February 01, 2004
The above picture shows a pileus cloud (background) and was taken at my Ft. Lauderdale, Florida home on an August afternoon. Pileus is Latin for "cap" or "skull cap." These clouds are observed to ascend quite rapidly -- rising at velocities of 40 feet (about 12 m) per second and sometimes even faster. As this type of cloud rises through the atmosphere, it forces air ahead of it, in a similar way that a train pushes air ahead of the locomotive. If the layer of air above a rising cumulus cloud is nearly saturated, the additional lift from below is often sufficient to saturate it completely, forming a smooth and rounded cloud.
Pileus clouds are occasionally seen just before a lightning storm. Note the iridescence along the upper rim of the pileus cloud in the photo.