### Instruction:
When do avalanches occur?

### Response:
Avalanches are primarily caused by unstable snow conditions. Although a mountainside covered in snow might appear as a uniform surface, it is actually made up of many layers of snow from different storms, each of which might have very different compositions. For example, a "dry" snow from a very cold storm might land directly on top of a sun-baked layer which turned to ice. The point at which these layers meet is called an interface, and is where slipping or sliding of one layer on top of another is most likely to occur. This occurrence is what can trigger an avalanche. Snow conditions on a slope can also evolve over time with wind and sun and other weather patterns, so even a once-safe slope might increase in risk under certain conditions. People recreating in the outdoors in the winter can benefit from learning about these situations from avalanche professionals.