Changing temperatures and seasons can cause a remarkable weather event called Virga. LPM News' Bill Burton spoke with WAVE 3 meteorologist Tawana Andrew about the occurrence also known as jellyfish clouds.
This transcript was edited for brevity and clarity.
Tawana Andrew: Today we're talking about precipitation that doesn't quite reach the ground.
Bill Burton: This is one of my favorite things to talk about, and I'm glad we finally have a chance to come around and talk about this. It's Virga. Tell us about it.
TA: So Virga occurs when rain or ice or snow descends from the bottom of a cloud and it evaporates or sublimates before ever reaching the Earth's surface. So this phenomenon typically happens when you have falling droplets or ice crystals falling through a layer of air that is dry or warm and that keeps the precipitation from actually reaching the ground. So Virga is sometimes referred to as jellyfish clouds, because when you look at them, you have very puffy tops. Kind of looks like the top of a jellyfish, and that precipitation that is evaporating looks very streaky underneath it, and looks like the tentacles of a jellyfish.
BB: I like that. Now I can picture it clearly.
TA: So if you ever see that happening in the sky, now you know what's going on. Now, part of the reason that we have Virga happening in our atmosphere is that each level of the atmosphere has distinct levels of temperature and moisture. So when you have that precipitation falling from the cloud base, often thousands of feet above our heads, usually what happens it enters warmer, more moist air. However, in certain situations, you have that layer of drier air, closer to the surface, and it evaporates all of that moisture. This can even happen on the leading edge of showers and thunderstorms, that initial wave of rain moving in, if it's pushing into dry air, a lot of that will evaporate, and eventually the air will become saturated enough for the rain to actually reach the ground. Now, Virga is very cool in terms of temperatures, because it literally cools the air as that rain falls and evaporates, the air below the clouds will cool in temperature as the water vapor actually absorbs all of that heat from the surrounding air. Now this plays a role in cloud formation as well, because those little particles that end up evaporating or sublimating, they will be drifted downstream by the wind, and they can act as nucleation points for water droplets and ice crystals and help to create another cloud downstream. Now, usually Virga is for those quiet days where there's not much going on. However, they can lead to the formation of what's called a micro burst, and those happen as that rain transitions into water vapor, it pulls all that heat from the surrounding air, and that results in a sudden downward rush of cooler air, and that can cause some severe turbulence for aircraft, and it scared a lot of people. Even brought down power lines in Midland, Texas on April 22. On that day, a weather station in Midland Texas reported at 111 mile per hour wind gusts.
BB: And all because of Virga. That's just plain scary.
TA: All because of Virga. So the National Weather Service there, referred to it as a Virga bomb, also known, we're naming things a little bit better now.
BB: I like that one. That's catchy.
TA: So they called it a Virga bomb, or a dry microburst. And because of rain at the upper levels of the atmosphere falling into a significant layer of drier air that all that rain quickly evaporated, that cooler, denser air descended around 20,000 feet, and as it's falling that 20,000 feet, it's speeding up. So once the air reached the ground, it's spread out in all directions at 111 miles per hour. So that does not happen often.
BB: Thankfully.
TA: I want to clarify. But sometimes the weather is weird and things like that happen.
BB: Sometimes the weather is weird. Words to live by.
TA: And by the way, sometimes you can actually notice Virga yourself by checking a weather app. So you may look at the weather app, and you look at the radar and you see it's showing rain or snow over your area, but you're looking up and there's like, it's not raining, it's not snowing, that may be Virga happening above your head, because all of that moisture falling into the dry air, and so radar is picking up what's happening over your head, but not quite what's reaching the ground. So keep that in mind as you're heading out and about, and you look up, you see some sky jellyfish, as I'll call them. And now you know what Virga is.
