Where is here?
Mammal body temps tend to be over 90 so they will still stand out. Once the sun goes down, you will have differential cooling of materials that will provide you with a decent image as thermal scopes tend to work on relative heat relationships.
Humidity does affect thermal, usually negatively for things like ground and plant life that don't generate their own heat. A solid overcast day with continued mist will make background items grey out, though things like animals will still stand out just fine.
Thermal does not see through glass.
Night vision is great if you have an IR illuminator. Gen 3 won't need an illuminator most of the time out in the open, but on solid overcast nights with no backlighting from a nearby city, hunting at night under a solid tree canopy, even Gen 3 can be rendered virtually useless without supplemental illumination such as from an IR light.
Without an IR light, night vision will not see into the shadows.
Camouflage that works visually will also trick you with night vision in may cases. A brown coyote will still blend into brown grass, for example.
With an IR illuminator, you can spot animals such a predators at distances well beyond what you would be able to actually see them with the scope due to eye shine. It is a pretty neat trick. You can use the IR illuminator to see into the shadows as well. However, using an IR illuminator on your rifle means that when you shoot, you will get some whiteout due to your IR light reflecting off of your gun smoke. During very calm conditions, this can last for several seconds. During a light breeze, only 1 or 2. During a heavy breeze, you might only notice a brief flash.
Given current technology available to the civilian market, night vision will produce a clearer image than thermal. Thermal will see farther than night vision. It is much easier to spot animals with thermal than night vision. It can be easier to make proper identifications with night vision than with thermal. If you need to navigate in a vehicle with a windshield, then you need night vision.
In the long run, they both have advantages and disadvantages. Most hunters prefer thermal for hunting, however.
