The Washington PostDemocracy Dies in Darkness

Stunning mammatus clouds south of Washington, D.C. Thursday evening

Mammatus in south Stafford (Isha Renta via Facebook)
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From Manassas to Warrenton to Fredericksburg, mammatus, the dynamically drooping cloud-forms, made for a dramatic evening sky.

It was an unmatched display of these bubbly, pouch-like clouds since the July 6, 2012 show which I posted about. See: Mammatus clouds bubble up in Washington, D.C.’s evening sky (PHOTOS)

As I described, mammatus clouds are relatively rare, but an indicator of high levels of atmospheric instability.  They typically signal the worst weather has moved away or is somewhere else. Indeed, these developed in the wake of a line of intense thunderstorms that passed south of the District late Thursday afternoon.

Here’s a set of beautiful mammatus cloud sightings from readers…

How do these clouds form? See this description, courtesy University of Illinos:

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 clouds.