Dispersal
The distribution of pyroclastic ash depends largely on the direction of wind at intermediate and high altitudes between approximately 4.5 – 13 km. The general trend of pyroclastic dispersal is shown using isopachs (which are analogous to topographic map contours though they illustrate lines of equal thickness rather than elevation) and show the dispersal as elongated with wind direction.
The Krakatoa (Indonesia) eruption of 1883 produced an eruption column which rose to more than 50 km. An ash flow from this explosion was recognised 2,500 km west of the volcano. The total area of recognisable pyroclastic fall was greater than 800,000 km². The pyroclastic ash encircled the globe in 13.5 days and at altitudes of between 30 and 50 km the average velocity was 12 km/h. The ash remained in the upper atmosphere and produced brilliant sunsets for many years, lowered the global temperature by 0.5 °C for at least five years.
The 1912 eruption in the Valley of Ten Thousand Smokes (Alaska) covered an area greater than 100,000 km² to a depth of six mm.
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