uncommon conjuration fire 

Cast [>>] somatic, verbal

Range 30 feet; Area 10-foot burst

Saving Throw Reflex; Duration sustained up to 1 minute

A thin layer of ash and flame covers the ground in the area. The area becomes hazardous terrain. A creature that moves on the ground through the area takes 1 fire damage for every square of that area it moves into. A creature that ends its turn in the area must succeed at a Reflex save or take 1 persistent fire damage.

Each time you sustain this spell, the radius of the burst increases by 5 feet.

Heightened (+2) Increase the damage and persistent damage by 1.

Secrets of Magic, page 201.