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?