A character in very hot conditions (above 90° F) must make a Constitution saving throw each hour (DC 5, +1 for each previous check) or suffer 1 level of exhaustion. Characters wearing heavy clothing or medium or heavy armor have disadvantage on this saving throw. Creatures that are resistant or immune to fire damage automatically succeed on the saving throw. Creatures naturally adapted to hot environments also automatically succeed.
In severe heat (above 110° F), a character must make a Constitution save once every hour (DC 10, +1 for each previous check) or gain 1 level of exhaustion. Characters wearing heavy clothing or medium or heavy armor have disadvantage on this saving throw. Creatures that are resistant or immune to fire damage automatically succeed on the saving throw. Creatures naturally adapted to hot environments also automatically succeed.
For example, around 11 AM the temperature breaks above 90 degrees and the players start making checks. At 2 PM the temperature soars above 110. The players have already made 3 checks, but now the base DC is 10, so the adjusted DC is 13. By 5 PM the temperature drops below 110. The players have made 6 checks now, so the DC is 11 (5+6).