Evaporating rain in Nevada

Just because rain is produced in a cloud, doesn’t mean the precipitation will reach the ground. This is referred to as “virga” and this evaporation cools the air (the same reason you feel chilly when getting out of the shower), leading to strong downward motion because cold air is denser. This can result in periods of gusty winds and if the storm also produces lightning on a hot day, could create a serious fire threat in this dry environment.

This morning, in Elko, NV, a ballon was launched from the local National Weather Service office (as is done both morning and evening every day) that carries instruments to measure temperature, moisture, pressure, and winds in the atmosphere. The diagram below shows what this data looks like, with the red line showing temperature decreasing as you go up in the atmosphere and dewpoint temperature (a measure of the amount of moisture in the air) shown in green. The difference between the temperature and dewpoint is the humidity so the farther apart they are, the drier the atmosphere. This morning, you can see that there were some nearly saturated conditions in the mid-levels (where the green and red are closest) while below, near the surface, the lines are far apart indicating very dry conditions.


Brian Boyd sent us the following picture early this afternoon from near Elko, showing the cumulus clouds that were forming. The temperature at this time, according to Brian, was 95 degrees (Fahrenheit) with a relatively humidity of 9%; consistent with the dry atmosphere that was observed in the balloon data (called a sounding) earlier that morning. The heat from the sun was enough to create lift as the warm surface air rose and cooled to produce clouds at the level where the temperature dropped to the dewpoint.

Brian Boyd
Location: Elko, NV
Date: 2 July 2014

Nearly an hour later, from essentially the same view, Brian sent us another photo that showed that these clouds had increased in coverage, appearing darker and, as Brian told us, produced lightning.

Brian Boyd
Location: Elko, NV
Date: 2 July 2014

If we take a look at the visible satellite image from around this time (2245 UTC), you can see these clouds scattered across much of northeastern Nevada.


Now, if we take a look at the Elko radar data from this same time (but over a smaller area), you can see backscatter power returned from the raindrops in these growing cumulonimbus clouds near Elko (the blues, greens, and yellows).


Despite seeing what looks like precipitation on radar, if you look back at Brian’s second photo, and based on his observations, there wasn’t actually rain hitting the ground. It was evaporating in that very dry air below. That, combined with the lightning he observed, posed a serious fire threat for this region. Add to that the gusty winds due to the evaporative cooling, and you can see why this could be a concern.

Speaking of gusty winds and virga, we saw a video recorded yesterday south of Elko from the National Weather Service office in Reno, NV, a place also characterized by dry, hot low-levels recently and evaporating rainfall. In this video, you’ll also see the clouds building and, towards the end, you’ll see the “microburst” as strong winds hit the surface resulting from strong downward motion due to the evaporative cooling. Because there was no rain at the surface, this is referred to as a dry microburst.


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