virga

Virga is Latin for 'twig, rod, branch.' The meteorological use was coined in the twentieth century by scientists who saw the phenomenon — streaks of precipitation trailing from cloud bases that evaporate before reaching the ground — and reached for a visual metaphor. The trailing wisps looked like thin rods or branches hanging from the cloud. The Latin word fit.

The physics of virga requires dry air beneath a moist cloud layer. Rain or snow falls from the cloud base and enters air with low relative humidity. The precipitation evaporates (rain) or sublimates (snow) during the descent. The result is visible precipitation that hangs in the air like a curtain but never reaches the surface. In the desert Southwest of the United States, virga is more common than rain reaching the ground.

Virga can be dangerous to aviation. A pilot seeing virga may assume the air below is wet, but the evaporation process produces microbursts — sudden, powerful downdrafts that can cause aircraft to lose altitude rapidly. The Dallas-Fort Worth microburst of August 2, 1985, associated with virga from a thunderstorm, caused Delta Air Lines Flight 191 to crash, killing 137 people. Rain that does not reach the ground can still kill.

The word entered English meteorological vocabulary in the mid-twentieth century and remains primarily a technical term. Most English speakers have seen virga but do not know the word. The long, trailing wisps beneath desert thunderstorms are one of the most photogenic weather phenomena in the American West. The word is Latin. The sight is universal.