I live in Phoenix most days it is above 75 this time of year. Anytime it is cold or it rains my bathrooms in my upstairs smell so bad like sewer gas, it is overwhelming I have septic not sewer and I keep my tank cleaned out often, where is this smell coming from and any advise on how to get rid of it? Is this a venting issue.

The first thing I would check is the wax seal. It's possible for a wax seal to prevent water from seeping in but allow sewer gas to escape if a down draft occurs in the vent. Let me know. Cheers, Tom

Is it only on windy days that this happens? If so there might be too much grade on the arm right after your p-trap. Depends where exactly the smell is coming from too though.

Is it only on windy days that this happens? If so there might be too much grade on the arm right after your p-trap. Depends where exactly the smell is coming from too though.
I don't think so Trotter,
Even if the arm after the trap were pointed directed down, turning the "P" trap into a "S" trap there would still be a trap seal in place. I still think the best bet is the wax seal. Cheers Tom

If the trap arm is has too much of angle on it the P-trap will still hold water in normal conditions. But in a properly vented house, on windy days air entering the system will move the water in the trap, with too much of an angle you'll lose water from the trap and lose your seal. This is a common problem in really windy areas. There's specs on trap arm's in the code to prevent this from happenning. I was just throwing it out there as another possibility, and if the smell is coming from the sink.