May 18, 2020

What were the local weather impacts of the eruption of Mount St. Helens?

It was 40 years ago, at 8:32 AM on May 18, 1980 that Mount St. Helens exploded, producing an ash plume that rapidly expanded westward.   Within a few hours, much of eastern Washington was in mid-day darkness and by that evening the volcanic plume had reached Idaho.

What were the weather effects of this extraordinary eruption?   This is something I am in a position to talk about, after examining the issue in detail and co-authoring two articles with a colleague, Professor Alan Robock

Alan and I visited the eruption zone the following summer and it looked more like Mars than Earth (see below, I am the one on the left!)

The relatively primitive weather satellites at the time illustrated the growth of the dust plume, from near the initial eruption time (8:45 AM):

To its expansion across eastern Washington by 1:45 PM

Lights turned on in eastern Washington as day turned into night (take a look at the mid-day picture at Yakima).  Now that is impressive.

The thick volcanic cloud had a huge impact on surface temperature.  During the day, the volcanic plume reflected some of the solar radiation back to space and absorbed the rest, leaving little to reach the surface.  Thus during the day, the volcanic dust cloud cooled the surface. 

At night, the opposite was true.  The cloud absorbed infrared radiation leaving the surface and emitted infrared radiation back to the surface;  this, stopped or reduced the nighttime cooling.

All of you are familiar with similar effect with normal clouds--they cool during the day and warm at night.   But this effect was on steroids with the Mount St. Helens ash cloud.

Let me show you what happened at the surface, using a figure from my paper with Alan Robock (published in Monthly Weather Review in 1982). 

Below are the observed surface air temperatures at Yakima, Spokane, Great Falls, Montana, and Boise, Idaho for the days around the eruption.  The small vertical arrows show when the dust cloud reached the location in question.  At Yakima, May 17th had the normal rise and fall, but when the ash cloud reached them on the 18th, temperatures stopped rising, cooled a bit and then remained constant for over 12 hours.  Amazing.   Things slowly recovered the next few days as the ash cloud thinned and moved eastward.

Spokane had similar effects but were delayed by a few hours.  In contrast, Great Falls reached their normal highs, but the nighttime arrival of the volcanic cloud kept the temperatures up at night.

But exactly how much did the volcanic plume influence the temperatures?  We knew what the observed temperatures were, what we needed was to know what the temperatures would have been like without the volcanic eruption

How could we do that?   Then we got an idea.  Why not use the best objective temperature forecasts available--those from the National Weather Service Model Output Statistics system-- to determine what would have happened?  Then take the difference with the observed temperatures to get the volcanic influence.   And it worked!

Here is the difference between the forecast and observed surface air temperatures at 5 PM on May 18th (shading indicates cooling from the expected temperature).  Wow...about 8 degrees centigrade (14F) cooling.
And what about the effects at night?   Looking at the differences at 5 AM the next morning, shows warming of 7 C (13F) over western Montana and about half that over eastern Washington.  Our physical intuition was correct.
Alan and I also studied the climatic impacts of Mount St. Helens, as did several other investigators.  

We all found that the eruption had very little long-term, climatic impacts.  

Why?   Although it injected a lot of volcanic particles in the lower atmosphere, St. Helens was a low-sulfur volcano that did not put much sulfur dioxide into the stratosphere.  And sulfur dioxide, in the presence of water vapor, produces the long-lived stratospheric particles that result in sustained cooling.  

Finally, I have often thought about what would have happened if the eruption had occurred earlier in the year when the winds were more southerly.  Seattle would have been buried and crippled, and there would have been huge human impacts.


  1. I always felt it was good luck that it erupted during the daytime and there were only high clouds. Could just as easily have been in the dark or obstructed by clouds and no one would have seen or photographed the event.

    1. My father was outside his home, a few blocks North of Greenlake in Seattle, when he heard a muffled "whump" had reminded him of listening to German artillery fire, when he was a soldier during WW2, off in the distance..a couple hours later, he and I noticed the ash cloud, extremely low at first, on the southern horizon...what a unique event!

  2. Cliff,

    This takes me back to that time. It was like nothing I had ever seen and felt as far away as another planet from my home. But, it was fascinating, fierce and made a big impression on me.

    Thanks for the memories and nostalgia of all that happened at that time.

  3. Most will never forget. It was Sunday; I was walking back to my apartment in Moscow Idaho after church. I reached the crest of a hill and saw in the distance what looked like an unending squall line of thick clouds, black as obsidian, on a slow roll eastward across the Palouse. I got back to the apartment and, with my roommates, watched day turn to night as ash began to fall. We knew this was a once-in-a-lifetime occurrence. I'll steal Yeats' line from Easter 1916..a terrible beauty is born. Thank you for your discussions. I can take my brain elsewhere from the troubles of today.

  4. On Whidbey Island the explosion shook the house. Jolted out of bed, we thought the Naval Air Station was being bombed. Days later I helicoptered over St. Helens with a geologist. Unimaginable devastation. Stripped trees lying like spaghetti, rivers of mud. Mount St. Helens, Kennedy's Assassination, the Twin Towers ~ some of the events by which this life are measured.

  5. I remember a secondary eruption when when we went to downtown Portland to watch a bran new movie Airplane at the theater. We were standing in line to go in, and we heard an explosion, and couldn't see what or where it came from because of all the buildings. Just as we began to file in the theater, we could see the ash plume start to enclose around us from above . And just then, my thoughts turned to " Wow! We're going to watch a comedy movie while the world is coming to an end..." I came out to a different downtown Portland than the one I'd left before the movie. Everything covered in ash. It looked like a movie set.

  6. Why are those bozos on the news saying that we are in for "one of the worst fire seasons in Washington"? I see absolutely no reason to make such a pessimistic prediction, in view of the fact that we are at or above normal for snow-pack and rainfall. I suspect that, to get people to be careful, they try to justify using scare tactics and exaggerate the risk- which I know Cliff hates- so do I.

  7. Thanks for the memories! I grew up in Yakima and that's exactly how it looked....pitch black. Remember seeing a weird, large purplish/black/blue/dark gray "storm" cloud approaching, not knowing initially the mountain had blown. A short time later, I was watching a TV screen split between the Mariner game and Mt St Helens spewing ash into the sky.

  8. I was just a kid but I remember we heard the boom up here in Vancouver Canada

  9. "they try to justify using scare tactics and exaggerate the risk"

    But this is their modus operandi. It's a fun sport for them. They are having a hey day with the virus.

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