Is a creature that is not on the ground affected by the Sleet Storm spell’s risk of falling prone

area-of-effectdnd-5eflightspellstargeting

The spell sleet storm on page 276 in the Player's Handbook says, in part:

The ground in the area is covered with slick ice, making it difficult terrain. When a creature enters the spell's area for the first time on a turn or starts its turn there, it must make a Dexterity saving throw. On a failed save, it falls prone.

The fact that the ground in the area is covered with slick ice seems to be connected to the reason a creature might fall prone. On the other hand, the spell doesn't explicitly state that the affected creature has to be on the ground.

Are creatures that are not on the ground (e.g. flying or swimming) but are in the area of the spell still affected?

Best Answer

Freezing rain and sleet interfere with flying.

The effect of the slick ground is given explicitly:

The ground in the area is covered with slick ice, making it difficult terrain.

However, the spell's area includes more than just icy ground:

freezing rain and sleet fall in a 20-foot-tall cylinder with a 40-foot radius centered on a point you choose within range.

When it comes to falling prone, the only triggering condition is:

When a creature enters the spell's area for the first time on a turn or starts its turn there

It does not require that you be touching the ground, only that you enter the spell's area, which is:

a 20-foot-tall cylinder with a 40-foot radius centered on a point you choose within range.

So we conclude that a creature only needs to be in the area to trigger the dexterity save against falling prone, whether they be walking or flying. Narratively, this looks like freezing rain and sleet pelting the flying creature as it tries to stay aloft in the torrent.