System Reference Document 5.2.1
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Environmental Effects
Deep Water
Swimming through deep water (more than 100 feet 
deep) presents additional challenges because of the 
water’s pressure and cold temperature. After each 
hour of swimming in deep water, a creature that 
lacks a Swim Speed must succeed on a DC 10 Consti -
tution saving throw or gain 1 Exhaustion level.
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.
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 Re -
sistance or Immunity to Fire damage automatically 
succeed on the save.
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. Creatures with Resistance 
or Immunity to Cold damage automatically suc -
ceed on the save, as do creatures that are naturally 
adapted to living in ice-cold water.
Heavy Precipitation
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.
High Altitude
Traveling at altitudes of 10,000 feet or higher above 
sea level is taxing for most creatures because of the 
reduced amount of oxygen in the air. 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 (see “Travel Pace”).
 Creatures can become acclimated to a high alti -
tude by spending 30 days or more at this elevation. 
Creatures can’t become acclimated to elevations 
above 20,000 feet unless they are native to such 
environments.
Slippery Ice
Slippery ice is Difficult Terrain. A creature that 
moves onto slippery ice for the first time on a turn 
or starts its turn there must succeed on a DC 10 
Dexterity saving throw or have the Prone condition.
Strong Wind
Strong wind imposes Disadvantage on ranged at -
tack 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 strong wind in a desert can create a sandstorm 
that imposes Disadvantage on Wisdom (Perception) 
checks.
Thin 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. All creatures on broken ice fall 
through. Below the ice is frigid water (see “Frigid 
Water” above).