The sun doesn't instantly melt all the snow on mountains primarily because of the high specific latent heat of fusion required to change snow (solid ice) into water (liquid). Essentially, it takes a significant amount of energy to break the bonds holding the ice crystals together, even when the air temperature is above freezing.
Here's a breakdown of the factors involved:
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Specific Latent Heat of Fusion: This is the amount of heat energy needed to change a substance from a solid to a liquid without a change in temperature. Water has a relatively high latent heat of fusion. That means a lot of the sun's energy goes into breaking the bonds between water molecules in the snow before the temperature of the snow can rise high enough to turn it into water.
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Albedo (Reflectivity): Snow is highly reflective, meaning it bounces a large portion of the sunlight that hits it back into the atmosphere. This reflected solar energy doesn't contribute to melting the snow. Fresh snow can reflect as much as 80-90% of incoming sunlight.
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Angle of Incidence: The angle at which sunlight strikes the mountain slopes affects how much energy is absorbed. Steeper slopes may receive sunlight at a more direct angle, leading to more absorption and faster melting, while shallower slopes receive sunlight at a less direct angle, causing more reflection.
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Air Temperature: While air temperature plays a role, it's not the only determining factor. Even if the air temperature is above freezing, the energy from the sun might not be sufficient to overcome the latent heat of fusion, especially if the albedo is high.
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Surface Area: The vast surface area of snowpack on a mountain dissipates the sun's energy. The heat absorbed is spread across a large area, reducing its localized effect on melting.
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Altitude and Location: Higher altitudes are generally colder, slowing down the melting process. The aspect (direction the slope faces) of a mountain also impacts how much sunlight it receives. South-facing slopes in the Northern Hemisphere get more direct sunlight and melt faster than north-facing slopes.
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Air Convection and Evaporation/Sublimation: Some of the heat absorbed by the snow is transferred to the air through convection. Additionally, snow can directly transition from solid to gas (sublimation) or undergo evaporation once melted, both of which require energy and cool the surrounding snow.
In short, the combination of the high energy required to melt ice, the snow's reflectivity, and other environmental factors prevents the sun from quickly melting all the snow on mountains. If not for these factors, the rapid melting could lead to catastrophic floods.
