SPECTACULAR MAMMATUS CLOUDS
OVER HASTINGS, NEBRASKA.
Mammatus are pouch-like cloud
structures and a rare example of clouds in sinking air.
Sometimes very ominous in appearance, mammatus clouds are harmless
and do not mean that a tornado is about to form; a commonly held
misconception. In fact, mammatus are usually seen after the worst
of a thunderstorm has passed.
As updrafts carry precipitation
enriched air to the cloud top, upward momentum is lost and the air
begins to spread out horizontally, becoming a part of the anvil
cloud. Because of its high concentration of precipitation particles
(ice crystals and water droplets), the saturated air is heavier
than the surrounding air and sinks back towards the earth.
The temperature of the subsiding
air increases as it descends. However, since heat energy is required
to melt and evaporate the precipitation particles contained within
the sinking air, the warming produced by the sinking motion is quickly
used up in the evaporation of precipitation particles. If more energy
is required for evaporation than is generated by the subsidence,
the sinking air will be cooler than its surroundings and will continue
to sink downward.
The subsiding air eventually
appears below the cloud base as rounded pouch-like structures called
Mammatus are long lived if the
sinking air contains large drops and snow crystals since larger
particles require greater amounts of energy for evaporation to occur.
Over time, the cloud droplets do eventually evaporate and the mammatus
dissolve. Mammatus typically develop on the underside of a thunderstorm's
anvil and can be a remarkable sight, especially when sunlight is
reflected off of them.