What is a cumulo-pileus cloud? How rare is it, and under what conditions is it likely to be formed? Also any information on other rare cloud formations and conditions would be helpful.
A cumulus cloud is a detached, fair-weather cloud with a relatively flat base and dome-shaped top. A pileus cloud is a smooth-cap cloud that forms in a stable layer above a cumulus cloud when air is temporarily forced upwards by the vigorous thermal below. Its shape explains its name, which is Latin for a close-fitting cap used by ancient Romans. We don’t think it would be considered a rare formation. There are so many cloud variations that it is impossible to describe them all, let alone their rarity or lack thereof, but two come to mind. First are nacreous clouds, often called “mother of pearl” clouds because of their iridescence. They form in the stratosphere and are visible at night when illuminated against a dark sky by the Sun from below the horizon. They are frequently sighted in Antarctica and were first described in 1911 by a member of Scott’s last expedition. Second are noctilucent clouds, which are seen in latitudes higher than 50 degrees, usually about midnight. They look rather like cirrostratus clouds with a blue or yellow tinge. They appear to form at altitudes of about 50 miles and travel northeast at 100 to 300 mph, but it is not yet known if they consist of ice crystals or dust.