Source: Steam Community (opens in a new tab)

During the course of the game, an event may start that changes the conditions of the game or makes it more difficult.
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Solar flare
Exposure to radiation throughout the location except for safe or underground structures.
Toxic precipitation
If it starts raining during the event, a player with insufficient protection against toxins will start getting poisoned.
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Eclipse
Reduced natural light and consequently more difficult searches.
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Aggressive animals
Increased chance of animal attacks, in places where they may be.
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Gang activity
Increased chance of ambush or encounter with raiders.
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Cold snap
A sudden drop in temperature. Warm clothes or warming up by the fire will help prevent frostbite.
The duration and power of an event can have different meanings.