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How an Avalanche Forms.

Each year more than one million avalanches, or snowslides, barrel down Earth's mountains. They range from harmless bursts of sliding snow to powerful blasts--some striking with the force of 200 pounds of dynamite. How do scientists predict when an avalanche will slide? They study and measure the strength of a slope's snowpack, the accumulation of different layers of snow. The cross-section of a snowpack shows how its layers interact to form an avalanche.

Snowflakes fall and then form separate layers on the ground, The flakes, or snow crystals, on each layer tend to have the same general shape. But as snow piles up, and air and ground temperatures change, water molecules in crystals take on a new shape. At any one time the shape of the crystals determines how well they hold together.

If the crystals in a new layer of snow are cohesive, or "sticky," and form bonds with the layer below, the snow won't slide. If the bonds between the crystals either within or between layers break, the snow will begin to slip.

1 A cornice is an overhang of snow formed by wind. Gravity's force can cause it to break off. 2 Fresh snow adds extra weight co a snowpack--especially if it's falling fast, giving old snow buried beneath little time to adjust. How tightly one layer clings to another depends on the structure of its ice crystals. Rain or melted snow weakens the bonds between the crystals. 3 When water refreezes, a slippery layer called a sun or rain crust forms. 4 Fluffy snow crystals make an unstable base for heavier snow. 5 Surface hoar crystals are large, loosely bound flakes. They turn dangerous when buried by heavier snow. 6 Although old snow is usually tightly packed, it may not bond with other layers. 7 Grainy, sugarlike depth hoar crystals will slide and take all the above layers with them if their weak bonds break apart. These crystals form when water vapor rising from the "warm" snow nearest the ground refreezes quickly.

Three Types of Avalanches

A Soft-slab avalanches usually form when a relatively new layer of snow breaks off and slides. The slab breaks apart, dissolving into a huge cloud. The powdery mass can race downhill at speeds of 161-322 kilometers (100-200 miles) per hour, pushing a powerful blast of air ahead of it.

B Hard-slab avalanches are usually made up of old, dense snow compacted by wind. When the top layer slides, it breaks up into large, cementlike chunks--not powder. These deadly fragments--sometimes as big as a car--can reach speeds up to 80 km/h (50 mph).

C Wet-slab avalanches rarely develop unless there is liquid water mixed with snow--usually from spring thawing or rain. When a wet slab begins to slide, it won't rush or tumble down a mountain. Instead, it will creep along at no more than 16 km/h (10 mph).

Three Types of Avalanches

A Soft-slab avalanches usually form when a relatively new layer of snow breaks off and slides. The slab breaks apart, dissolving into a huge cloud. The powdery mass can race downhill at speeds of 161-322 kilometers (100-200 miles) per hour, pushing a powerful of air ahead of it.

B Hard-slab avalanches are usually made up of old, dense snow compacted by wind. When the top layer slides, it breaks up into large, cement-like chunks--not powder. These deadly fragments--sometimes as big as a car--can reach speeds up to 80 km/h (50 mph).

C Wet-slab avalanches rarely develop unless there is liquid water mixed with snow--usually from spring thawing or rain. When a wet slab begins to slide, it won't rush or tumble down a mountain. Instead, it will creep along at no more than 16 km/h (10 mph).

Use the diagram shown here to answer the following questions:

1. Which type of avalanche re,aches the highest speed?

2. Which is the slowest?

3. Which type of avalanche forms the largest--and deadliest--chunks of snow?

4. Which is the least common kind of avalanche?

5. Which type of avalanche pushes a strong blast of air along in front of it?

6. When a fairly new layer of snow breaks off and slides downhill, what type of avalanche does it create?

7. When a top layer of old, compacted snow slides, what kind of avalanche roars downhill?

8. Which type of avalanche causes snow tO become a powdery mass?

THREE TYPES OF AVALANCHES, p. TE5

1. soft-slab

2. wet-slab

3. hard-slab

4. hard- slab

5. soft-slab

6. soft-slab

7. hard-slab

8. soft-slab
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Publication:Science World
Article Type:Brief Article
Date:Mar 12, 2001
Words:755
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