Altocumulus
Ac
Altocumulus
© METOffice
Description
Altocumulus (lat. altus = high, cumulus = heap) is a mid-level heap cloud that occurs in the middle troposphere. It typically appears as a layer or field of white or grey, rounded cloud masses, often arranged in regular patterns. The individual elements have an apparent width of approximately one to five degrees when observed at an angle of more than 30 degrees above the horizon. Altocumulus can occur in various species and varieties, including lenticularis (lens-shaped), castellanus (turret-shaped), or floccus (tufted). It is one of the most common cloud types in the mid-latitudes and often gives the sky an impressive, structured appearance.
Identification Features
- White to grey, rounded cloud masses or patches at mid-level altitude
- Individual elements appear larger than those of cirrocumulus, typically one to five finger-widths at arm's length
- Often arranged in regular patterns, rows, or undulating bands
- May display self-shadowing, making them appear darker than cirrocumulus
- Frequently visible as an extensive field covering large portions of the sky
- Individual elements can be clearly separated or partially merged
- Distinguished from stratocumulus by their greater altitude and smaller individual elements
- When backlit, the edges of the cloud elements can appear brilliantly bright (iridescence possible)
Species of this genus
Highlighted: all species that occur with Altocumulus.
Meteorological Significance
Altocumulus in the morning, particularly the castellanus species, can indicate increasing instability in the middle troposphere and may herald thunderstorm development in the afternoon. When altocumulus occurs together with a thickening and lowering cloud deck, this can suggest an approaching warm front and imminent precipitation. Isolated, persistent altocumulus fields during stable high-pressure conditions are generally harmless and produce no precipitation. In conjunction with orographic lifting, lens-shaped altocumulus lenticularis can form, indicating strong upper-level winds. As a general rule of thumb: altocumulus in the morning can serve as an early warning for afternoon convection and possible thunderstorms.
Facts
- WMO-Code
- Ac
- Height layer
- Mid-level clouds
- Typical height
- 2.0 - 6.0 km
- Genus
- ALTOCUMULUS