Mammatus clouds over southwest Washington, D.C. Thursday evening (Jason Kopp)

Who doesn’t love mammatus clouds? These eerie, pouch-like structures that develop in sinking air are the atmosphere’s version of bubble-wrap: you want to extend your arms and pop them.

Although they’re an indicator of high atmospheric instability, they also usually signify that the worst weather has either moved off or is somewhere else

That was the case last night between 8 and 9 p.m. when mammatus were seen and photographed in both northern Virginia and the District. Thunderstorms were occurring in eastern Maryland.

Here are some of the dramatic pictures submitted to us via Twitter and Facebook - alllow a few seconds to load:

The University of Illinois explains how these clouds form:

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.

Did you notice these clouds last night?