ROCHESTER, NY (WROC) – Have you ever seen that classic, green signature on radar indicating rain should be falling and hitting the ground? Sometimes if the air is dry enough at low levels of the atmosphere, it won’t quite reach the surface. The phenomenon that appears instead is called virga.
Virga is defined by the National Weather Service as, “streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.”
The radar is able to detect the signal of rain above the surface due to the radar beam increasing in thousands of feet with height away from the radar site, where moist air aloft could be producing rain. Hence, the signatures appearing on radar. This can be very deceiving to meteorologists and to the public if the radar is displaying what looks like steady rain falling, but it’s not. This is why it’s important for meteorologists to always check observations at the surface to see if rain appearing on radar is reaching the ground. If the temperature is too far away from the dew point, it means the air could be too dry and will evaporate before reaching the surface.
Virga can occur during any time of the year in the form of rain or snow, and in some cases virga can occur before a microburst event, which is a sudden downpour of rain that builds up from the entrainment of dry air throughout mid levels of the atmosphere.
Here are a couple examples of when our radar showed precipitation, but it wasn’t reaching the ground:
Here’s an example of surface observations from the Greater Rochester international airport from the same day showing no rain collected (yellow box), the temperature and dew point difference (red box), and humidity (blue box). All of these factors tell us that the air is too dry at lower levels for any falling rain to reach the ground.