D&D 5E Optional Rules on Heat and Exhaustion

Borrowed shamelessly from the D&D Wiki on Heat Dangers and adapted for 5E:

HEAT DANGERS

Heat deals nonlethal damage that cannot be recovered until the character gets cooled off (reaches shade, survives until nightfall, gets doused in water, is targeted by endure elements, and so forth, and spends at least a Short Rest out of direct heat), as measured by Exhaustion Counters (PH 291).

• A character in very hot conditions  (above 90° F) must make a CON Saving Throw each hour in direct heat (DC 15, +1 Cumulated since last check) or suffer +1 level of Exhaustion.

• A character in severe heat (above 110° F) must make a CON save once every 10 minutes in direct heat (DC 15, +1 for each cumulative check) or suffer +1 level of Exhaustion.

• A character in lethal heat  (above 140° F, Fire, Lava, Boiling Water) takes physical damage, 1d6 per minute (no save).

Saving Throw Modifiers:

1. Characters wearing heavy clothing or armor of any sort take -2 penalty to their saves.

2. Characters with Survival Skill may receive their skill bonus on the Saving Throw.


THIRST

A character must have 1 gallon of water per day. If they don't, they suffer +1 Level of Exhaustion from dehydration.

Water skins are 5lbs full (PH 150) and contain 4 PTS of water (PH 153). 8 PTS make 1 gallon.

SANDSTORMS

Sandstorms are a heavily obscured area (PH 183), and characters suffer from the blinded condition.