Altostratus Clouds are located worldwide, but are more common in mid latitudes. These clouds are found in the mid levels of the atmosphere and can be just a thin veil ranging to a layer 2,000-3,000m thick. Generally Altostratus will contain ice crystals near the top and water crystals lower down. They are always a sign of a significant amount of moisture being held in those layers and cover a very large area, sometimes over thousands of square miles. Generally it is too thick, but when it is thin the Sun and Moon may be visible through Altostratus.
Altostratus Clouds form at warm fronts and is the result of condensation when warm moist is lifted above cooler air. When it is sufficiently thick Altostratus can produce rain or snow over a very large area.
When stratus cloud covers the entire sky it can be difficult to determine whether it is low level or mid level cloud. As a general rule of thumb if you can make out a texture in the cloud deck then it is likely to be low level stratus, if there is no discernible structure and appears smooth and featureless then it is likely to be mid level altostratus clouds.
Summarised facts about Altostratus Clouds:
Altostratus clouds usually form ahead of a front producing widespread and often continuous precipitation.
These clouds generally form between 2,000 and 3,000m and often produce long, steady rain.
Altostratus are thinner if formed at higher altitudes but are heavier and more dense if closer to the ground.
They are formless gray to bluish clouds that form a thin veil over the sun and moon.
Altostratus are common in the advance of a warm front, preceding nimbostratus clouds.
Altostratus clouds are potentially dangerous because they can cause ice to form on the wings of aircraft that fly trough them.
There are a number of features allowing the observer to distinguish various stratus clouds from each other:
Stratus clouds bring much lighter precipitation (drizzle) than nimbostratus;
Altostratus clouds are lighter in colour and less opaque than nimbostratus, so sunlight can be seen through them;
Cirrostratus clouds never bring precipitation and have a thin, whitish, veil-like structure, characteristic for cirrus;
Stratocumulus bring only light precipitation and have a clearly visible base with easily distinguished separate cloud elements.
If the altostratus clouds cover a large portion of the sky, are approaching from the direction of the wind at the clouds' level, and are increasing in coverage, then widespread precipitation could be expected in the area.
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