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During my eleven year circumnavigation, I observed virga thousands of times.

The only thing I knew about virga was that it was precipitation that did not reach the ground. That scant level of knowledge was adequate to get me safely around the world.

I confess that I did not understand the significance of virga, and more importantly, I did not understand it is associated with microbursts.

Virga is a streak or shaft of precipitation coming from the base of a cloud, and the precipitation can be in the form of ice crystals or droplets of rain.

If the virga is caused by ice crystals, the ice sublimates (turning to vapor without melting to form water) before it hits the ground. Sublimation of ice crystals is more likely to occur with high altitude virga associated with cirrus clouds.

If the virga is caused by rain drops, the rain evaporates before it hits the ground. Water droplet virga is more likely to be associated with storm clouds at a lower altitude where it is warmer.

What I did not know during my circumnavigation is that virga can form at any altitude where there are clouds.
Virga is associated with cirrocumulus, altocumulus, altostratus, nimbostratus, cumulonimbus, and cumulus clouds.

I only focused on the virga associated with cumulus and low lying clouds. I completely ignored the virga at higher altitudes.

When water evaporates and ice sublimates to water vapor, it absorbs heat rendering the surrounding air cooler, and the cold air is more dense, and it falls toward the ground. The falling rain and ice crystals create a downdraft, and the cold air created by evaporation and sublimation is additive to and reinforces the downdraft.
When the downdraft is severe, the result is a microburst or downburst.

In a microburst, people at ground level experience a blast of cold air radiating out in all directions from the shaft of falling air.

The shaft of cold descending air associated with virga is what causes microbursts.

On rare occasions you can have a "heat burst" of warm air if the virga and descending air column starts at high altitude. The shaft of falling cold air collides with warm air near the surface pushing it downward, and there is a paradoxical heat burst which is more common in desert regions.

Pilots give virga wide berth. A strong downdraft associated with virga can cause a plane to lose several thousand feet of altitude in a few seconds.

Glider pilots especially have to avoid virga. At their low air speed, they could remain in the descending air column long enough to get slammed into the ground.

To concisely sum up what happens in a virga associated microburst:

Precipitation falls from the base of a cloud, and as it falls, it evaporates. As evaporation happens, the air cools becoming more dense and sinks rapidly toward the ground. The downdraft hits the surface at speeds of more than 60 mph. Because there is no rain reaching the ground, this is called a dry microburst.

Microbursts have had winds clocked at speeds up to 150 mph.

Virga is also responsible for seeding of storm cells in adjacent clouds. Ice crystals and water droplets from virga can be blown to neighboring clouds and act as nucleation particles for a new thunderhead to form.

Virga are sometimes called the jellyfish of the skies when they have a puffy top and tentacles of precipitation dangling beneath the top.

The shape of falling virga is determined by the intensity and direction of the wind in the layer of air through which it falls.

Virga is also out of this world.

NASA has used LIDAR to detect virga on Mars. High altitude snow and ice crystals in the Martian atmosphere have a similar pattern to the virga found on earth.

Virga is also found on Venus and Jupiter.

The takeaway for the cruising sailor is this.

Pay attention to Virga.

It can reveal the wind speed and direction of high altitude, medium altitude and low altitude winds by looking at the deflection of the virga.

Virga is the atmosphere’s way of telling you that you are at risk for a dry microburst.

Significant virga may not result in 60 knot winds, but the downdrafts can easily reach 30 knots or more.

When you see significant virga in unsettled conditions, consider putting a reef or two in the mainsail until the virga disappears and the winds settle down into their normal pattern.

Virga is Mother Nature’s way of letting you know that you are in the presence of unstable air, and you should take appropriate precautions.

Dr. Dave

Captain Dave - David J. Abbott M.D.





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