Cap Cloud

Photo credit – Anne Burgess – CC-BY-SA

Cloud of the Day – Cap Cloud

Cap cloud (link to a super photo on the Astronomy Photo of the Day website) is similar to pileus in that it forms on top of another structure. In the case of pileus, the other structure could be a cumulonimbus cloud or a volcanic plume. Cap clouds, on the other hand, are found on permanent structures, such as mountain peaks. Cap cloud is defined at Weather Online as a “. . . stratiform, orographic cloud that hovers above or over an isolated mountain peak.” “Orographic” means that the lifting mechanism that raises the moisture-laden air to the condensation level is the terrain. And “stratiform” means that the cloud is more layer-like than vertical in its development. In these ways, cap cloud is also related to altocumulus lenticularis, another stratiform cloud formed by orographic lift.

Photo credit – euphro – CC-BY-SA

Cap clouds form when warm, moist air is forced up over a mountain peak. When the condensation level is at or near the summit, a cloud forms in close enough proximity to merit the name. When the winds are strong enough, they smooth it out into the classic lenticular form. An interesting effect can be seen sometimes when you’re on the lee side of the mountain. As the air flows over the top of the mountain and down your side, it warms up again and evaporation causes the cloud to disappear partway down. If you look closely, you can see the cloud constantly flowing over the mountain, then vanishing, an effect I was lucky enough to see at Cook Mountain in New Zealand.

Photo credit – Bojan Dodoc

Cap cloud is not associated with precipitation, given that the air is necessarily drier at lower elevations.

Photo credit – Colin Sue Brown

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