Pileus Clouds Above Singapore
December 18, 2005
On my way to holiday in Singapore, I spotted a building cumulonimbus cloud with a satiny pileus cap cloud above. Atop the main pileus, several smaller caps soon became visible. Pileus caps are composed of ice crystals and typically form high in the troposphere just ahead of a rapidly rising convective tower -- a layer of air is forced upward in the shape of a dome or cap. Water vapor in this dome condenses directly into ice crystals, forming the pileus. The lifted layer above the convective tower must be nearly saturated or else the pileus cap won't form. Photo taken on October 16, 2005. See also the Earth Science Picture of the Day for August 30, 2005.