What determins raid spawns

So its been about a week since Tyrnatiar came back, and in that time my whole raid chat's seen only one (this is a discord that's running all hours of the day because theres 300 soemthing plus people across an entire city) It's been Partly cloudy or foggy most days, and while the fog is good for mewtwo raids, most of us are really just wanting to do tyrnatiar raids and really not sure why there aren't any Its nt like we're getting all the other tier 4's, we just arent getting tier 4 raids period, tons of 1,2,3 and 5, but no 4's. So are tier 4's just a lot less common than the others, or is this just a week of really bad RNG?