Extreme Heat:
When the temperature is at or above 100 degrees Fahrenheit, a creature exposed to the heat and without access to drinkable water must succeed on a Constitution saving throw at the end of each hour or gain one level of exhaustion. The DC is 5 for the first hour and increases by 1 for each additional hour. Creatures wearing medium or heavy armor, or who are clad in heavy clothing, have disadvantage on the saving throw. Creatures with resistance or immunity to fire damage automatically succeed on the saving throw, as do creatures naturally adapted to hot climates.
Armor and Extreme Heat:
Characters exposed to the heat and without access to water must make a Constitution saving throw at the end of each hour of exposure (DC 5 + 1 per hour after first) or gain a level of exhaustion. Those wearing Medium or Heavy armors have disadvantage on this save. Anyone foolish enough to wear metal armor in extreme heat also requires double their normal allotment of water.
Resistance to Fire and Heat:
When wearing medium armor you do not need to roll with disadvantage. You only need to make a check every two hours instead of every hour.