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.