Snow that falls on low ground does not lie for long and soon melts away in the first warmth of early spring. On higher ground snow remains for a longer time but even there it is usually all melted away by May. But there are places where even the summer sunshine cannot banish the snow. This is on mountains like the Alps at heights of more than 3,000 meters.

This height is known to geographers as the snowline or the limit of persistent snow. It varies according to location on the globe: in the tropics, for example, the snowline is much higher, at about 5,500 meters, and in the polar regions it is practically at sea-level.

If all the snow that falls on the Earth were to stay on the ground winter after winter, all the highest mountain-tops would be covered many times over. But snow only stays on valleys and hollowed-out mountainsides to form snowfields.

When snow falls it is light and feathery. A piece of snow of this type measuring a cubic meter weighs about 75 kilograms. But as the snow heaps up on the ground its weighs causes the bottom layers to freeze into a hard glassy mass and the weight of a cubic meter rises to about 900 kilograms. The upper slopes of all the world’s mountain ranges are covered in these masses of snow. Once it finds an outlet this frozen snow begins to move slowly like a gigantic river of ice and a glacier is born.