WEATHER BLOG: Rain on radar but none on my head!
Have you ever checked your phone app or maybe the radar on a website and noticed it was showing rain, only to look outside your window and notice there's no raindrops? It's a common sight, especially in winter and fall when the air is dry.
The first thing to look at is what data is going into the radar. There's two main types of radar data- base reflectivity and composite reflectivity. Base reflectivity gives you only the lowest scan of the atmosphere. In other words, this is the best one to use if you're looking for what's happening at the surface. Composite reflectivity combines all layers, from the surface to several thousand feet up. This can make rain look heavier than it actually is, or make it look like it's raining when it's not at the surface!
WHAT IS VIRGA?
Raindrops fall from the clouds several thousand feet up in the atmosphere. Since radar scans at an angle, it detects these raindrops several thousand feet up above the earth.
The atmosphere from the surface to the base of the clouds is an ever changing variable of temperature, dewpoint and humidity. In the winter, the air, especially at the surface is often very dry. This causes rain drops (or snowflakes) to evaporate before the hit the ground!
Meteorologists refer to this phenomanon as virga. The word is derived from the latin word virga meaning "twig" or "branch" because of how it looks.
Ever seen a wispy layer below a cloud? That's probably rain (or snow) falling from the cloud but evaporating.
In the winter months, when we're dealing with a borderline snow setup, with temperatures near 32, this evaporation of water can actually cause the temperature to cool. This is a phenomenon known as evaporative cooling. Latent heat is released, causing the air temperature to cool. From time to time, this will allow the air to get cold enough to then get snow instead of rain! This process can sometimes take hours, and is often the reason at the start of a snowstorm, it's hard to get any precipitation to fall, regardless of what radar shows.
So the next time you're playing Scrabble, break out "Virga!" Your friends and family will probably argue that's not a word, but just refer them over to this blog and smile...
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Meteorologist Ricky Matthews