Heat

Make a HT or HT based Survival (Desert) roll (whichever is better) every 30 minutes. This is modified by -1 for Light encumbrance, -2 for Medium, etc.
Failure costs 1 FP. On a critical fail you suffer heat stroke and lose 1d FP
As usual when you go below 0 FP you start to lose 1 HP per FP.
You cannot recover until you move into cooler surroundings.
At hotter temperatures you can lose 1 extra FP for every fail, or even 2 extra.

Intense Heat
After 3 x DR seconds make a HT roll every second.
On failure you lose 1 FP.

Sunburn
After a day of full sun on unprotected skin an albino will be near death, and a light skinned being will be very uncomfortable (1d-3 damage). A darker skinned individual may itch, but aren't in much danger.