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.

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.

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 …