(meteorobs) on the subject of clouds

[Bruce McCurdy]

To Arlene south of Troy, I sent your cloud question to Alister Ling,
astronomer, meteorologist, and skywatcher extraordinaire. His response
follows. regards, Bruce

> Arlene, it's not easy to be sure from your description. Stationary clouds
> are quite common in hilly terrain. As long as the atmosphere is moist
enough
> at the right altitude for a prolonged time, the cloud will form above a
peak
> or range of hills. The basics of it is that a non-saturated airmass is
> forced to rise over terrain, and cools because it ascends into lower
> pressure and expands. If there is sufficient moisture in the original
> parcel, the cooling will eventually hit the parcel's dewpoint and become
> saturated and a cloud forms. As the parcel goes back down on the other
side
> of the hill, it is compressed by the higher pressure below and warms up
> above the dewpoint and the cloud evaporates.
>
> The odd thing, at first glance, is that the cloud is stationary, but the
air
> is constantly moving through it. In some ways, it is like a breaking wave
in
> a river, attached by a rock in the stream. The wave is stationary but the
> water moves through it.
>
> The shape you describe is fairly unusual for an "orographic cloud".
> Otherwise, I can't think of much else it could be.
>
> Good observing,
> Alister.
> (meteorologist)