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Describes the process where a gas changes directly to a solid without going through the liquid state.

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Dangerous Deposit

Dangerous Deposit


License: CC BY-NC 3.0

Former President Gerald Ford, pictured here, was an avid skier. Do you like to ski? If you do, then you probably know that skiing can be a dangerous sport. Running into other skiers or trees is one risk. Avalanches are another.

Why It Matters

  • An avalanche is a sudden downhill rush of a large mass of snow. One of the biggest causes of avalanchesis surface hoar, or surface frost.
  • Surface hoar forms when water vapor in the air contacts cold surfaces and changes directly to ice crystals. This change of state is called deposition.
  • Credit: kcxd
    Source: https://www.flickr.com/photos/by-sgillies/4612006879
    License: CC BY-NC 3.0

    Surface hoar is a fragile layer of frost that accounts for many avalanches each year [Figure2]


  • Surface hoar often forms on top of snow. If more snow falls on top of the frost, the frost forms a weak layer of ice crystals within the snowpack. This increases the risk of avalanches.
  • Why does it matter? You’ll see why when you watch the following video. It shows three expert skiers barely escaping from an avalanche in the Swiss Alps.


Show What You Know

Learn more about surface hoar and avalanchesat the link below. Then answer the questions that follow.

  1. Describe how surface hoar forms. Explain the conditions needed for it to form.
  2. In what type of climate is surface hoar especially likely to form? Why?
  3. Describe specific places where surface hoar might form.
  4. Why is a layer of frost in snowpack difficult to detect? What is the best way to detect it?
  5. How and why does surface hoar cause avalanches?

Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ Credit: kcxd; Source: https://www.flickr.com/photos/by-sgillies/4612006879; License: CC BY-NC 3.0

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