Extreme Cold and Heat Rules in 5e
At 0 degrees Fahrenheit or lower a hero makes a DC 10 Constitution save at the end of each hour or gains 1 Exhaustion level; at 100 degrees or higher without drinkable water the DC starts at 5 and rises by 1 each hour. A blizzard has no hit points and no Armor Class. It still kills more heroes than most goblins, because the rule behind it hits every hour and every miss makes the next roll harder. Here are the SRD's environmental effects, quoted, with the odds worked out.
Why weather snowballs
Nearly every hazard here costs Exhaustion. The condition reads:
D20 Tests Affected. When you make a D20 Test, the roll is reduced by 2 times your Exhaustion level.
A saving throw is a D20 Test. So the first failed save against the cold makes the second one 2 harder, and the second makes the third 4 harder. Six levels and the character dies. A Long Rest removes only one. Every number below includes that feedback.
Extreme cold
When the temperature is 0 degrees Fahrenheit or lower, a creature exposed to the extreme cold must succeed on a DC 10 Constitution saving throw at the end of each hour or gain 1 Exhaustion level. Creatures that have Resistance or Immunity to Cold damage automatically succeed on the save.
A character with Constitution +2 passes the first hour on an 8 or better, 65%. Left exposed, here is where they stand:
| Exposed | Con +0 | Con +2 | Con +2, blanket |
|---|---|---|---|
| 4 hours, average levels | 2.1 | 1.6 | 0.6 |
| 8 hours, average levels | 4.8 | 3.9 | 1.3 |
| 8 hours, dead | 46.5% | 24.6% | 1.6% |
Two pieces of the Adventuring Gear list change everything. A Blanket (5 SP, 3 lb.):
While wrapped in a blanket, you have Advantage on saving throws against extreme cold
And a Bedroll (1 GP, 7 lb.):
While in a Bedroll, you automatically succeed on saving throws against extreme cold
So a night in the cold costs nothing if everyone sleeps in a bedroll. The danger is the day: walking, fighting, or keeping watch outside the bedroll. Silver and White Dragonborn have Cold Resistance from their ancestry, and Protection from Energy grants it to one creature for up to an hour of Concentration. Either one passes the save outright.
Extreme heat
When the temperature is 100 degrees Fahrenheit or higher, a creature exposed to the extreme heat and without access to drinkable water must succeed on a Constitution saving throw at the end of each hour or gain 1 Exhaustion level. The DC is 5 for the first hour and increases by 1 for each additional hour. Creatures wearing Medium or Heavy armor have Disadvantage on the save. Creatures that have Resistance or Immunity to Fire damage automatically succeed on the save.
Heat starts gentle and climbs. Drinkable water switches the rule off entirely, so the real question is how long a party goes dry:
| Dry hours, Con +2 | Light or no armor | Medium or Heavy armor |
|---|---|---|
| 4, average levels | 0.8 | 1.5 |
| 8, average levels | 2.8 | 4.6 |
| 8, dead | 7.2% | 36.2% |
| 10, dead | 32.2% | 78.1% |
The Fighter in Chain Mail is the one at risk. Red, Gold and Brass Dragonborn and the Infernal Tiefling ("You have Resistance to Fire damage") cross a desert without a roll.
Frigid water
A creature can be immersed in frigid water for a number of minutes equal to its Constitution score before suffering any ill effects. Each additional minute spent in frigid water requires the creature to succeed on a DC 10 Constitution saving throw or gain 1 Exhaustion level.
The same DC as extreme cold, but every minute instead of every hour. A hero with Constitution 14 has 14 safe minutes. Ten minutes past that, the Con +2 hero averages 4.8 Exhaustion levels and is dead 51.9% of the time. Get out of the water.
Thin ice and slippery ice
Thin ice has a weight tolerance of 3d10 × 10 pounds per 10-foot-square area. Whenever the total weight on an area of thin ice exceeds its tolerance, the ice in that area breaks.
Tolerance runs from 30 to 300 pounds and averages 165. The SRD gives no body weights, so the GM decides what a hero weighs; with gear, a few hundred pounds is common. The odds that one 10-foot square breaks:
| Weight on the square | Ice breaks |
|---|---|
| 150 lb. | 35.2% |
| 200 lb. | 71.7% |
| 250 lb. | 94.4% |
| 300 lb. | 99.9% |
Spread out: one hero per square, never two. Anyone who falls through lands in frigid water.
Slippery ice is Difficult Terrain, and a creature entering it for the first time on a turn, or starting a turn there, must succeed on a DC 10 Dexterity saving throw or fall Prone. The Difficult Terrain post has those odds.
Rain, wind and altitude
Everything within an area of heavy rain or heavy snowfall is Lightly Obscured, and creatures in the area have Disadvantage on all Wisdom (Perception) checks. Heavy rain also extinguishes open flames.
A Perception +3 lookout against DC 15 drops from 45% to 20.25%. Ambushes in a storm work.
Strong wind imposes Disadvantage on ranged attack rolls with weapons. It also extinguishes open flames and disperses fog. A flying creature in a strong wind must land at the end of its turn or fall.
A +5 Longbow against AC 15 falls from 55% to 30.25%. The rule says "with weapons", so Fire Bolt and other spell attacks fire as normal. The wind also grounds every flier, including the party's familiar.
Each hour such a creature spends traveling at high altitude counts as 2 hours for the purpose of determining how long that creature can travel
Above 10,000 feet, four hours of walking uses up the eight-hour day, and anything more triggers the forced-march saves in Travel Pace. Thirty days at altitude acclimates a creature, but not above 20,000 feet unless native there.
At the table
- Pack a bedroll for every hero. It wins the cold night outright.
- Carry water into the heat. It wins the hot day outright.
- Put the heavy-armor hero in light clothes for a desert crossing, and armor up at the destination.
- Count the minutes in icy water: Constitution score, then a DC 10 save every minute.
- On ice, one hero per 10-foot square.
- In strong wind, casters attack and archers wait.
The Book of Dice covers the d20 and how Disadvantage moves your odds in Part III, The Rolling.
This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License.
