Altocumulus opacus
Ac op
Altocumulus opacus
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Description
The name "opacus" (Latin: opaque, shady) describes a variety of altocumulus whose cloud field is so dense that it completely obscures the sun or moon. It is an extensive layer of contiguous cloud patches or rolls at medium altitude, covering a large portion of the sky. In contrast to the variety "translucidus", opacus allows no sunlight to pass through and appears noticeably darker. This variety occurs more frequently in moister air masses and when the cloud layer has greater vertical depth.
Identification Features
- Closed or nearly closed cloud cover at medium altitude
- Sun or moon completely obscured and not visible through the cloud layer
- Recognizable structure of patches, rolls, or elements on the underside
- Gray to dark gray coloration, particularly on the underside
- Distinct shadowing between individual cloud elements
- Individual elements typically 1 to 5° in size in the sky (rule of thumb: larger than a finger held at arm's length)
- No optical phenomena such as halos (distinguishing feature from altostratus)
Occurrence by genus
Highlighted: all genera with which opacus occurs.
Meteorological Significance
Altocumulus opacus indicates increasing moisture and instability in the middle troposphere. This cloud frequently appears ahead of a warm front or with approaching disturbances and can be a precursor to precipitation within the next few hours. If the cloud layer transitions into altostratus or nimbostratus, persistent rain or snowfall is to be expected. In summertime situations, opacus can also signal an impending deterioration in weather with thunderstorms, particularly when the clouds develop castellanus turrets.
Facts
- WMO-Code
- Ac op
- Height layer
- Mid-level clouds
- Typical height
- 2.0 - 7.0 km
- Genus
- ALTOCUMULUS