Weather forecasters seem to have a difficult job at times so it might just pay to give them a bit of slack and allow them a bit of leeway every now and then. However they do have their own 'Get out of jail free card' that can be whipped out at a moments notice for those days when it seems like it should have rained, indeed it may have looked like it rained and yet nobody actually felt the rain on the earth's surface. This get out of jail free card comes in the form of VIRGA.
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| Virga over Mt Ainsle 29 August 2012 |
Virga is of course rainfall which evaporates before reaching the ground. The word has its roots from a Latin word meaning twig or branch and the reason behind this definition becomes obvious when one observes the tendrils and extensions created from the base of clouds by the precipitation. In Canberra we had an extravaganza of virga just last Wednesday.
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| More virga falling over Canberra 29 August 2012 |
The two key ingredients for virga are obviously rainfall and a layer of fairly dry air where the raindrops can evaporate into vapour. Virga is responsible for those high hooked cirrus clouds we see with an approaching front, but can also be observed lower in the atmosphere. On Wednesday the virga was generally falling from altostratus clouds which are what are referred to as mid-level clouds.
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| Virga from altostratus under cirrocumulus |
Some of the more devastating features associated with virga are so-called dry thunderstorms with strong wind gusts (dry microbursts) and dry lightning. It's always a worry as a source of ignition for bush fires, but we'll leave that till the summer.
For now, virga can remain the forecaster's friend and also the get out clause when the forecast said rain yet nobody got wet.
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