Ever since its launch, Pokémon Go did something very smart (or very lazy): it did not teach us how it works. Sure we get the basics here and there but most of the time you need to discover things on you own. One big mystery is how to find those pesky monsters themselves!
We know there is a radius but you might have also noticed that it changes color. If you happen to be in an area where a Pokémon appears and want to catch a specific one then following this guide by Artigius might do the trick.
So far it seems to be working, I’ll try more later tonight once the heat dies down.