May 19, 2013

Weather Impacts of the Mount Saint Helens Eruption

Yesterday was the 33rd anniversary of the eruption of Mt. St. Helens, an event that devastated the mountain and surroundings and caused terrible problems on nearby rivers.

But there is another story of the eruption and one far less known:  its impact on local weather.

The eruption produced a huge volcanic dust cloud that was mainly blown downwind (towards eastern Washington) by the prevailing winds.  Here are a few weather satellite images that day (May 18,1980 at 8:32 AM), that clearly shows the eastward dispersion of the volcanic dust.

Imagine if the volcano had erupted a few months earlier when the winds are more typically from the south--the nearby Puget Sound region would have been crippled.

What were the meteorological impacts of this dust cloud?   I decided to investigate this with the help of Professor Alan Robock of the University of Maryland (now at Rutgers). 

Here are the temperatures at Yakima and Spokane during that period (this is from a paper we wrote in Monthly Weather Review).  As the plume went over Yakima, day turned to night.  There was a slight cooling as the sun was obscured and then the temperature remained virtually constant for over 12 h.  Why?   The thick volcanic cloud acted as a very effective blanket:  solar radiation couldn't get in, infrared radiation couldn't get out.   The cloud hit Spokane a bit later in the day (they had more time to warm) and was a bit thinner there, so the impacts were less.

How much did the cloud influence temperatures that day?  We estimated this by taking the difference between a very skillful forecast system (MOS, Model Output Statistics) and what actually happened.  The next plot, which shows the estimated cooling in Celcius at 5 PM that day,  shows what we found.  Over portions of eastern Washington the volcanic cloud caused temperatures to cool by around 8 C (14.5F), with cooling of roughly 9 C extending to the Idaho border.

The dust cloud rapidly spread into Idaho, Montana, and Wyoming (although thinning as it moved eastward).   Those locations were able to warm up during the day and then the dust spread over during the night.   The volcanic cloud, like all clouds, reduced the amount of infrared cooling (we cool at night because the earth radiates infrared radiation day and night, and at night there is no warming from the sun).  Thus, the cloud caused the temperatures to be much warmer than they would be otherwise (there were few meteorological clouds). 

Here is the proof:  the estimated temperature changes due to the volcanic cloud at 5 AM the next morning:  8-12 degrees (C) warmer over western Montana!
The weather effects of the Mt. St. Helen's dust cloud rapidly weakened during the next few days as the dust thinned and moved to the east. 

Interestingly, this eruption had virtually no climatic effects.  The reason:  the effluent from the volcano had relatively little sulfur content (SO2).  Injecting this gas into the stratosphere is the main way to produce long-lived volcanic hazes that spread around the planet and cool the lower atmosphere for a few years.  In fact, some folks would like to try cooling our warming planet by doing this artificially--injecting large amounts of particles into the stratosphere to reflect some of the solar radiation.  But that is the subject of another blog!

Announcement:   I will be teaching Atmospheric Sciences 101 (WEATHER) at the UW this fall.  This class is accessible folks 60 or older at very little cost (the UW Access Program) and, of course, to regular UW students.  This class will give you a good basic understanding of the atmosphere and Northwest weather.


  1. Wow. I must say I love that first satellite image - it brings home what an impressive eruption that was.

    I was in college at the time. I remember we were actually disappointed the plume didn't come our way (the day of the eruption I was at my folk's house in Spanaway). We saw the video from Yakima and some of the small towns east of the mountains - I imagine if it had travelled towards Puget Sound instead, we might not have been all that happy.

  2. Wow, Cliff. Thank you for the heads up on the Access Program.

  3. I'm a bit surprised that the default assumption is that the ash absorbed IR radiation strongly. Or are you saying it reflected it? Macroscopic "rock" particles scatter IR light pretty well but I don't have any experience with micron sized particles, although many do. Can you say more about this? Thanks.

  4. Amazing post.

    Hey, I'm sure you've heard about the Shawnee Oklahoma tornado today (and yesterday). This timelapse video is amazing. F4 Tornado 05-20-13

  5. Yes, if you would Cliff, please consider a tornado topic: how they form, why the middle of our country gets so many, is there any way to stop them in theory, and why do people live in tornado alley?


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