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Why the eruption of Mount St. Helens dramatically altered temperatures, but not for long

Mount St. Helen's erupts on July 22, 1980 in Washington State.
The Associated Press
Mount St. Helen's erupts on July 22, 1980 in Washington State.

“An improving trend” is in store this Memorial Day weekend. The clouds, rain and cool temperatures we’ve been experiencing over the past several days will yield to something a little less cloudy. You can expect dry conditions in most places and it will warm up considerably, to as high as 70 degrees on Monday, says KNKX Weather expert Cliff Mass.

But it will still be pretty cloudy.

As you look up at those clouds, consider how different they are than the ones that formed around the blast zone of Mount St. Helens after its eruption on May 18, 1980 — 40 years ago this week.

Cliff Mass was a professor at the University of Maryland at the time. He says he was so excited and fascinated by the volcano in Washington state that he produced several papers with a colleague, on how the eruption affected local weather.


“As the plume — the dust veil from the volcano — went over Eastern Washington, it turned day into night,” Mass says. “And that had a huge impact on temperatures.”

Together with his colleague, Professor Alan Robock, he documented the mechanics and extent of that effect. Mass says the plume blocked out the normal warming that would happen on the earth’s surface during the day.

“Basically, all the solar radiation was either reflected to space or was absorbed by the cloud. And in the total darkness, the temperatures stopped rising,” he says. 

Mass says the temperatures in much of Eastern Washington remained constant for about 12-18 hours, resulting in “profound cooling — from Eastern Washington, all the way into Idaho.”

And he says at night, the opposite happened.

Normally at night, the earth’s surface cools as it radiates infrared energy to space. But the thick volcanic clouds kept the heat in.

“It acted like a blanket,” Mass says.


Mass and Robock looked at the forecasts back then to figure out the magnitude of that effect; they compared the predicted temperatures before the eruption with the reality afterwards. They found a pretty dramatic difference.

“During the day in Eastern Washington, the temperatures were suppressed by 8 degrees centigrade — so that’s like 15 degrees Fahrenheit,” Mass says.  “And then during the night, it was warmed up by probably on the order of 10-12 degrees Fahrenheit.”


But as it turned out, the effects of Mount St. Helens’ volcanic plume were relatively short lived. Many eruptions in recent decades have had profound cooling effects that lasted for years. This kind of impact is considered a change in the local climate. 

Here, there was a key ingredient missing, says Mass: sulfur, which can form sulfur dioxide particles that deflect heat and light, especially when a volcano shoots it high up into the atmosphere.

“Mount St. Helens didn’t do that,” Mass says. Much of the blast stayed in the lower half of the atmosphere. And it didn’t produce much sulfur.

“And so, after a few weeks there was virtually no sign — in the weather, or climate, or anything else — of the volcano,” Mass says.

Fascinating as it remains 40 years later, when it comes to meteorology, “It did not have a long-term effect,” Mass says.

You can listen above to hear the full discussion. 

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to