The year 10 class at Marlborough Girls' College asks :-

Snow will not form in air as warm as 4C but large snowflakes can fall into air that warm and not melt completely. Why?

Erick Brenstrum, a meteorologist with the MetService, responded.

Snow typically forms in air with temperatures of minus 10C or colder, which is some distance above the Earth's surface. As snow falls below the level where the temperature is zero - known as the freezing level - it begins to melt. The energy needed to change ice particles to liquid water is taken from the air, which becomes colder. If enough snow falls, the warmer air is eventually cooled to zero and the falling snow stops melting. Gradually the layer of air at zero is deepened and may reach the ground.

Then snow will fall to ground level and, if the ground is not too warm, the snow will settle.

Paradoxically, for heavy snow to sea-level usually requires a current of relatively warm air from the north to feed into the storm. This is because warm air can hold more water vapour than cold air. As the warm air rises inside the storm it encounters lower surrounding air pressure and consequently expands which causes it to cool. This in turn causes some of the water vapour to form ice particles.

In contrast, if the air through the whole depth of the atmosphere comes from the deep south, snow may fall to sea-level, but only in small amounts. Once on the ground, snow acts as an insulator because of the air trapped between snow crystals. On a clear night with no wind, the top surface of the snow may cool to minus 15C or lower while the temperature at the base of the snow stays around zero.

Sheep trapped under the snow can form small snow caves when their body heat melts some snow and so remain sheltered from the extreme cold.