By Dr. Grant Woods
Almost every whitetail hunter knows weather conditions can impact deer activity. Some of the hunts when I've seen the most deer active have been just before or just after a major change in the weather.
However, it seems few hunters consider how weather conditions impact the ability of deer to detect hunters. Most of us consider wind direction. We attempt to place our stands downwind of where we expect to see deer. However, there is another factor in addition to wind direction and speed that can be a big factor in whether we see deer from our stands.
The factor I am talking about is humidity. A simple definition of humidity, from Merriam Webster, is the "amount of water vapor in the air." But understanding relative humidity isn't quite that simple. If you want to dig into the details, you can educate yourself using this information from Utah State University.
If you'd rather not spend time understanding the details of relative humidity, simply understand that warm air can hold more moisture than cold air. In fact, the amount of water molecules air can hold approximately doubles for each 20-degree increase in temperature.
Moisture and Odor
Let's get to the bottom line. How much moisture is in the air is a huge factor in how well scent will disperse and how well a deer can detect odors. The less moisture there is in the air, the less likely deer are to smell you.
To illustrate this point, imagine filling a glass with water and then placing a few drops of food coloring into the glass. The food coloring moves from the top to the bottom, gets caught in swirls of disturbance and gets thinner as it moves further away from where the coloring was initially dropped at the top of the water.
Odors from hunters and their gear are also impacted by the temperature and amount of moisture in the air. Changes in temperature and moisture have a huge impact on both the strength of odors and also how quickly they dissipate. When the temperature drops, air and scent molecules become more dense.
This is why helicopters can lift more in cold air compared to hot air. Colder (denser) air also means there can be a higher concentration of odor molecules per square inch. Similarly, water vapor traps scent molecules, enhancing the odor they release and slowing their diffusion into the air. A combination of cool, moist air results in scent that is heavy and closer to the ground. This is why it's much easier for a dog to point a quail or track a rabbit during the cool morning hours versus during the heat of the day.
As another example, a few years ago I shot a deer and hit it too far back. I lost the blood trail around 10 a.m. In Missouri (where I was hunting), it's necessary to get the local conservation officer's permission to use a dog (on a leash) to trail a wounded deer. By the time I got permission and got our dog to the last place I'd found blood, the day had gotten very warm. Initially, the dog took the trail another few yards and started wandering. She was clearly having trouble finding the trail. It was obvious we were not making any progress, so we decided to give it a break and come back later that afternoon.
Once back in the woods that afternoon, we put the dog on the last sign. She immediately took off, pulling on her leash and dragging her handler through the woods. She ran straight to the deer! The dog's blood trailing skills didn't improve during the break. And the trail was hours older. But as the temperature cooled, the air got denser which allowed her to simply pick up the existing odors from the blood trail that much better!
The Freeze Factor
Cool, moist air certainly makes it easier for dogs, deer and man to detect odors. If you live in a neighborhood where folks put their trash out or there's some type of manufacturing around that gives off odor, you've probably noticed you can smell it best when it's cool and moist. However, you can be downwind of a pig or poultry farm when it's really cold and dry outside and barely smell the odor.
That's because it is also true that air can be too cold for odor molecules to be carried by air. As temperatures continue to drop, the moisture in the air freezes to a solid. Eventually, the scent becomes trapped in dry solids and cannot be detected easily by either man or beast. It is for this reason that dogs may have trouble tracking in subfreezing temperatures but do well on cool, rainy days. For the same reason, deer may not bust you due to odor on very cold or very dry days.
If you live where the temperatures and/or humidity are low during the late season, it may be a great time to hunt. You can scout and hunt more without deer detecting odors from you or your gear as easily as they can during cool, moist conditions.
The weather not only impacts when deer are active, but how well they can detect you with their sense of smell. This is simply another reason I'm an avid weather watcher during deer season. I don't simply watch wind direction; I also watch the humidity and temperature.