Do you have an adjustable bridge that allows you to raise them up? If the bottom of your lenses are close to your face, that is the cause of your problem. A cap with a long brim also can trap the heat and humidity and your glasses will start to fog
I have that problem if the glasses fit too close to my face. My current shooting glasses tend to do that, so I took a pencil eraser and attached it to the top of the glasses to serve as a spacer between the glasses and bridge of my nose. Problem went away.
I used to have similar issues with my regular glasses in hot weather but since switching over DeCot Hy-Wyd's the issue has disappeared; the glasses properly sit up and slightly away to breathe better than standard glasses. I also wear my cap to catch sweat and bend the bill upwards to prevent blocking my sight and trapping heat. Blinders can also trap heat so ditch them if not necessary for you. Another thing to think about is breathing through your nose and not your mouth; it can cause fogging with the gun mounted up to your face. Most clubs face north so there isn't any reason to keep the bill on your hat down. I'm on station 4; noticed the flipped bill. Sorry the quality sucks - it's from a video. Sometimes I even wear the old school cycling caps with the small flipped bills; they're designed to catch sweat and are very comfortable.
For us 4 eye'd Folk, A cheap and easy way to keep glasses from fogging.
Take a small bar of "IVORY" Soap...ONLY "IVORY" Bar soap will work.
Coat your lenses both sided DRY with the IVORY, then wipe the lenses clear with a clean soft towel...I use a Micro-Fiber towel.
This will last a day or so no matter how humid....However, If raining coat as often as necessary.
My Mail Man gave me this tip years ago.
Rain-X is good stuff. Should be applied to any windows, or glass you have to see through in wet conditions.
That being said, air circulation is a must. When the humidity is high, no matter the temperatures, trapping warm humid air behind the cooler lens of your glasses is going to cause the condensation to form. Circulating the air behind the lenses, to even out those temperatures is crucial.