The change in temperature over snowpack depth.

Temperature gradient is a very important in the snowpack because when temperature changes more than about 1 degree centigrade per 10 centimeters, the snow metamorphoses into weak, sugary, faceted crystals, which cause the lion’s share of avalanche accidents. Large temperature gradients usually occur when cold, clear weather causes the snow surface to become very cold, or if the snow is especially shallow—or both. (See faceted snow.) Luckily, temperature gradient metamorphism is completely reversible; when you take away the temperature gradient, the crystals start to metamorphose back into rounded, well-bonded grains.