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Monsoon: Highly regular seasonal winds that blow steadily from land to sea during the cool season and from sea to land during the warm season. It was first applied to winds over the Arabian Sea, which blow for six months from the northeast and for six months from the southwest. The word is derived from the Arabic mausim, meaning "season".

The primary cause of the monsoon is the much greater annual variation of temperature over large land masses compared with the neighboring ocean surfaces. In the winter, air over the land is colder (generating higher pressure) than air over the water (lower pressure), and wind blows from higher to lower air pressure; that is, from land to water in the winter. In summer, the situation reverses: Air over land is hotter than air over water, and the monsoon blows the other way; that is, from water to land in the summer.

Moist oceanic air moving inland (the summer regime) generates abundant warm-season rainfall in areas subject to the monsoon. By extension, the word "monsoon" is applied to the rain which it brings, though this is technically incorrect.

On Sunday, May 18, 1980, around 8:32 a.m. -- 30 years ago -- Mount St. Helens, dormant since about 1855, erupted violently. The mountain had been rumbling for several weeks prior to May 18 and volcanologists from the U.S. Geological Survey knew an eruption was imminent, but they had no way of knowing exactly when it would blow. Magma had been rising into the mountain and a massive bulge had developed on its north face (see photograph).
 
Here's a picture of Geologist Don Swanson measuring growth of the bulge on the north slope of Mount St. Helens on April 27, 1980---three weeks before the eruption:

USGS photo

Measurements indicated the bulge was expanding by 1.5 meters (4.9 feet) per day. At 8:32 a.m., a magnitude 3.3 earthquake beneath the mountain caused the unstable north flank to slide, essentially unleashing the magma chamber. The eruption began. One blast pushed upward, but a much larger horizontal blast occurred simultaneously.

Geologist Dan Swanson was one of many scientists monitoring Mt. St. Helens and he has the comments:

"We thought the eruption would be vertical like a normal volcanic eruption, although we knew there was a possibility that there could be a flank failure. But we thought these things were rare. We didn't know much about it. A big lesson was that, indeed, there can be a massive failure of volcano flank. The result of the failure is a debris avalanche. People started recognizing debris avalanches around the world, like at Mt. Shasta in California. A light bulb came on. Within one to two years roughly 100 avalanches were recognized around the world. It was a marvelous kind of awakening."
 
"Mt. St. Helens eruption was important because it was so well observed. It provided a kind of model for jump-starting volcano monitoring around the world. Existing observatories got increased funding and people learned how to respond."