From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon outbreaks (Japanese: 大量発生 mass outbreak), also known as swarms, mass outbreaks, or wild outbreaks, are a feature introduced in Generation II that allows the player to catch Pokémon that are usually rare or otherwise unobtainable. They appear in every generation after their introduction except Generation VI.
Pokémon outbreaks originated in Generation II. In that generation, if the player registered a certain Trainer in the Pokégear, that Trainer would occasionally call to tell the player that he saw or heard about a large number of Pokémon in a certain area; for the next 30 minutes after such a call (in real time, not play time), the swarming Pokémon would appear in the given area with much greater frequency than the usual 1%.
In Generation III, in addition to making rare Pokémon temporarily more common, this feature can be combined with record mixing to allow the player to catch Pokémon that cannot be caught at all normally.
The player may view televisions in houses to see if and where an outbreak is occurring. Outbreaks are determined randomly each day and last for 24 hours. During an outbreak, the swarming Pokémon will have an appearance rate of 50%.
The record mixing feature of Pokémon Ruby, Sapphire, and Emerald allows players to give an outbreak they've seen to other players, regardless of whether that outbreak would naturally occur in the player's game. The traded outbreaks will be announced on television in the following days after mixing records.
The Seedot and Nuzleaf that break out in Emerald will know the Egg Move Leech Seed.
Pokémon outbreaks were largely expanded in Generation IV. Once the player attains the National Pokédex, every day in a particular location, a "swarm of Pokémon" appears, which changes the probability of finding the given Pokémon from 0% to 40%. Outbreaks last only one day and are random events. Information about Pokémon outbreaks can be found in Sandgem Town by talking to Lucas/Dawn's sister or by watching the Sinnoh television DPPt, or by listening to Professor Oak's Pokémon Talk HGSS.
When players all join the same group, they will all experience the same outbreak on any given day.
The species of an outbreak seems to be determined using a formula, because it is not chosen randomly when the player starts the game that day. It is also possible that the species is determined the day before, though there is no additional evidence to support this. If the DS clock is changed or a different DS is used, then the game will detect this and will not change the species swarming until 12:00 AM.
Many of the Pokémon obtained through Pokémon outbreaks are outside of the Sinnoh Pokédex, and are usually otherwise exclusive to other regions.
Due to the high encounter chances for Pokémon that appear during outbreaks, they make for longer and better chains when using the Poké Radar.
The feature continues to appear in HeartGold and SoulSilver, where the wild Pokémon available include all those from Generation II, plus Ditto, Chansey, and Pokémon from Hoenn and Sinnoh. Information on swarms can now be found by listening to Professor Oak's Pokémon Talk on the Pokégear radio.
The gates in Unova have a monitor inside that will alert the player to where an outbreak is taking place.
Unlike in previous games, the Pokémon outbreaks can be manipulated by soft resetting the game, but only if the player has not saved their game at all that day. When the game is restarted, there will be a different Pokémon swarming.
During winter, the monitor may occasionally not report on outbreaks, which is due to parts of Route 8, where Croagunk can be found, becoming frozen during Winter and not allowing the Pokémon to be caught there. All outbreaks in these games have a level range of 15-55.
All outbreaks in these games have a level range of 40-55 and an encounter rate of 40%.