Stratiform
| Stratus cloud | |
|---|---|
Stratus cloud
|
|
| Abbreviation | St |
| Symbol |  |
| Genus | Stratus (layered) |
| Variety | altostratus cirrostratus nimbostratus |
| Altitude | Below 2,000 m (6,000 ft) |
| Classification | Family C (Low-level) |
| Appearance | horizontal layers |
| Precipitation cloud? | Drizzle, freezing drizzle or snow grains |
Stratus clouds are low-level clouds characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds that are formed by rising thermals. More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color from dark gray to nearly white. The word "stratus" comes from the Latin prefix "strato-", meaning "layer". Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog formed either through the lifting of morning fog or through cold air moving at low altitudes over a region. Some call these clouds "high fog" for the fog-like cloud. While light rain may fall, this cloud does not indicate much meteorological activity.
Stratus clouds form when a sheet of warm, moist air lifts off the ground and depressurizes, following the lapse rate. This causes the relative humidity to increase due to the adiabatic cooling.
Stratus clouds can also form in a similar manner to fog when the ambient air temperature decreases, increasing the relative humidity. Once the temperature drops below the dew point, a stratus cloud can form.
Stratus clouds look like featureless gray to white sheets of cloud. They can be composed of water droplets, supercooled water droplets, or ice crystals depending upon the ambient temperature.
Stratus nebulosus clouds appear as a featureless or nebulous veil or layer of stratus clouds with no distinctive features or structure. They are found at low altitudes, and is a good sign of atmospheric stability, which indicates continuous stable weather. Stratus nebulosus may produce light drizzle, or flakes of snow. Stratus fractus clouds on the other hand, appear with an irregular shape, and forms with a clearly fragmented or ragged appearance. They mostly appear under the precipitation of major rain-bearing clouds. These are the nimbostratus clouds, and the cumulonimbus clouds, and are classified as pannus clouds. Stratus fractus can also form beside mountain slopes, without the presence of nimbus clouds (clouds that precipitate), and their color can be from dark grey to almost white.
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Wikipedia