Zones.

The map is divided up, and the zone you are standing in decides how hard it is trying to kill you.

Safe

Traders, storage and a place to plan. Nothing spawns on you here and you cannot be shot at.

Livable

Ordinary ground. The infected are around, but the pressure is low enough to build and hold something.

Radiation

Health drains the whole time you are inside. Bring protective gear, take what you came for, and leave.

Everywhere else

Outside the mapped zones the world stops looking after you at all. This is where the better loot is.

An in-game map of Albuquerque, with the trader and spawn areas marked
A map in hand names what is around you — the settlement you are in, and where the traders and the spawn sit inside it. There is no minimap: this, your compass and your own memory are all you get.

Weather.

Weather is not decoration here. Three of the seven kinds do something to you directly, and you can see all of them coming.

Sky What it does to you
Clear Nothing. Travel while it lasts.
Rain Wet and dark. Harder to see, harder to be seen.
Mist Visibility collapses and the fog carries sound. You will hear things you cannot place.
Storm Heavy rain and lightning across the map.
Radiation rain Takes health every few seconds while you are out in it, and leaves you nauseous. Get under cover.
Supercell Constant lightning and wind strong enough to drag you off your line. Tornadoes form inside it.
Nuclear fallout Poison and hunger for as long as it lasts. The worst sky the server has.

Events.

These fire on their own timers and everyone online gets them at once. Some are an opportunity. Some are a reason to be somewhere else.

Supply drop

A crate lands somewhere on the map, every half hour to an hour. Good loot, and everyone else saw the same announcement you did.

Zombie horde

The most frequent one, every fifteen to thirty minutes. A mass of infected arrives somewhere and starts moving.

Roaming horde

Fifteen to twenty-five infected walking a route behind a dreadnought. Do not try to hold ground against it alone.

Bombardment

Shells land across an area for a while. Being indoors helps. Being elsewhere helps more.

Two that arrive another way.

Tornado

Not on a timer — a supercell throws one. It touches down about sixty blocks away and stays for two minutes. If the sky has already turned, you have had your warning.

The nuke

A player sets this one off. Everyone online gets a countdown on their screen before it lands, which is the only reason anyone survives it.

A nuclear fireball rising over a burning treeline at night
The nuke The countdown ends and the treeline goes up. The fires it leaves do not put themselves out.
A tornado touching down between tower blocks in a ruined city
Tornado Sixty blocks away and closing. Two minutes on the ground is long enough to lose whatever you were standing next to.
Shells falling through the sky above a fenced compound
Bombardment You see them before you hear them. The broken leg in the corner is what happens if you are still outside.