Hi Bob,

I wanna ask a question about thunderstorms in your RPL/PPL book.

It said that the developing of growing stage. During this stage, all of the air movement is upwards within the cloud. The edges and tops are growing rapidly and large water droplets and often snow flakes are suspected in the strong updates. There is no precipitation but virga is sometimes present.

Next page, talking about microburst, there is one sentence: A visual indication is often the presence of virga as small water droplets are pulled from the cloud base by the descending cold air.

My question is in what condition or circumstance the virga will happen during the developing stage of thunderstorms, since there are all updrafts and droplets are supported by the updrafts? This confused me a bit.

Thanks

Chao

Even though, in the developing stage, the air movement is upwards there are still some areas where the updrafts are weaker than in other areas. It is still possible that water droplets can be falling even though they are in weak updrafts. The speed a droplet falls at depends upon its mass and surface area. If a droplet is large enough, it can end up falling faster than the updraft that contains it. In that case, it actually leaves the cloud base. These droplets often evaporate in the warm air beneath the cloud base before they reach the ground, producing the phenomena known as 'virga'.

In the mature stage, in the vicinity of microbursts , the downdrafts can be so powerful that they pull even small droplets out of the cloud base. These droplets were not really ready to fall from the cloud except for the action of the powerful downdrafts. These small droplets readily also evaporate in the warm air beneath the cloud base also producing virga.