The added weight of wind drifted snow.
Wind Loading:

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 easily 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.

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab