Virga Rain: Explaining Virga And How It Occurs
You may not have heard of virga before, but the chances are good that you already saw the phenomenon it refers to more than once.
This meteorological event is actually a very common occurrence and appears throughout the world wherever precipitation takes place.
If you ever observed a cloud and noticed wispy streaks of rain forming at its base, only to disappear into "thin air," you saw what is commonly known as virga. And your eyes are not deceiving you.
Even a weather radar can clearly identify the presence of rain in a cloud, yet nothing reaches the surfaces. It may seem inexplicable, but there is a very logical explanation for this event.
In this post, we look at what virga is, its characteristics, as well as how it forms.
The introduction already provided a clear indication of what virga entails. Before continuing, though, one needs to establish a concise definition of what precisely the phenomenon is.
What Is Virga?
Virga is a meteorological phenomenon where precipitation, typically bands of rain, can be observed forming at the base of a rain cloud but disappear before reaching the ground as a result of evaporation or sublimation.
Simply put, virga is precipitation that forms at the base of a cloud but never reaches the ground. Depending on height, the type of precipitation can be solid (snow) or liquid (rain), but usually manifest in the form of rain.
The virga rain is visible as wispy light-colored streaks or tails that extend below the cloud base before disappearing in mid-air.
Virga clouds are also commonly known as jellyfish clouds due to their shape. The puffy structure and dark base of a cumulus type cloud represent the body, while the light streaks of rain below it resemble the tentacles of a jellyfish.
The types of clouds associated with this occurrence are, in general, the type that produces precipitation. They include cumulus, nimbostratus, cumulonimbus, stratocumulus, and altocumulus clouds.
How Virga Occurs
Rain forming at the cloud base level and then suddenly "disappear" in mid-air is quite common, as mentioned in the introduction. The reason you don't see it more often is that it does not always appear in view or at an angle, which makes it more visible.
The rain does not actually disappear. It merely changes from a liquid (rain) into a gas (water vapor) through a process of evaporation or sublimation, making it invisible to the naked eye.
The following steps provide a concise summary of how virga occurs in general:
- 1Clouds form when moist air cools down to the point of condensation. The resulting microdroplets making up a cloud are also the source of precipitation.
- 2The microdroplets crash into each other and combine to form larger waterdrops. When they become too heavy, gravity causes them to fall to the ground as rain.
- 3In the case of virga, raindrops falling from a cloud usually encounters a layer of warm, dry air. The heat and lack of humidity in the air cause the rain to evaporate or sublimate (turning it back into its gaseous state).
- 4Evaporation takes place when precipitation changes from its liquid form to gas (raindrops to water vapor). Sublimation takes place when precipitation turns from its solid state directly into a gas (snow to water vapor).
- 5Virga typically occurs in deserts or other regions with low humidity. It also forms at high altitudes where small ice crystals quickly turn into water vapor as a result of adiabatic compression.
- 6Evaporation is a cooling process, meaning the virga rain that evaporates leaves a pocket of cold air behind. The cold air can accelerate to the ground at a rapid pace, resulting in a dry downburst, which creates dangerous, unstable weather.
- 7Unstable weather conditions also occur higher in the atmosphere, where virga is the result of adiabatic compression. This heats the air, which causes pockets of warm & cold air at the same altitude, the perfect recipe for turbulent conditions.
- 8Virga rain is also known as fallstreaks due to its appearance and location beneath the cloud base.
Virga forms in many different ways, but the general steps in which it occurs as well as the characteristics it display, follow the same path as illustrated in the steps above.
Virga is a common experience, observed almost everywhere precipitation takes place. The rain, which drops halfway from the cloud base before disappearing, remains a strange sight. This article, however, illustrated how the phenomenon occurs in a perfectly logical way.
The focus of this post was to explain what virga is, how it occurs, and highlight its most essential characteristics.
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Until next time, keep your eye on the weather!