The added weight of wind drifted snow.

Wind erodes snow from the windward (upwind) side of obstacles, such as a ridge, and deposits the same snow on the leeward (downwind) terrain. Wind loading is a common denominator in most avalanche accidents. And no wonder because wind can deposit snow 10 times more rapidly than snow falling from the sky. Moreover, wind-drifted snow is ground up by bouncing along the snow surface and when it comes to a rest it is often much denser than non-wind loaded snow. In other words, it not only adds significant weight on top of buried weak layers but it forms a slab that can propagate a fracture very easily. Wind can turn very safe snow into very dangerous snow in a matter of minutes. Wind is usually the most important weather factor in avalanche accidents.

Luckily, we can often recognize wind loaded slopes:

Wind deposited snow Wind eroded snow
What does it look like? Smooth and rounded, sometimes called “pillows” chalky, dull appearance Sandblasted, etched look
Where does it form? Lee terrain (downwind of an obstacle such as a ridge). Often, a cornice overhangs the slopes Windward terrain (upwind side of an obstacle, such as a ridge). Often a cornice faces away from the slope
What does it feel like? “Slabby” or “punchy,” meaning that denser and stiffer snow overlie softer snow Rough, difficult to travel on
What does it sound like? Sometimes sounds hollow like a drum Noisy from the rough texture

It’s very important to memorize the look, feel and sound of wind loaded slopes.

Always avoid steep slopes with recent deposits of wind drifted snow unless you are experienced, and have checked it out very carefully.