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High clouds (base above 6,000 m); "cirrus" or "cirro-"
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Middle clouds (base between 2,000 - 6,000 m); "alto-"
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Low clouds (base below 2,000 m); no prefix
Fog is a cloud so low it touches the ground |
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Cloud names are based on: their shape, altitude and whether they are producing precipitation.
Clouds come in 3 basic shapes:
- cumulus (heaped & puffy)
- stratus (layered)
- cirrus (wispy)
Clouds occur in 3 altitude ranges (specifically, the altitude of the cloud base):
High clouds (base above 6,000 m); "cirrus" or "cirro-"
- Cirrus
- Cirrocumulus
- Cirrostratus
Middle clouds (base between 2,000 - 6,000 m); "alto-"
- Altocumulus
- Altostratus
Low clouds (base below 2,000 m); no prefix
- Stratus
- Nimbostratus
- Cumulus
- Stratocumulus
- Cumulonimbus
Fog is a cloud so low it touches the ground
Clouds whose names incorporate the word "nimbus" or the prefix "nimbo-" are clouds from which precipitation is falling.
Contrails are clouds formed around the small particles (aerosols) which are in aircraft exhaust. When these persist after the passage of the plane they are indeed clouds, and are of great interest to researchers. Under the right conditions, clouds initiated by passing aircraft can spread with time to cover the whole sky. The presence of contrails should be specifically flagged in your observations.
Clouds provide one of the keys to understanding the weather. Knowing about clouds, their shapes and changing patterns, will help you forecast weather changes.
Clouds form when the air is cooledwhen the air is "lifted" by:
At the higher altitudes the air expands and cools causing water vapour contained in the air to condense into tiny but visible water droplets. If the droplets grow large enough they form raindrops and fall to the ground.
The atmosphere consists of air masses which are different notably in temperature and moisture content. In general the air masses do not mix but are separated by relatively narrow transition zones called fronts. A cold front is part of a system along which cold air is advancing; a warm front is that part of a system along which cold air is retreating. The fronts are frequently characterized by clouds and produce some of the stormiest weather. The type and extent of the clouds depend on the air mass characteristics. Some fronts have few clouds when the air is very dry.
Clouds frequently rise to an altitude of 6 km, less frequently to about 10 km, but in the case of very severe storms may extend upwards at least 18 km. Temperatures at these heights fall to -50 to -55°C, even in the summer.
The first sign of an approaching warm front is usually a layer of cirrus that begins to cover the western or south-western sky. This layer thickens to a cirrostratus and perhaps, also altocumulus begin to form. Light rain may begin at this point becoming more intense in the dark grey nimbostratus with fog often developing. Stratus cloud and some thick stratocumulus may also form. Sudden heavy downpours may occur, along with thunder or lightning and sometimes hail, indicating cumulonimbus are embedded in the solid layer of nimbostratus. The thunderstorms may last for hours. Temperatures rise slowly with the advance of the front, but more rapidly as the front passes and wind shifts in the clockwise direction. Then the cloud cover begins to break up but the fog often lingers.
There are few clouds ahead of a cold front to warn of its arrival, although occasionally a layer of altocumulus and/or stratocumulus may spread out in advance. The front itself is usually marked by a line of towering cumulus and showers. In the most severe cases a line or wall of cumulonimbus marks the front and can be accompanied by thunderstorms and hail. The tops of the cumulonimbus often spread out ahead of the front into dense cirrus called an anvil. Temperatures fall and the wind shifts in a clockwise direction while the front passes. The band of cloud and rain is usually quite narrow and passes within a few hours. Afterwards, some non-frontal clouds, such as stratocumulus or cumulus, may develop but just as frequently the skies clear.
Unstable - summer
The sun heating the ground in turn heats the air just above the
ground. The warmer air becomes buoyant and rises and cools. As the
air cools, the water vapour begins to condense and small fluffy
cumulus clouds form. If the cumulus
continue to grow they may become towering
cumulus and eventually, cumulonimbus clouds with heavy rain,
lightning and sometimes hail. These storms often last for only an
hour or so.
Unstable - winter
In winter, areas of open water are warmer than the layer of air
passing over the water. These areas then act as heat sources similar
to the land heating process of summer. Clouds may form above the open
water and give rise to snow flurries and snow squalls. If the area of
open water is large enough, towering cumulus clouds usually develop
and snow flurries or significant heavy snowfalls can persist downwind
from the water as "streamers".
Stable type clouds
When the air is not as unstable as described above, extensive layers
of stratocumulus clouds may develop.
Strong surface winds can cause sufficient turbulence to lift and cool
the air, causing the moisture to condense. Small individual cloud
elements can form and may thicken into a solid layer.
Cumulus or towering
cumulus clouds that form over warm water may flatten out over
land and become stratocumulus. Warm air
lying over a cold land surface may also be cooled during the night
and form a fog or stratus
layer cloud.
Orographic clouds
When winds blow moist air up a mountainside, a hill or even a gentle
slope, the air is forced to rise, causing in order: cooling, water
vapour condensation and fog or cloud formation. The cloud activity
will depend on the wind speed, the air moisture and its stability.
Cumulus, towering cumulus and even cumulonimbus clouds may form on
the windward side of the slopes. On the leeward side, lens-shaped
altocumulus clouds may form,
particularly if the winds are moderately strong.
People who earn their living in weather sensitive occupations such as farmers and fishermen, are very aware of the different cloud types and become proficient at interpreting their changing patterns. By observing clouds as well as changes in temperature, wind direction and barometric pressure you can improve your understanding of the weather and its causes and perhaps anticipate forthcoming changes which will affect your activities.
