Mammatus clouds seen over Montgomery County’s Agriculture Reserve on Thursday. (Martin Radigan Photography via Facebook)

In the wake of the vigorous thunderstorms that swept through Loudoun and Montgomery counties Thursday evening, curious bubbly clouds filled the dusk sky.

They were mammatus — stunning pouch-like, bulbous clouds that hang menacingly from a layer of mid-level clouds.

These were some of the most well-defined, picturesque mammatus we’ve ever seen in the region.

Mammatus clouds seen over Clarksburg, Md., on Thursday. (Momo Couture via Facebook)

In the old days, mammatus were decreed as sure signs of a tornado nearby — check out any meteorology textbook dating back 40 to 50 years or so. But since then, we’ve learned that these are common apparitions of rather ordinary thunderstorms, rendered even more dramatic by the lighting of a low-setting sun.

These cloud pouches are created by “upside down” convection. With ordinary convection, warm air rises into cooler air aloft — like a hot-air balloon with the internal burner flaming. Mammatus represents blobs of cloudy air chilled by evaporation or sublimation, then sinking as bubbles of dense (heavy) air into a warmer air layer below. This particular thermal setup portends nothing about a thunderstorm’s severity or tornado-generating proclivities.

Mammatus clouds seen over Landsdowne, Va., on Thursday. (Jason Demers via Facebook)

In fact, we have commonly observed mammatus clouds developing on the underside of a very humdrum, stratocumulus cloud layer on a cold and dreary winter day.

Below find more beautiful photos of these clouds from our readers …

Mammatus clouds seen over Mount Airy, Md., on Thursday. (Cindy Snipes via Facebook)

Mammatus clouds seen over Poolesville, Md., on Thursday. (Kevin Williams via Facebook)

Mammatus clouds seen over Germantown, Md., on Thursday. (Arthur Simpson via Facebook)

Mammatus clouds seen over the Kentlands, Md., on Thursday. (Steven D Martin via Facebook)

Mammatus clouds seen over Germantown, Md., on Thursday. (Freddy De Grano via Facebook)

Mammatus seen over Monrovia, Md., on Thursday. (KipOffice Guy via Facebook)

Mammatus clouds and a rainbow seen over Ashburn, Va., on Thursday. (Bobby Hiddy via Facebook)

Mammatus clouds on Thursday. (Regina Campbell via Facebook)

Mammatus clouds over Leesburg, Va., on Thursday. (Cortney Witmer via Facebook)