The Meath Weather Cam

Cirrus

Cirrus clouds are characterized by thin, wisplike strands, often accompanied by tufts, leading to their common (non-standard) name of 'mare's tail'. Sometimes these brownish clouds are so extensive that they are virtually indistinguishable from one another, forming a veil or sheet called "cirrostratus". Sometimes convection at high altitudes produces another form of cirrus called "cirrocumulus", a pattern of small cloud tufts which include droplets of supercooled water.

Many cirrus clouds produce hair like filaments made of the heavier [ice] crystals that precipitate from them. These "fall streaks", a form of virga, often indicate the difference in the motion of air (wind shear) between the upper part of the cirrus cloud and the air below it. Sometimes the top of the cirrus cloud is moving rapidly above a slower layer of air, or the streak is falling into a faster moving lower layer. The directions of these winds can also vary.

Cirrus usually form at altitudes above 8000 meters (26,000 ft). At this altitude, aircraft leave condensation trails that can turn into cirrus clouds. This happens when hot exhaust, mostly water, freezes leaving a visible trail. Streaks may appear straight when wind shear is absent, giving the clouds the appearance of a comma (cirrus uncinus), or tangle, an indication of high-level turbulence. The falling ice crystals evaporate before reaching the ground.

Cirrus clouds trap and reflect infrared radiation (heat) beneath them (greenhouse effect), but also reflect sunlight to some extent (albedo). It has not yet been determined whether the net effect of cirrus clouds is to warm or cool the earth. Much of the difficulty lies in modeling the albedo effect of clouds composed of various size and shape crystals. Older models tend to underestimate the albedo effect of cirrus. Refinements of these models will improve climate predictions.

If there are many cirrus clouds in the sky, this may be a sign that a frontal system or upper air disturbance is approaching; usually meaning the weather will change within the next 24 hours. Cirrus clouds can also be the remnants of a thunderstorm. A large shield of cirrus and cirrostratus typically accompany the high altitude outflow of hurricanes/typhoons. Cirrus clouds have also been observed to develop after the persistent formation of contrails from aircraft.

Cirrus uncinus

Cirrus uncinus is a type of cirrus cloud. Its name is derived from Latin, meaning curly hooks. Also known as mares' tails, these clouds are generally sparse in the sky, and very thin.

The clouds occur at very high altitudes, at a temperature of about minus 40-50 degrees Celsius. They are generally seen when a warm or occluded front is approaching. They are very high in the troposphere, and generally mean that precipitation, usually rain, is approaching.

Kelvin–Helmholtz Instability

Kelvin–Helmholtz instability can occur when velocity shear is present within a continuous fluid or when there is sufficient velocity difference across the interface between two fluids. One example is a wind blowing over a water surface, where the wind causes the relative motion between the stratified layers (i.e, water and air.) The instability will manifest itself in the form of waves being generated on the water surface. The theory can be used to predict the onset of instability and transition to turbulent flow in fluids of different densities moving at various speeds. Hermann von Helmholtz studied the dynamics of two fluids of different densities when a small disturbance such as a wave is introduced at the boundary connecting the fluids.

Cirrostratus

Cirrostratus clouds are thin, generally uniform clouds, composed of ice-crystals, capable of forming halos. When thick enough to be seen, they are whitish, usually with no distinguishing features. When covering the whole sky and sometimes so thin as to be hardly discernible, this may indicate a large amount of moisture in the upper atmosphere. Cirrostratus clouds sometimes signal the beginning of a warm front and thus may be signs that precipitation might follow in the next 12-24 hours. These clouds are located above 6,000m (20,000ft). Compare cirrostratus with other stratus cloud formations at lower altitude: altostratus, nimbostratus, and low altitude stratus clouds.

Cirrocumulus

Cirrocumulus clouds are high-altitude cloud, usually occurring at 16,000-40,000 feet. Like other cumulus clouds, cirrocumulus clouds signify convection. Unlike other cirrus clouds, cirrocumulus include liquid water droplets, although these are in a supercooled state. Ice crystals are also present, and typically, the ice crystals cause the supercooled water drops in the cloud to rapidly freeze, transforming the cirrocumulus into cirrostratus. This process can also produce precipitation in the form of a virga consisting of ice or snow. Thus cirrocumulus clouds are usually short-lived.

Properly, the term cirrocumulus refers to each cloud, but is typically also used to refer to an entire patch of cirrocumulus. When used thusly, each cirrocumulus element is referred to as a "cloudlet".

By appearance, the cirrocumulus is a small, white patch or tuft without a gray shadow. It occurs in patches or sheets along with other cirrocumulus. These often are organized in rows like other cumulus, but since they are so small, cirrocumulus patches take on a finer appearance, sometimes referred to colloquially as "herringbone" or "mackerel". As cirrocumulus consists of water droplets, it does not have the distinctive veil-like or fibrous look of other Family A clouds, and consists of separate "cloudlets" that resemble altocumulus.

The cirrocumulus is distinguished from the somewhat similar altocumulus in several ways, although the two cloud types can occasionally occur together with no clear demarcation between them. Cirrocumulus generally occur at higher altitudes than altocumulus, and thus the "cloudlets" appear smaller as they are more distant from observation at ground level. They are also colder. Cirrocumulus clouds never cast self-shadows and are translucent to certain degree. They are also typically found amongst other cirrus clouds in the sky, and are usually themselves seen to be transforming into these other types of cirrus.