1. Metamorphism Due To Direct Weather Effects Learning Outcomes Understand the effects of direct weather on the snowpack. Understand melt-freeze and its effect on the snowpack. Know the conditions that are conducive to melt-freeze metamorphism. American Institute for Avalanche Research & Education Level II Avalanche Course

2.  This lesson discusses metamorphic processes in the snowpack.  It includes changes on the surface or in the upper layers of the snowpack due to the direct effects of weather.  The formation of crusts due to the effects of wind, rain, temperatures, solar radiation, and riming is discussed as well as melt-freeze metamorphism.  In addition, I review decomposition and fragmentation, an early stage metamorphic process that occurs at/near the surface and is strongly influenced by direct weather factors. American Institute for Avalanche Research & Education Level II Avalanche Course Metamorphism Due To Direct Weather Effects

3. Review A particle of new snow in the air a snow crystal. On the ground and part of the snowpack the common term is snow grain. The snowpack consists of various layers created by differences in snow crystals, timing of snowfall, and variations in weather conditions at the time when the snow fell. Once on the ground, the layers of the snowpack are not static, the snow grains that make up the layers change over time, which in turn changes the characteristics of the layers themselves. The process of change in the snow grains is called metamorphism and the changes that occur are driven primarily by weather. The effects of weather can be divided into two general categories: direct weather effects and indirect weather effects. Metamorphism Due To Direct Weather Effects

4. Direct weather factors:  Wind  Rain  Temperatures  Solar radiation American Institute for Avalanche Research & Education Level II Avalanche Course The most common result of direct weather effects on the layers at or near the surface in winter and early spring is the formation of crusts. Later in the season, when temperatures climb, solar radiation is strong, and the snowpack melts but does not refreeze the result is wet, slushy snow.

5. As wind redistributes the snow it changes the grains. Wind deposited snow breaks up into smaller parts and pack together more tightly which forms a harder, denser layer a wind crust or wind slab at the surface of the snowpack. Metamorphism

6. High levels of humidity and warm temperatures enhance the formation of wind slab and crust Warm snow and humid air tend to increase bonding between grains as they are moved by the wind. Metamorphism

7. Snow grains that have been broken into small pieces and packed by wind are properly referred to as “broken” grains or wind crust, or wind slab depending on the extend of the effect. If the snow surface is cold, rain may freeze immediately upon striking the surface. A crust formed in this manner is easy to differentiate from a melt- freeze crust which forms from a different process and has a distinct appearance. Metamorphism The result is a rain crust. Rain crusts from freezing rain have a clear, icy appearance and, when well defined, are smooth and glassy.

8. Rain can infiltrate and wet the surface snow without completely melting the snow grains into liquid water. In this case, snow grains are surrounded by a film of free water. When this mass freezes, a crust forms. This is not a true rain crust but is actually a melt-freeze crust. A melt-freeze crust has a white, opaque appearance and does not look like frozen water like a freezing rain crust but looks like frozen snow, which, in fact it is. Metamorphism

9. If the snowpack is not cold enough to create a freezing rain crust and not dense enough to prevent penetration, or if rainfalls are extensive, liquid water (rain itself or rain mixed with melted snow) can percolate into the snowpack. While generally limited to the upper layers, percolation can, penetrate deep into the pack or even to the ground. When liquid water percolates into the snowpack, melt channels form and water pools. Metamorphism Around the channels and pools, melt- freeze grains are often formed where liquid water invades the surrounding snow. When channels and pools freeze, they form vertical and horizontal, irregular crusts or chunks of ice (ice lenses).

10. High temperatures warm the surface or very top layer of snow which causes crystals to break into smaller parts and pack closer together, forming a crust. If temperatures rise above the freezing point, a melt-freeze crust will form when the temperatures drop again. These crusts can look similar to rain crusts, especially if melting has occurred. However they are referred to as a temperature crust if the mechanism is known. Metamorphism The solar radiation from strong sun warms or melts the surface snow and can have similar affects as temperature. Crusts formed in this manner are referred to as sun crust if the mechanism is known.

11. Rain can infiltrate and wet the surface snow without completely melting the snow grains into liquid water. In this case, snow grains are surrounded by a film of free water. When this mass freezes, a crust forms. This is not a true rain crust but is actually a melt- freeze crust. A melt-freeze crust has a white, opaque appearance and does not look like frozen water like a freezing rain crust but looks like frozen snow, which, in fact it is. Metamorphism Field symbols crust melt-freeze crust ice (ice lenses)

12. The depth to which direct weather affects are felt is not clearly defined. The strongest effects are at the surface or in the upper layer(s). This depends on the intensity of the weather and the characteristics of the layers being affected. Effects of direct weather Metamorphism Discussion

13. Over a period of days or longer, when a combination of solar radiation and temperatures cause the snowpack to undergo a number of melting/freezing cycles, melt-freeze metamorphism occurs. Here, free water forms in the snowpack during the warm part of the cycle, increasing in amount with each successive cycle. Melt freeze grains form which are increasingly spherical and clear and “icy” in appearance. Melt-freeze metamorphism With each successive melt-freeze cycle large grains grow at the expense of small grains (not unlike rain drops on a pane of glass coalescing).

14. In the advanced stages of melt-freeze metamorphism there are no small grains left, and the large grains that remain are very uniform in size and shape. It takes a number of strong cycles to get things to this stage and you can tell when it has happened because this is when corn skiing starts to become good. Melt-freeze metamorphism Melt-Freeze Metamorphism - there is a significant amount of free water in the snowpack and the grains are completely surrounded by water in the melt phase and strongly bonded in a lattice of ice when in the frozen stage.

15. The effect of melt freeze metamorphism is relatively easy to assess as it is due to direct weather effects.  Primarily sun  Warm temperatures  Occurs at or near the surface. It creates very strong snow in the frozen phase. When melted, the snow is very weak. One needs little in the way of experience, skills, or tools to assess the effects of melt- freeze metamorphism. Melt-freeze metamorphism Field Symbol wet grains (melt-freeze grains in melt cycle) O

16. The conditions that promote melt-freeze metamorphism: High daytime temperatures: Adds heat to the snowpack Strong solar radiation: Warms the snowpack further Cold nighttime temperatures: Allows freezing to occur Recurring cycle of melting during the day and freezing at night: Quickly increases the water content of the snow and causes grains to become larger and more uniform.

17. The conditions that promote melt-freeze metamorphism are: Clear nights: Enhances cooling of the snowpack due to radiation loss High density snow: High water content Rain: Adds water to the snowpack Sunny aspects: Increase solar radiation effect Steeper slopes: Increase solar radiation effect

18. Melt-freeze grains, melted and frozen clusters

20. Melt-freeze grains, frozen state

21. Metamorphism Due to Direct Weather Effects Summary · Understand the effects of direct weather on the snowpack. · Understand melt-freeze and its effect on the snowpack. · Know the conditions that are conducive to melt-freeze metamorphism.

22. Metamorphism Due to Direct Weather Effects Conditions that promote melt-freeze: · High daytime temperatures · Strong solar radiation · Cold nighttime temperatures · Recurring cycle of melting and freezing · Clear nights · High density wet snow · Rain · Sunny aspects · Steeper slopes