Sewer gas from stack, stinking out back yard

We have been having an ongoing problem with sewer gas outside the house. Those cold winter days you can even see steam blowing out the roof vents. When the wind blows from the south the way the roof is the stink blows right down to the patio.

Is there a check valve that can be installed on the roof vents? I thought about raising the vent height but I doubt it would help. Our chimney is already tall and the smoke gets pushed down to the ground. We have some big trees that create downdrafts. Any ideas welcome.

Here is one possible cure...

http://www.sweetfilter.com/

I had this same problem. I installed an Air Admittance Valve (AAV) on the top of the 4 inch vent on the roof that is over the bathroom. The AAV allows air to go into the vent but does not allow it to come out. They are used inside of buildings instead of cutting holes through the roof or where you do not want the vent to show. When I was picking it out at Lowe's the salesman said it was not approved to be used outdoors. He said it would deteriorate and not hold up in the sun. I told him I do not care and when it deteriorates I will saw it off and put on another one. It has been up there for twelve years. There is a plastic screen around the edge that has started to fall apart. It has worked and there have been no odors at the back door or on my patio since I installed it. Mine is a Studor vent.

I would not use a air admittance valve in this application.

When sending water to a closed container it is very desirable for air to be able to escape...