What affects Pokemon spawn rates?

Question

After the recent event where the original 150 Pokemon spawn rates were boosted, I assumed it might affect the spawn rates of those Pokemon. However, it appears it didn't have that effect. So far all I have seen of the originals are Pidgey and Rattata, which makes sense since they are a very common Pokemon.

I understand that bringing the other generation's spawn rates will saturate the field somewhat, but it appears that isn't the case.

What, if anything, affects a Pokemon's spawn rates?

Answer 1

Spawn rates are affected by:

• Individual Pokemon. Some Pokemon spawn much more frequently than others, for example Rattata is a relatively common spawn compared to Snorlax.
• Weather. If a weather boosts that type, Pokemon of that type are more likely to spawn
• Nests. If there is a nest for a particular Pokemon, that Pokemon is much more likely to spawn until the nest is re-rolled. Here's a list of Pokemon that can have nests.
• Location of the spawn point. It's believed that certain spawn points are more likely to spawn Pokemon of certain types. Spawn points near water being more likely to spawn Water-Pokemon near water is the most well-known example. Beyond water though, the rest is based on observation and speculation with questionable evidence. For example at a creek near my old house pre-Gen 2, Poison-type Pokemon apart from Grimer and Koffing (rare Poison-types) were common spawns but Water-types weren't, but I wouldn't say that's the case for every creek.
• Regionals. They only spawn in certain countries
• Events. Obviously events can make certain Pokemon much more likely to appear, for example Community Day

Answer 2

S2 Cells determine the specific spawn locations of any Pokémon. For instance, you may have noticed the Pokémon that spawn near your home or work location seem to generate in the -exact- same location. This is based on an S2 Cell.

During special events, such as the Kanto festival which recently ended, additional spawn points are enabled on cells to give the appearance of more available Pokémon. Take note between events of your frequent playing locations, and you will see a (roughly) 2x spawn rate per S2 Cell area.

I cannot find a specific reference at the moment, but the spawn locations were determined by historical cellphone data for a particular location, IIRC. For example, City-Metro areas, Downtowns, Shopping Malls, and Highway Rest Stops often have an incredible number of active spawns compared to a residential area. I will look for a source on this and provide it if I am able.