I live in Cheyenne Wyoming and during the winter time on cold windy days I get a strong sewer odor from two of the three bathrooms in the house. One up stairs and one in the basement. I have a thought that the wind is blowing down the vent pipes and drying up the "P" traps. Could this be? I did see an item on the web that you put in the drain called "SureSeal" at http://www.thesureseal.com/Default.aspx?&gclid=CM_cibuAxKwCFcvJKgodaT2arQ Has anyone used this and do you think it would work. I tried closing the drains and the odor still comes out. I guess it is seeping by the drain stopper. Any ideas would be greatly appreciated.

Also on another note, It only happens when it is cold. When it is windy during warmer times of the year (Short as they are) this does not occur so I do not feel a cap would help.

Hi John and welcome to the Plumbing Page at AMHD.
I think you're going at this from the wrong end. First off you don't need drain seal. That's more for floor drains that have lost their trap primer and the water has evaporated from the trap.
Sewer gas will raise during warmer weather and fall during a cold snap. Bearing that in mind I would suggest installing vent caps, (see image ) on your vents to eliminate any down drafts.
If you still have a problem then a smoke test might tell you where the smell is coming from. Good luck, Tom

Thank you very much. I will give that a try. I appreciate your quick reply