Why,when the temperature goes above zero degrees centigrade,does any snow not all melt at once?
Here is a simple experiment to find the answer. Get a cookie sheet and place some snow at one end. Then lift the end with the snow off the table just slightly. Watch as the snow melts. To explain: Snow is cold, obvious yes. A bunch of snow in a pile has the ability to keep itself cold even when the temperature is above zero. Our experiment should melt fairly fast, since there is only a little snow and the temperature of the room is going to be much warmer. Yet, if you watch the experiment you should notice the edge of the pile melting first, causing the pile to move down the tray. Snow that is packed hard, like at a ski lodge, will take longer to melt than loose fluffy snow. This is because as loose snow melts, the water has an easier time flowing downward, causing more snow to melt. To see this within the experiment, hold a hair dryer -at low speed- about 5cm above the pile. Of course the snow will melt from the center outward. Hope this helps.
Contrary to popular belief, snow is a good insulator (mainly because of the air trapped within). To melt, snow has to absorb energy from the environment. Initially, this energy will actually cool the surface of the snow and turn it into ice (wind chill, etc. has an effect). The reflectivity of the snow would stop most of the radiated heat from the sun from imparting more energy to the snow. Only after the snow has been exposed to temperatures significantly above zero degrees Centigrade for a prolonged period will the snow melt.
