Predicting the different types of snow

The Economist : Eskimos have numerous ways to describe different types of snow, whether its ‘sticky,’ flakey, wet, or dry. And as the recent snowstorms on the East coast of the US have shown, the type of snow that falls on the ground (heavy and wet with the first snowstorm, leading to power outages, felled trees, and blocked roads for days, and light and fluffy for the second, which caused less disruption) can have a big impact on whether utilities and governments can still provide core services.Forecasting what sort of snow will fall is not easy. But Jim Steenburgh and Trevor Alcott from the University of Utah think they have found a correlation between weather conditions at the time of a snowfall and the amount of water in the snow that fell.This is called the snow-to-liquid ratio (based on the area of snow cover and how much liquid it would produce when melted), and the higher the ratio, the drier and more powdery the snow.Steenburgh and Alcott found that temperature plays a large part in controlling snow density. Where that temperature pertains, though, is crucial. It is the temperature above the crests of the range, where the snow crystals form, that seems to matter most. When this is around freezing point, the snow formed is dense (ie, the value of the ratio is low). As the temperature falls, snow gets drier and fluffier because of the formation of dendrite crystals that contain lots of spaces. As conditions get colder still, though, the crystals become more compact again, and the snow gets heavier.