Keep Your Eye On Mt. St. Helens' Ash

Although most of the ash injected from the Mt. St. Helens eruption fell to the surface within a day or so, a tiny fraction was thrown high enough to be above the clouds and weather system. This dust, and secondary particles which form from sulfur gases also injected from the volcano into the atmosphere, can linger in the atmosphere for months. The resulting high-altitude cloud of volcano debris is blown around the world in the jet stream to be eventually spread out to cover the entire northern hemisphere.

In the past, such veils of dust in the stratosphere have created very unusual effects. The most obvious are spectacular sunsets, tinged with violent streaks of red and purple and caused by sunlight reflected from the high-altitude dust layer. The most spectacular sky colorations appear when the sun is three to four degrees below the horizon. Consequently, those people living in central and northern Alaska and in northern Canada will not see the prettiest sunsets until August and September, since the sun will not dip low enough below the horizon until then.

As ash eruptions go, the Mt. St. Helens eruption was moderately respectable. In fact, the material ejected was roughly equal to that of Mt. Vesuvius which erupted in 79 A.D. to destroy the city of Pompeii.

Vastly larger quantities of ash were erupted from the Indonesian volcano Krakatoa in 1883 and the Alaskan volcano Katmai in 1912. Vivid green, blue and red sky scenes in the tropics soon followed the Krakatoa eruption, and beautiful sunsets were seen for three years afterwards. Katmai's ash cloud did nearly as well by producing fiery sunsets up to two years after the eruption. The eruptive products from Katmai absorbed enough sunlight that the light reaching the earth's surface was reduced by six percent.