Altocumulus undulatus
Ac un
Altocumulus undulatus
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Description
The name "undulatus" (Latin: wavy) describes the characteristic wave structure of this Altocumulus variety. Altocumulus undulatus appears as an extensive cloud layer at mid-levels, marked by parallel, wave-like bands or rows. These wave patterns form through wind shear at the interface between air layers of differing temperature and velocity, a phenomenon known as Kelvin-Helmholtz instability. The clouds can appear as either a translucent or opaque layer, often producing an impressive, regular striped pattern across the sky. The waves can occur in both thin, semi-transparent forms and in thicker, closed sheets.
Identification Features
- Clearly recognizable parallel wave structure or roll pattern at mid-level altitude
- Alternating lighter and darker stripes reminiscent of a rippled water surface
- Cloud elements arranged in regular, elongated rows
- Typical altitude between 2,000 and 6,000 meters above ground
- Individual cloud elements appear smaller than a clenched fist held at arm's length (distinguishing feature from Stratocumulus)
- Wave lines often extend across large portions of the sky
- Can consist of both individual puffs and continuous sheets
- Occasionally with gaps between wave crests through which blue sky is visible
Occurrence by genus
Highlighted: all genera with which undulatus occurs.
Meteorological Significance
Altocumulus undulatus indicates pronounced wind shear and dynamic instability in the mid-troposphere. The presence of these clouds shows that different air masses are sliding over one another at differing speeds or directions. This cloud form frequently occurs ahead of warm or occlusion fronts and may indicate an approaching deterioration in weather conditions within the next 12 to 24 hours. In stable high-pressure systems, however, Altocumulus undulatus can also occur without subsequent precipitation and may dissipate rapidly. For aviation, this cloud form serves as an indicator of possible turbulence at mid-level flight altitudes.
Facts
- WMO-Code
- Ac un
- Height layer
- Mid-level clouds
- Typical height
- 2.0 - 6.0 km
- Genus
- ALTOCUMULUS