You conjure up a roiling mass of flame that envelopes an area within range. The area can be up to 30 feet long, up to 10 feet wide, and up to 10 feet tall. Each creature in that area must make a Strength saving throw. On a failed save, a creature takes 8d8 fire damage and is knocked Prone. On a successful save, a creature takes half as much damage and isn’t knocked Prone. The fire then continues to move 20 feet per round on initiative 20 out across the ground in a straight line.

Each creature that ends its turn in the area of Burning Air must succeed on a Strength saving throw or take 8d8 fire damage and be pushed 20 feet following the line of movement of the Burning Air. A successful save halves the damage and the creature is not pushed. A creature takes the same damage and effect when it enters the area for the first time on a turn.

Any creature in the area of effect must spend 2 feet of movement for every 1 foot it moves. Ranged attacks into or through the area effect are at disadvantage.

The Burning Air disperses gas or vapor, and it ignites candles, torches, and similar unprotected items in the area. 

As a bonus action on each of your turns before the spell ends, you can change the direction in which the line moves.