Snow Melting
Why does snow melt in the sun at temperatures below freezing?
The rate of snow melt is dependent on energy availability, which is mostly in the form of radiation. Cold snowpacks have a negative energy balance, but warming causes the snowpack to become isothermal and additional energy results in positive energy balance and melt. Daily snow melt in forested areas is considerably less than melt in open areas, as forests protect the snow cover from solar radiation and wind. Canopy warming can increase longwave radiation, but the net effect of forest is reduction in melt. Rain falling on snowpack may accelerate its melt rate, but intense sunshine of late spring and summer is the principal melting energy source.
Most operational procedures for snow melt prediction rely on ambient air temperature as the index of the energy available for melt. The temperature index is usually used to characterize the level of the energy balance because it is superior to other simple methods for the full energy balance at the snow surface.
As long as snow remains bright, the sunshine bounces off and is not retained to cause any melting. Two factors other than sunshine which directly spells the demise of snow are air temperatures above freezing and contact with dark material that absorbs the heat of the sun. Early in the spring, the air temperatures usually are not significant in the melting process, so sunshine by itself does little to deplete the winter's accumulation of snow. But if you sprinkle some ashes, sand or other dark material on the snow, within a few days the snow will start disappearing. This is because those dark materials absorb and conduct the solar radiation they've received.
The process that explains why ice melts first around the stems of plants which extend through the ice lies in the combination of direct and reflected solar radiation falling upon the shrub stems. Shrub bark is quite dark and readily absorbs much of the sun's energy rays rather than
The rate of snow melt is dependent on energy availability, which is mostly in the form of radiation. Cold snowpacks have a negative energy balance, but warming causes the snowpack to become isothermal and additional energy results in positive energy balance and melt. Daily snow melt in forested areas is considerably less than melt in open areas, as forests protect the snow cover from solar radiation and wind. Canopy warming can increase longwave radiation, but the net effect of forest is reduction in melt. Rain falling on snowpack may accelerate its melt rate, but intense sunshine of late spring and summer is the principal melting energy source.
Most operational procedures for snow melt prediction rely on ambient air temperature as the index of the energy available for melt. The temperature index is usually used to characterize the level of the energy balance because it is superior to other simple methods for the full energy balance at the snow surface.
As long as snow remains bright, the sunshine bounces off and is not retained to cause any melting. Two factors other than sunshine which directly spells the demise of snow are air temperatures above freezing and contact with dark material that absorbs the heat of the sun. Early in the spring, the air temperatures usually are not significant in the melting process, so sunshine by itself does little to deplete the winter's accumulation of snow. But if you sprinkle some ashes, sand or other dark material on the snow, within a few days the snow will start disappearing. This is because those dark materials absorb and conduct the solar radiation they've received.
The process that explains why ice melts first around the stems of plants which extend through the ice lies in the combination of direct and reflected solar radiation falling upon the shrub stems. Shrub bark is quite dark and readily absorbs much of the sun's energy rays rather than