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Tuesday, September 07, 2010

Today's Weather

General Weather: Normal summer weather , but dust haze at first.

Wind: Mainly n'ly 05 to 10kt reaching 10 to 15kt at times.

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Education Center


	Education Centre

General Meteorology :	Bahrain Climate - Air Masses - Cold Front - Fog - High Pressure - Humidity - Introduction to Weather - Large Thunderstorms - lightning - Low-Pressure - Meteorology - Occlusion Fronts - Rain - Sea Breaze & Land Breaze - Temperature - The Water Cycle - Tornadoes - Warm Front - Wind

Clouds :	Cirrus - Clouds - Cumulus - Cumulonimbus

Radar :	General - Radar Technology - Attenuation in the atmosphere - Velocity measurements - Sources of error - Optimizing radar characteristics - Radar installation - Precipitation measurements

Clouds
Cirrus
Cirrus clouds are the highest of all clouds, and are sometimes a foreteller of bad weather to come.
High-level clouds form above 20,000 feet (6,000 meters) and since the temperatures are so cold at such high elevations, these clouds are primarily composed of ice crystals. High-level clouds are typically thin and white in appearance, but can appear in a magnificent array of colours when the Sun is low on the horizon.
The most common form of high-level clouds are thin and often wispy cirrus clouds. Typically found at heights greater than 20,000 feet (6,000 meters), cirrus clouds are composed of ice crystals that originate from the freezing of super cooled water droplets. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation. Often however, they mark the high-altitude front of an approaching depression, with rain from low-altitude stratus cloud following with 12 hours.

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Clouds
There are three main types of clouds: cirrus, stratus, and cumulus. The words alto for high clouds and nimbus for rain clouds are sometimes added.
High clouds are found from between 16,500 and 45,000 feet (5,000 to 13,700 meters) and include (from highest to lowest) cirrus, cirrocumulus, and cirrostratus. A cirrus cloud appears in delicate, feather-like bands, sometimes in tufts, and is usually white. Cirrocumulus clouds look like very small round balls or flakes. Sometimes cirrocumulus clouds form the pattern of what is called a "buttermilk" or "mackerel" sky. Cirrostratus clouds sometimes form tangled webs or thin whitish sheets. When cirrostratus clouds cover the sky, a large ring or halo is sometimes seen around the sun or moon. This is caused by the bending of rays of sunlight or moonlight as they pass through the ice particles which make up the clouds.
Middle layer clouds are found from between 6,500 and 23,000 feet (2,000 to 7,000 meters) and include (from highest to lowest) altocumulus, altostratus, and nimbostratus. Altocumulus clouds are rounded puffs of cloud larger than cirrocumulus. Altostratus clouds cover the sky with a greyish veil through which the sun or moon may shine as a spot of pale light. Nimbostratus clouds are thick, dark, and shapeless and usually bring rain or snow.
Low clouds are found from between ground level and 6,500 feet (2,000 meters) and include (from highest to lowest) stratocumulus, stratus, cumulus, and cumulonimbus. Stratocumulus clouds are large and lumpy, round or rolled-looking, and often cover the entire sky. Stratus clouds are generally dark and appear as streaks across the sky or as a grey layer hanging above the Earth. Stratus clouds are generally shapeless.
Cumulus clouds range in size from the small puffball-like forms to huge dome-topped thick piles of "woolpack" that often develop into thunderclouds. These storm clouds, cumulonimbus, may range in thickness from about 1 to 3 miles (1.6 to 4.8 kilometres) in Great Britain to 8 or 9 miles (12.9 or 14.5 kilometres) near the equator.

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Cumulus
Cumulus clouds look like fluffy balls of cotton wool, and generally only develop during the daytime.
Cumulus clouds look like white fluffy balls of cotton wool and mark the vertical extent of convection or thermal uplift of air taking place in the in the atmosphere. The level at which condensation and cloud formation begins is defined by the flat cloud base, and its height will depend upon the humidity of the rising air. The more humid the air, the lower the cloud base.
Small cumulus clouds are evidence of relatively stable air during fair weather. Once condensation begins, uplift continues only for a short distance before the temperature of rising air falls below that of the surrounding atmosphere. Since cumulus clouds have no existence apart from thermal convection, they are diurnal clouds when over land, forming mid morning and disappearing again after sunset. They may also form over sea when the sea surface temperature is warmer than the air passing over it. Then similar convection currents can develop.
Young fair weather cumulus have sharply defined edges and bases while the edges of older clouds appear more ragged, an artefact of cloud erosion. Evaporation along the cloud edges cools the surrounding air, making it heavier and producing sinking motion or subsidence outside the cloud. This downward motion inhibits further convection and the growth of additional thermals from below, which is why fair weather cumulus typically have expanses of clear sky between them. Without a continued supply of rising air, the cloud begins to erode and eventually disappears. Fair weather cumulus typically have a lifetime of 5 minutes to about half an hour.

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Cumulonimbus
Cumulonimbus clouds can be beautiful but often bring with them heavy rain, hail and lightning.
Cumulonimbus clouds are much larger and more vertically developed than cumulus clouds which form in a more stable atmosphere. They can exist as individual towers or form a line of towers called a squall line often present at cold fronts. Underneath they are dark and menacing. At a distance they rise up like huge white mountains when the Sun shines on them, and are commonly topped with anvil-shaped heads. Fuelled by vigorous convection of air in an unstable atmosphere the tops of cumulonimbus clouds can easily reach 12 km or higher. Lower levels of cumulonimbus clouds consist mostly of water droplets while at higher elevations, where temperatures are well below 0°C, ice crystals dominate.
During the formation of the larger cumulonimbus, condensation droplets are carried up and down several times within the convection currents inside the cloud before being released, coalescing to form raindrops. The more vigorous the thermal currents inside the cloud, the larger the raindrops which form. Light showers can fall form modest-sized "cauliflower" clouds whilst the larger cumulonimbus clouds can produce heavy downpours. The largest cumulonimbus clouds of all are found in thunderstorms. In these clouds up-currents are so severe that splitting of raindrops and ice crystals can occur before re-coalescing and falling to the ground. It is believed that this may contribute to the build up of electric charge and the occurrence of lightning.

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