Virga happens when rain or snow falls out of a cloud, then dries up on the way down and never reaches the ground. Rain evaporates; snow sublimates instead, turning directly from ice back into vapor without melting first (National Weather Service, Glossary; American Meteorological Society, Glossary of Meteorology). It is common enough in dry climates, and common enough in the Sky Islands region specifically, that it is worth its own short explanation here.
Why it has that hooked shape
Virga most often falls from mid-level clouds such as altocumulus or altostratus. It can also fall from a tall, cumulus-type cloud, whenever its precipitation passes through a dry layer of air on the way down. Virga usually has a hooked or comma-like shape rather than a straight streak: nearly straight down right under the cloud, curving toward horizontal lower down. Wind shear, a change in wind speed or direction with height, can help bend it. The main cause, though, is what happens to the droplets or ice crystals themselves. As they evaporate or sublimate on the way down, they get smaller, and smaller droplets and crystals fall more slowly. The lower part of the streak lags behind the upper part, and the whole thing curves (American Meteorological Society, Glossary of Meteorology).
Why it can look like rain, on radar and in person
A weather radar detects the falling droplets or ice themselves, not whether they actually land. Virga shows up on radar as a real echo, the same way rain that reaches the ground does. From a distance, virga can also look like an ordinary rain shaft, a gray curtain hanging beneath a cloud. The difference only becomes obvious directly underneath it, where the ground stays dry.
This is exactly what happened over central Arizona in September 2026. Radar showed what looked like light rain over parts of the region. Weather stations across the area reported 0.0 inches, though. Traffic camera images showed dry roads under the same radar echo (Sky Islands Climate, “That wasn’t rain, that was virga”).
Virga and dry microbursts
Virga is not purely a visual phenomenon. It has a practical safety angle too. Evaporating water pulls energy from the air around it as heat, which is why the air cools wherever precipitation evaporates. That cooler air is also denser than the air around it. Instead of blending in, it drops straight down (National Weather Service, Nashville, Tennessee, “Downbursts”). Once that dropping air hits the ground, it has nowhere else to go, so it spreads out fast in every direction. That burst of spreading wind is a downburst.
When virtually none of the rain makes it to the surface, meteorologists call the resulting downburst a dry microburst. When heavy rain does reach the ground along with it, the same kind of event is called a wet microburst instead. Dust plumes and virga are both signs that a dry microburst may be underway (National Weather Service, Glossary). Either type of microburst can produce damaging winds, even though a dry one leaves little or no rain behind as evidence. Both are also a recognized hazard to aircraft.
Virga and the rain shafts that lead into a dry microburst can visually resemble a tornado or funnel cloud from a distance. Because of that, National Weather Service storm spotter training in Phoenix lists rain shafts and virga among the features spotters need to learn to tell apart. Both can otherwise be mistaken for the real thing (National Weather Service, Phoenix, Arizona, “Skywarn Spotter Training, Standard Class”).
Why it’s common in the Sky Islands region
Out of the whole country, dry microbursts and the virga that can precede them show up most often in the western United States and the High Plains. Thunderstorm cloud bases in that part of the country typically sit much higher above the ground than they do in more humid regions. Underneath those high bases sits a deep layer of dry air, reaching all the way down to the surface (National Weather Service, Nashville, Tennessee, “Downbursts”).
That deep, dry layer is exactly what lets a storm’s rain evaporate so completely before it reaches the ground. It is a routine feature of the desert Southwest, not an unusual one. It shows up especially often in the weeks before the summer monsoon fully establishes itself, and again in the gaps between storms once it has. Both are times when moisture and clouds sit well above the surface, but the air near the ground stays dry.
Sources
National Weather Service. “Virga.” Glossary. https://forecast.weather.gov/glossary.php?word=VIRGA.
American Meteorological Society. “Virga.” Glossary of Meteorology. https://glossary.ametsoc.org/wiki/Virga.
National Weather Service, Nashville, Tennessee. “Downbursts.” https://www.weather.gov/ohx/downbursts.
National Weather Service. “Microburst.” Glossary. https://forecast.weather.gov/glossary.php?word=MICROBURST.
National Weather Service, Phoenix, Arizona. “Skywarn Spotter Training, Standard Class.” https://www.weather.gov/media/psr/Skywarn/Standard%20Class%20Presentation.pdf.