What condition leads to the formation of ice pellets?

The formation of ice pellets occurs when a shallow layer of above freezing temperatures is situated above a layer of below freezing air. When snowflakes fall through this warmer layer, they partially melt and turn into raindrops. As these raindrops continue to fall and pass through the below freezing layer closer to the surface, they freeze into ice pellets before reaching the ground.

This specific process relies on the unique structure of the temperature layers in the atmosphere, where there is a clear distinction between the warmer air at a certain altitude and the colder air closer to the ground. The ice pellets that result from this interaction often appear during winter months when these temperature profiles are commonly found.

The other conditions mentioned do not facilitate the formation of ice pellets as effectively; for instance, a deep layer of below freezing air would not allow for any melting to occur, while a deep layer of above freezing air at the surface would result in rain rather than ice pellets.