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In winter, snow-forming clouds are still mostly made of liquid water droplets, even when the temperature is below freezing. The water is said to be super-cooled, simply meaning that it is cooled below the freezing point. As the clouds get colder, however, the droplets start to freeze, and fall as snowflakes. This begins to happen around −10°C, but it's a gradual process; the droplets don't all freeze at once.

Warm-Blooded Animals and the Cold

Our bodies produce heat at a constant 37°C, with a slight change of 0.5°C lower in the morning and 0.5°C higher in the evening. A body temperature of above 38°C denotes a fever, and hypothermia sets in once the body temperature drops below 35°C. Extra energy is needed when the body is confronted with cold temperatures which draw that heat away. Humans put on clothes to retain that heat, while animals (unless cold-blooded) are protected from the outside temperatures by fur and layers of fat. When animals lose their insulation layer, they are at risk; their survival depends on adequate food intake. The availability of their food source determines whether or not they will survive.