Virga has been happening as long as there has been an atmosphere with a spectrum of vertical humidity stratification. We can see virga streaks in the sky, which reminds us that the radar isn’t lying. Airplanes will fly through it as they take off or land, and on some occasions, precipitation can reach the top of high-rise buildings, but not the sidewalk below. You can throw the Frisbee, jog with your dog, and do any other normal outdoor activity while rain or snow falls above us. When virga is occurring, you will see precipitation on radar, but the ground will be dry. Asymmetry in reflectivity around the radar site indicates varying altitudes of sublimationĮxample of virga over and around a radar site.As precipitation gets closer to the ground, the reflectivity will get closer to the radar site.If approaching precipitation encircles the radar site but stops “approaching”, virga is confirmed.Distance to reflectivity is proportional to the altitude of evaporation/sublimation.Heavier precipitation can saturate the lower levels more quickly allowing the rain or snow to reach the surface. Virga always makes rainfall forecasting very difficult, especially if the precipitation is light. In this case, snow virga was streaking across the North Texas sky and showed up well on radar also. Virga is precipitation that evaporates or sublimates before reaching the ground. To an observer on the ground, precipitation that is evaporating aloft will appear as streaks extending down from the cloud (see the first image). If the air below cloud base is very dry, precipitation will evaporate before reaching the ground (snowflakes sublimate raindrops evaporate). The second condition listed above requires the layer of air below cloud base to be sufficiently moist for precipitation falling from the clouds to make it all the way to the ground. Since this discussion is about virga, we are only concerned with the second item and we will leave the topic of precipitation formation for another day. Relatively high moisture content between the ground and the cloud base.First and foremost, high moisture content in the layer of rising air (depth varies).Rather, there must be sufficient moisture in at least two critical layers: Necessary and sufficient moisture for precipitation is not based on the humidity outside our front door here at ground level. If there is sufficient moisture in the area of rising air, clouds and precipitation may form. The fluid dynamics associated with atmospheric disturbances cause the downstream air (ahead of the disturbance, before it arrives at a location) to rise, and the upstream air (behind the disturbance, i.e., after the disturbance has passed) to sink.
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