Definition
An accessory cloud is a subsidiary cloud formation that is physically attached to, or closely associated with, a larger parent cloud system but possesses distinct morphological or dynamical characteristics. It is typically observed in conjunction with convective storm systems, where it may develop on the periphery or frontal region of the main cloud mass.
Overview
Accessory clouds are most commonly noted in association with cumulonimbus (thunderstorm) clouds. They can appear as horizontal, wedge‑shaped formations such as shelf clouds, or as localized vertical protrusions like wall clouds. Although they share the same atmospheric environment as the parent cloud, accessory clouds often have differing temperature, humidity, and wind shear conditions that give rise to their unique appearance. Their presence is frequently used by meteorologists as an indicator of storm intensity, wind direction, and potential for severe weather phenomena such as tornadoes or downbursts.
Etymology / Origin
The term combines the adjective “accessory,” meaning supplementary or attached, with “cloud,” the meteorological term for a visible mass of condensed water vapor or ice particles suspended in the atmosphere. The phrase entered the meteorological lexicon in the mid‑20th century, appearing in textbooks and glossaries that described various components of convective storm morphology.
Characteristics
| Feature | Description |
|---|---|
| Relationship to parent cloud | Physically connected to the main cloud’s base, flank, or top; may extend outward or upward. |
| Formation mechanisms | Result from localized variations in updraft strength, temperature gradients, and wind shear that cause condensation separate from the central updraft of the parent cloud. |
| Typical types | • Shelf cloud – a low, horizontal wedge that precedes a gust front. • Wall cloud – a lowered, rotating region beneath the main updraft, often a precursor to tornadoes. • Cap cloud – a thin, lens‑shaped layer that tops a cumulus tower, indicating strong updrafts. |
| Visual appearance | Varies by type: shelf clouds appear as a smooth, sweeping horizon line; wall clouds are darker, lower, and may show rotation; cap clouds are translucent and dome‑shaped. |
| Meteorological significance | Their development can signal changes in storm dynamics, such as intensifying updrafts, shifting wind directions, or the onset of severe weather. |
| Lifespan | Generally short‑lived, persisting from a few minutes to tens of minutes, depending on the evolving storm environment. |
Related Topics
- Cumulonimbus – the primary thunderstorm cloud to which many accessory clouds are attached.
- Shelf cloud – a classic example of an accessory cloud formed ahead of a gust front.
- Wall cloud – a rotating accessory cloud associated with tornadic potential.
- Gust front – the leading edge of cooler air outflow that often generates shelf clouds.
- Mesocyclone – a rotating updraft within a supercell thunderstorm, frequently linked to wall cloud formation.
- Cloud classification – the broader system (e.g., the World Meteorological Organization’s International Cloud Atlas) that categorizes cloud types, including accessory formations.
Note: While the concept of an accessory cloud is recognized in meteorological literature, detailed quantitative studies on their formation and evolution remain an active area of research.