Rules for terrain within an environment or area.

DIFFICULT TERRAIN

While moving through difficult terrain, you use 2 feet for each foot of movement.

TERRAIN HAZARDS

Rules for terrain hazards.

AVALANCHE

An avalanche is a mass of snow and debris falling rapidly down a mountainside. A typical avalanche is 300 feet wide, 150 feet long, and 30 feet thick. Creatures in the path of an avalanche can avoid it or escape it if they're close to its edge, but outrunning one is almost impossible.
When an avalanche occurs, all nearby creatures must roll initiative. Twice each round, on initiative counts 10 and 0, the avalanche travels 300 feet until it can travel no more. When an avalanche moves, any creature in its space moves along with it and falls prone, and the creature must make a DC 15 Strength saving throw, taking 1d10 bludgeoning damage on a failed save, or half as much damage on a successful one.

When an avalanche stops, the snow settles and buries creatures in it. A creature buried in this way is blinded and restrained and has total cover. The creature gains one level of exhaustion for every 5 minutes it spends buried in the snow. It can try to dig itself free as an action, breaking the surface and ending the blinded and restrained conditions on itself with a successful DC 15 Strength (Athletics) check. A creature that fails this check three times can't attempt to dig itself out again.

A creature that isn't restrained or incapacitated can spend 1 minute freeing another creature buried in the snow. Once free, that creature is no longer blinded or restrained by the avalanche.

FOG

Fog lets you see up to 300 feet.

FRIGID WATER

You can be immersed in frigid water for a number of minutes equal to your Constitution score before you become chilled. Each additional minute you spend in frigid water requires you to succeed on a DC 10 Constitution saving throw or become exhausted. Creatures with resistance or immunity to cold damage automatically succeed on the saving throw, as do creatures that are naturally adapted to living in ice-cold water.

RAIN

A rainy day halves how far you can see.

UNDERWATER

While completely immersed in water or a similar type of liquid, a creature is considered submerged.

WATER CURRENT

A creature that is immersed in a water current moves up to the speed of the current at the end of its turn. 

ICE

A creature that walks on ice must succeed on a DC 10 Dexterity saving throw or be knocked prone. A patch of thin ice can withstand a number of 3d10 × 10 pounds before it breaks.

VACUUM

A creature can't breathe while in a vacuum environment. A suffocating creature that is in a vacuum environment loses 2d6 Hit Points at the end of each of its turns.