Water from Snow: The Fuel in a Litre of Meltwater, and Why Eating Snow Does Not Work

Melting snow is cheap in fuel and expensive in the body, and this paper prices both sides of that contrast. Turning one kilogram of snow at minus 10 degrees Celsius into one litre of drinking water at 15 degrees Celsius requires 417.7 kilojoules of heat: 20.9 kilojoules to warm the ice to its melting point, 334.0 kilojoules to melt it, and 62.8 kilojoules to warm the water. On a stove of 40 percent efficiency burning wood at 14.4 megajoules per kilogram, that is 72.5 grams of wood per litre — 0.181 kilograms for a full day's 2.5 litres, worth about 26 seconds of gathering work at 25 kilograms per person-hour — plus the handling of 5.0 litres of settled snow, or 10.0 litres of fresh snow, for every litre of water obtained. Eating the snow instead moves the entire energy bill into the body and raises it: the body must warm the meltwater to 37 degrees, not 15, so each litre costs 509.8 kilojoules, or 121.8 kilocalories, equal to 1.77 hours of resting heat production. A day's water eaten as snow costs 304.5 kilocalories, and if that heat were drawn from the body's core without compensation, the core temperature of a 75-kilogram person would fall by 4.90 degrees, from 37.0 to 32.1 degrees Celsius — deep hypothermia. The claim that a person without fuel can simply eat snow for water does not survive the arithmetic. Eating snow is not a way of obtaining water without paying for the melting; it is a way of paying for the melting with the body's own reserves, at a worse rate, in the organ that can least afford the loss.