Paragliding Weather the Virga Phenomenon

Have you noticed how sometimes streaks “hang” from under the clouds? This phenomenon is called virga, it can often appear under cumulonimbus clouds. It is the same rain, but it evaporates before reaching the surface of the earth. The reason for this is low ambient humidity. That is, dry air in the way of drops tends to saturate with moisture and "absorbs" them. So the rain simply evaporates on the way to the earth. Due to the evaporation, the surrounding air becomes colder - a cold air mass goes down and suppresses new air flows rising under the cloud. This becomes the main reason for the cessation of the growth of the cumulonimbus cloud.

Have you ever seen a radar image that suggested it should be raining over a certain location, but it wasn't? This sometimes occurs and it is not a problem with the radar. The radar is not lying, the interesting part is that the rain is not hitting the ground. If there is a dry air mass in place, a lot of occasions rain cannot completely penetrate that dry layer before it evaporates. The rain that doesn't make it to the ground is called virga.

Below is a graphic showing a cross-section of a saturated environment (rain) intersecting a dry air mass. The deeper into the dry air mass you go, the less chance the rain has of reaching the ground. The radar will still detect the rain aloft, but you may not see it at the surface level. This scenario is one reason why it is hard to break drought conditions once they set in. The rain that you do get is reduced because of the dry air in place.