A substance sublimates when it reaches its triple point, the atmospheric pressure and temperature at which it can exist in a solid, liquid and gaseous state. Sublimation is the transition of a solid directly to a gas without passing through the liquid state.
Atmospheric pressure is the force exerted on a surface by the weight of the air above it. One standard atmosphere is a unit of pressure equal to about 14.7 pounds per square inch. At one standard atmosphere, water transitions from solid to liquid to gas as it is heated from below its freezing point. The pressure at water's triple point is 0.006 standard atmospheres, and at temperatures above its melting point at or below this pressure, water sublimates.
The most common substance to sublimate is solid carbon dioxide, or dry ice. Carbon dioxide has a triple point at a pressure higher than 1 standard atmosphere, the normal atmospheric pressure on Earth, and a melting point at minus 109.3 degrees Fahrenheit. Dry ice gets its name from the fact that it sublimates, and never becomes liquid, under normal atmospheric conditions. To prevent sublimation, dry ice is kept at extremely low temperatures to keep frozen, or high pressures to keep in a liquid state.