Category Archives: Virga

Incredible Seattle sunset

Last night, Seattleites were treated to a spectacular sunset. I, Angela, was attending the Seattle Sounders game and admit that I was quite distracted by the clouds, as were many others at the stadium. Here was one of the views I had from my seat:
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Angela Rowe
Location: Seattle, Washington
Date: 14 July 2014

What causes these vibrant colors? First, let’s take a look at the visible spectrum. The sun’s rays are made up of a range of wavelengths and therefore colors. The cool colors (blues) are the shorter wavelengths, while the reds are the longer wavelengths.
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This simple diagram from the National Weather Service online education course (JetStream) shows what happens to the sun’s rays as the sun sets in the sky. At lower angles (B and C), the sun’s rays have to pass through more of the atmosphere, therefore more of the shorter wavelengths are scattered by the air. This leaves the longer wavelengths (the reds and oranges), which can be reflected off of clouds in the mid- and upper-levels of the atmosphere.
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While there are many colorful sunsets (and sunrises) in Washington and beyond, the conditions last night were particularly conducive for a colorful show due to the mid-level clouds present over much of the area. Earlier in the evening, water vapor satellite imagery showed an upper-level low pressure system sitting off the coast, spinning counter-clockwise and pumping moisture into the area, seen as the pink colors in this image.

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This moisture was primarily concentrated in the mid-levels, as could be seen as a thickening altostratus layer covered the sun over the city. Note there are some darker clouds underneath as moisture continued to move in from the southwest.
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Showers and thunderstorms were prominent to the south, especially over northern Oregon, but the lower levels over Seattle and much of NW Washington remained dry. Every day, twice a day, the National Weather Service launches instruments on balloons called radiosondes that measure the temperature, moisture, pressure, and winds as the balloon ascends into the sky. Here is an example of what this balloon data looked like last evening around this time, with the red line indicating temperature and the blue dewpoint. The dewpoint is the temperature at which the air would have to be for saturation to occur. The higher the number, the more moisture in the air. Where the dewpoint and temperature are far apart indicates dry conditions (low humidity), while areas where they are nearly the same are where you might expect clouds. So, if you look at this example, you’ll see that most of the atmosphere near Seattle was very dry, with the exception of right near the surface and in between pressure levels of 400 and 350 mb. This corresponds to heights above the surface between roughly 7.5 and 9 km, where many of the clouds were observed on this particular evening.
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So as I entered the stadium around this time, I quickly became distracted by the mammatus clouds that were forming overhead. The dry air below the moist levels helped create these brief, but beautiful bulbous clouds and it became increasingly difficult to pay attention to the game.

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In order to keep track of what was going on around the stadium, I kept an eye on the radar. Some of the showers from the south looked like they were making their way toward Seattle, as can be seen in this radar image during the game.
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At the stadium itself though, some light drizzle could be seen, but overall the rain was evaporating before reaching the ground. This is referred to as virga and can be seen in these pictures that I took from the stadium around this time.
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So as you might have guessed, I was quite distracted by the sky throughout the game. The Sounders ended up beating Portland in an exciting 2-0 game, but I bet a large portion of the 64,000+ fans at the stadium were also paying attention to the sky. In fact, social media sites exploded with pictures of this fiery sunset, from the stadium and throughout other areas near the Puget Sound (http://blogs.seattletimes.com/today/2014/07/sunday-nights-seattle-sunset-captured-on-twitter/).

Later on at night, lightning flashed in the distance as storms persisted over the southern Olympic Peninsula. The low was moving onshore, bringing with it cold air aloft that created an unstable environment, in addition to the moisture available, to maintain these storms. Living in a place like this, where there aren’t a lot of thunderstorms, I’ll certainly take what I can get.
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This morning, I was curious to see how much rain did end up reaching the ground across the area last night. The CoCoRaHS network (www.cocorahs.org) is a volunteer organization where people report rain at their house. This map shows the reports for the 24-hours ending this morning, with most locations near Seattle receiving less than one-hundredth of an inch!
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And finally, for your viewing pleasure, here’s a link to time lapse video of the clouds and sunset as viewed looking west from the University of Washington’s Atmospheric Science building rooftop. Can you see the different cloud layers before the colorful finish to the day? Click here to see this time-lapse video!

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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.

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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.

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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.

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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.

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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).

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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.