Tuesday, 19 August 2014

Barely heard of it, now can’t get away from it.

Despite my long standing dislike of meteorology I actually faired reasonably well on that section in the written exam, with a respectable 80 odd %. Most of the questions I got wrong had very little to do with my understanding of weather phenomena and more to do with Transport Canada’s dicey wording.

Still the one I will admit to getting wrong through a lack of knowledge was to do with a weather phenomenon that simply doesn’t occur in the UK, it is called virga.

For those of you who can’t be bothered with the wiki article, it is basically rain that falls from the clouds but evaporates before it hits the ground. It indicates highly unstable air. That much I knew already.

The question, though, wanted to know about the type of instability associated with it. I narrowed it down to two answers, basically up or down drafts. In a futile attempt to apply some logic to the situation I picked “up”; evaporation = heat and heat rises right?

Apparently not, virga causes downdrafts. So now I know. I’m not unduly concerned about my lack of knowledge, in the couple of years that I’ve been flying I’ve never really encountered it, as far as I can tell it is very much a phenomena of the prairies.

Again, apparently not. Shortly afterwards the word was mentioned in two consecutive weather briefings I received, with rain bearing clouds being picked up on the radar but no precipitation being reported on the ground stations. I don’t know what information they have but the weather specialist assured me that the clouds were at 6000ft and the virga wouldn’t be an issue below 3000ft.

As I was planning to do low level airwork (forced approaches and the like) I was good to go. Sure enough though while I was up there I could see the cloud bases and the streams of precipitation falling but stopping a ways below the cloud.

I’ve seen similar conditions, in the air, on at least two other occasions now. I don’t know if it is due to the weird summer we are having, or that now I’m aware of it, I’m seeing it everywhere.

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