Altocumulus radiatus
Ac ra
Altocumulus radiatus
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Description
The variety "radiatus" (Latin: ray-shaped) describes clouds whose elements are arranged in parallel bands or streaks that appear to converge toward a single point on the horizon due to perspective. Altocumulus radiatus appears as extensive fields of mid-level cumulus clouds, whose individual cloudlets or rolls are arranged in strikingly regular, parallel rows. This arrangement is caused by uniform wind flow in the middle troposphere, which aligns the cloud elements in the direction of the wind. The appearance is often reminiscent of a ribbed or striped pattern that covers the sky with impressive regularity. Depending on the lighting and time of day, these cloud streaks can produce spectacular light effects, especially at sunrise and sunset.
Identification Features
- Parallel bands or rows of altocumulus elements that appear to converge toward a single point on the horizon
- Cloud elements at mid-level altitude, typically appearing as small to medium-sized, whitish or gray cloudlets
- The streak structure often extends across large portions of the sky
- The converging perspective is the key distinguishing feature from other altocumulus varieties
- The individual cloud elements can range from thin and translucent to thicker and opaque
- Clearly higher than stratocumulus but lower than cirrocumulus, with individual elements appearing larger than those of cirrocumulus
- Frequently observed in association with strong, straight upper-level winds
Occurrence by genus
Highlighted: all genera with which radiatus occurs.
Meteorological Significance
Altocumulus radiatus indicates an organized, uniform flow in the middle troposphere and often develops in connection with wind shear or gentle wave formation. When occurring in isolation, this cloud form is generally weather-neutral and indicates stable conditions. However, if cloud cover increases, the bands become denser, or the cloud transitions into altostratus, this can signal an approaching warm front and impending precipitation. In combination with falling air pressure, altocumulus radiatus is a harbinger of deteriorating weather within the next 12 to 24 hours. When occurring in isolation with stable pressure, however, the weather tends to remain fair.
Facts
- WMO-Code
- Ac ra
- Height layer
- Mid-level clouds
- Typical height
- 2.0 - 6.0 km
- Genus
- ALTOCUMULUS