April 15, 2017

The Beauty and Danger of Evaporating Precipitation

With our wet spring and winter, folks may think that all the precipitation falling out of our clouds is reaching the surface, but this is not true;  substantial amounts evaporate before hitting the surface and such evaporation of falling precipitation can be a beautiful sight to behold.

It also has a name:  virga.  And is seen as thin tendrils of precipitation falling out of cloud bases, frequently with curved and angled trajectories.  Virga can be made up of either ice crystals or rain drops.  Sometimes it reaches the surface (all too often around here!) and sometimes the precipitation evaporates before reaching the surface.

This week Greg Johnson of Skunk Bay Weather shared some wonderful images and videos that I would like to show you--like the image shown below.

The amount of evaporation (liquid to vapor) or sublimation (ice crystal to vapor) of the falling precipitation varies, depending the relative humidity of the air below, the precipitation rate, the height of cloud base, and precipitation type.    Low relatively humidity obviously results in greater evaporation or sublimation.

One thing that is so beautiful about virga is the often curved nature of the falling precipitation, which is also called fall streaks.  Why?  Because winds often change with height (generally faster aloft) and thus the falling precipitation, which moves with the winds, is pushed along by varying amounts as it falls.

Virga has a distinctive beauty and this video proved by Greg Johnson is wonderful to watch.

Marching Virga.... April 12, 2017 from SkunkBayWeather on Vimeo.

The precipitation from virga cools by evaporation (and sublimation) as it falls, particularly if there is dry air below cloud base.  Cool air is denser than warmer air and so it sinks, accelerating down to the surface.    

Undoubtedly you have experienced that effect.  On a threatening day, did you ever feel the temperature chill and the winds pick up right before the rain starts?   That is the cool, downdraft air from virga, whose precipitation evaporated before it reached the surface.  Eventually, the air becomes saturated and the precipitation reaches the ground.

Evaporating precipitation associated with virga can also be deadly.    Intense evaporation and cooling results in a very strong downdraft that spreads out when it hits the surface (see figure). Called a downburst (or microburst), these phenomena can produce winds of 50-100+ mph and has brought down commercial jets.

Like so many things in life, virga can be beautiful, but it has another, more dangerous, side.

For those that are interested, I will be giving a talk at Bellevue College next Tuesday (April 18th) on the politicization of climate science at 12:30 PM.  This is open to the public. And they have pizza.


How will Northwest Weather Change Under Global Warming?  Help Us Determine the Local Impacts of Climate Change

Society needs to know the regional impacts of climate change and several of us at the UW are trying to provide this information with state-of-the-art high resolution climate modeling.  With Federal funding unavailable, we are experimenting with a community funding to build this effort.  If you want more information or are interested in helping, please go here.  The full link is: https://uw.useed.net/projects/822/home    All contributions to the UW are tax deductible.


  1. this almost sounds like the phenomenon now called "micro - bursts," which (if memory serves) brought down a commercial airliner at O'Hare many years ago. Those were accompanied by thunderstorms, but until they were identified, were difficult to detect in advance.

  2. Eric, from the article:

    "Called a downburst (or microburst), these phenomena can produce winds of 50-100+ mph and has brought down commercial jets."

  3. Yep, it's the same thing. The diagrams are the same ones used on pilot exams. The numbers in the second one are tied to questions that ask you to describe what the winds are doing to aircraft performance at the various locations.

  4. John and Peter - thanks for the clarification, somehow I missed that last snippet in the article.

  5. I've always been puzzled when viewing the radar images on Wundermap. Sometimes they show precipitation near my home when there is none that is occurring or has occurred in the last little bit. May this explain that?

    Mind you, I've also infrequently seen them showing precipitation when the skies are perfectly clear. I've never figured out that.

  6. I just read about Greg Johnson and the Skunk Bay web cams. Combined with Cliff Mass' comments, it's a Master Class in climatology. Thrilled to find it.

  7. Cliff, here's one for your Friday radio piece: After a forecast for an all-clear day on Sunday, I was disappointed to see, with careful observation, that cirrus clouds were moving into this area from the southwest- yet at the same time, the NOAA wind reports were from the north from Bellingham and east through the passes. How is this possible? The wind can't blow in opposite directions at once. I can imagine a slight difference in wind direction with altitude, but opposite? Seems very unlikely- and it would required total subduction of one air mass under another, which ought to result in violent T-storms. What's going on?


  8. Billy Williams, forecaster at NWS Sioux Falls, SD, has long referred to microbursts with virga as "Vicious Virga." ;)


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