague edges that are difficult to discern and lack the texture common to other cirrus clouds. Quite often the only sign of a cloud formation will be a slight diminution of the intensity of the sunlight.
Occasionally, snow showers fall from cirrostratus formations, but these usually evaporate before reaching the ground.
Cirrus Kelvin-Helmholtz appear as a slender, horizontal spiral of cloud and are the most distinctive of the sort. However they tend to dissipate after only a few minutes after forming, so therefore are rarely observed.
The shape of this kind of cirrus is a result of a particular type of wind shear. In general, wind shear occurs when one layer of air slides across another layer moving at a different speed or in a different direction. This gives rise to vertical eddies that produce a regular pattern of air waves.
In most cases wind shear creates a series of gently undulating cloud formations along the top of the waves. In the case of Cirrus Kelvin-Helmholtz the eddies are more powerful, and carry the cloud over the peak and down the other side, so that the waves break in the manner of ocean waves. As these waves complete a circulation, they create a distinctive corkscrew pattern.
These clouds are named after a German physicist who first described this form of instability in liquids in the late nineteenth century.
Cirrus Kelvin-Helmholtz are quite common in the upper troposphere, but generally there is insufficient moisture present to generate cloud and render the visible pattern.
The presence of this cloud indicates a degree of wind shear that is likely to produce moderate to severe turbulence at cloud level.
Cirrocumuluses are seen as a milky veil of high level cloud that may stretch for hundreds of miles across the sky. They occur when a large area of moist air at a high level of the atmosphere reaches saturation and forms ice crystals. The difference between cirrocumulus and cirrocumulus is the presence of instability at cloud level. This gives the cloud its cumuliform appearance.
By itself this formation does not have ant great significance however if there is a steady increase in this cloud over a period of time, it may indicate the approach of a frontal system.
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