Friday, October 16, 2009


A cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae. Clouds are studied in the nephology or cloud physics branch of meteorology.
On Earth the condensing substance is typically water vapor, which forms small droplets or ice crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths. They thus appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases, hence the gray or even sometimes dark appearance at the cloud base. Thin clouds may appear to have acquired the color of their environment or background and clouds illuminated by non-white light, such as during sunrise or sunset, may appear colored accordingly. Clouds look darker in the near-infrared because water absorbs solar radiation at those wavelengths.

As air parcels cool due to expansion of the rising air mass, water vapor begins to condense on condensation nuclei such as dust, ice and salt. This process forms clouds. Sometimes an elevated portion of a frontal zone forces broad areas of lift, which form cloud decks such as altostratus or cirrostratus. Stratus is a large dark low cloud deck that tends to form when a stable cool air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions. Clouds can also be formed due to lifting over mountains and other topography

Cirrostratus cloud
Cirrostratus clouds are thin, generally uniform clouds, composed of ice-crystals, capable of forming halos. They are usually located above 5.5 km . 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 to 24 hours.

Circumhorizontal arc
A circumhorizontal arc is an optical phenomenon, an ice-halo formed by plate shaped ice crystals in high level cirrus clouds.
The current accepted technical names are circumhorizon arc or Lower symmetric 46° plate arc. The term 'fire rainbow' coined recently by a journalist is not recognised. It is misleading as the arc is not a rainbow and is not related to fires.
The complete halo is a huge and beautiful multi-coloured band running parallel to the horizon with its center beneath the sun. The distance below the sun is twice as far as the common 22-degree halo. Red is the uppermost colour. Often, when the halo forming cloud is small or patchy, only fragments of the arc are seen.
There is a myth that the halo is rare. How often it is seen depends on location and in particular latitude. In the United States it is a relatively common halo seen several times each summer in any one place. In contrast, it is rare in mid-latitude and northern Europe.
For the halo to form the sun must be very high in the sky, at an elevation of 58° or more. Cirrus cloud or haze containing relatively large plate-shaped ice crystals must also be present. The sun altitude requirement has the consequence that the halo is impossible to see at locations north of 55°N or south of 55°S (although a lunar circumhorizon arc might be visible). In other latitudes it is visible for a greater or lesser time around the summer solstice. Slots of visibility for different latitudes and locations can be looked up on graph. For example, in London, England the sun is only high enough for 140 hours between mid May and late July. Contrast that with Los Angeles with the sun higher than 58 degrees for 670 hours between late March and late September. When the cloudiness of Europe is also taken into account, the halo becomes more than 10-20X more likely to be seen in the United States.
The halo is formed by sunlight entering horizontally-oriented flat hexagon ice crystals through a vertical side face and leaving through the near horizontal bottom face. There is no requirement that the plates be thick. In principle, Parry oriented column crystals can also produce the arc although this is rare.
The 90° inclination between the ray entrance and exit faces produces the well-separated spectral colours and, if the crystal alignment is just right, makes the entire cirrus cloud appear to shine.
A circumhorizontal arc can be difficult to distiguish from an infralateral arc when the sun is high in the sky. The former is always parallel to the horizon whereas the latter curves upwards at its ends.

Cloud iridescence
Cloud iridescence or irisation is the occurrence of colors in a cloud not dissimilar to those seen in oil films on puddles. It is fairly common. The colors are usually pastel and need searching for but sometimes they can be very vivid. Iridescence is most frequent near to the sun and the glare masks it. It is most easily seen by hiding the sun behind a tree or building. Other aids are dark glasses or observing the sky by its reflection in a convex mirror or in a pool of water.
Iridescent clouds are a diffraction phenomenon. Small water droplets or even small ice crystals in clouds individually scatter light. Large ice crystals produce halos - the latter are refraction phenomena not iridescence. Iridescence should similarly be distinguished from the refraction in larger raindrops that gives a rainbow.
If parts of the clouds have droplets (or crystals) of similar size the cumulative effect is seen as colors. The cloud must be optically thin so that most rays encounter only a single droplet. Iridescence is therefore mostly seen at cloud edges or in semi transparent clouds. Newly forming clouds produce the brightest and most colorful iridescence because their droplets are of the same size. When a thin cloud has droplets of similar size over a large extent the iridescence takes on a structured form to give a corona - a central bright disk around the sun or moon surrounded by one or more colored rings.
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