September 22, 2023
Many moons ago, I watched a big, heavy- antlered bluegrass buck slink through a dense cedar thicket on his way toward a large bean field. As he neared the edge of the thicket, he eased down into a sinkhole the size of a basketball court that led directly into the lowest point of the beans.
Up until this point, my scouting had paid off in a big way, and now only 25 yards separated man from beast. What little wind existed on this particular evening was steady and out of the east. It wasn’t blowing directly in my face, but it was a perfect just-off wind — the kind that gives mature whitetails the confidence to move in daylight.
What I didn’t know about at the time, however, was a natural phenomenon that caused the hefty-framed Kentucky buck to smell me from upwind, turn tail and run. At that moment, several old adages started racing through my mind, and they all pertained in this situation. Don’t count your chickens before they hatch. Expect the unexpected. Learn from your mistakes. All were relevant in that moment . . .
When the deer spooked and ran, I quickly embraced the tenet about learning from one’s mistake. I was unaware of how the deer detected me, but I knew that something in my hunting equation wasn’t adding up. Later I discovered that the deer had taken advantage of a physical process called the thermal factor — something I had yet to master.
Simply put, thermals are pockets of warm air that start to rise when the sun heats the ground. These columns of air can begin emerging as soon as the sun peaks over the horizon, provided the thermals are generally east-facing and receive early direct sunlight. As the sun continues to rise throughout the morning and more of the landscape becomes sun-kissed, thermal activity increases. It continues throughout the day until the sun starts to dip below the horizon and the temperature begins to drop. As the sun begins to fade, those same air pockets start to cool and recede back toward terra firma.
Thermals are the reason western hunters approach from above their quarry in the morning and from below in the evening. But to say thermals are caused simply by rising warm air in the morning and falling cool air in the evening would be an oversimplification. The factors and variables that influence thermals are nearly endless. But, with a solid understanding of the complexities of thermals, we deer hunters can take advantage of them.
The thermal factor is present even on cloudy, stormy or cold days, because the ground can still absorb solar radiation even when it’s dark and gloomy. Water can also affect the air around it. For example, a cool-water spring flowing down a hillside can suck thermals in and carry them downhill. A secluded pond in thick cover may also have a surface temperature that is cool enough to create a thermal pull.
Certainly, the intensity of the thermals in these examples is not equivalent to those in the Rocky Mountains. However, even small, isolated terrain features on flat land can provide a thermal advantage to the discerning hunter. Terrain and topography influence wind and thermals, too. Thermals become more complex when a gazillion topographical elements are factored in, and wind can cancel out thermals at least in the short term. Ultimately, I try to analyze a situation in the context of the thermal factor. The sun heats up the ground, and the resulting warm air lifts an air pocket up. In the evenings, as the air columns cool, they begin to fall.
Another important aspect of thermals is how they function. Think back to your childhood when you spent hours outside on a hot summer day blowing soap bubbles by the hundreds. Those little magic pockets of soapy air rise up individually, not collectively. Thermals work in the same fashion. They may rise in one spot, but 10 feet away under the canopy of an old white oak, there may be no thermal activity at all.
The color of the ground even plays a role. Dark colors absorb the sun’s heat more readily, so rising pockets of air will occur sooner over dark areas of land than over light-colored earth in the same conditions. Rolling hills, creeks, wind direction and speed, cloud cover, ridges, sinkholes, ponds, rivers, timber and barns are just a few of the almost endless number of factors that influence thermals.
A circling buzzard conjures up an image of thermals in action. As long as the old dirty bird stays within the warm column of air created by a thermal, it can gain altitude without so much as a wing beat. Outside the pocket, the buzzard’s God-given aerodynamics and built-in lift will only take it so far, and the bird will have to expend energy to climb.
Hunting the leeward, or downwind, side of a ridge is a technique used by many experienced hunters. Rightly so, I might add, as mature bucks often visit such locations. Like many aspects of deer behavior, there is more going on than meets the eye. There is a reason why mature bucks funnel into and feed securely on leeward ridges. Rising or falling thermals, along with swirling winds that are often created by topography (in this case leeward ridges), give that old monarch a sniff in several directions.
And although the basketball-court-sized sinkhole I referenced earlier is not the same as a leeward ridge, it has a similar effect; and crafty whitetails use it to stay a step ahead of harm’s way.
Because of all these conditions, the lowest point on the edge of a crop field or food plot is almost always the first to get my attention when trying to locate a mature animal in a feeding pattern. Even during the rut when giant bucks will make a mistake or two, they often frequent these low spots around doe-feeding areas to scent-check for estrus does. Bucks don’t have to understand the dynamics of thermal expansion to know that the lowest point of the field edge gives them the thermal advantage, and leeward edges can do the same.
Through years of trial and error, I have learned that I can’t always predict precisely how a thermal will manifest or how it will behave. To help me identify and understand a thermal, I sometimes use a canister of surplus smoke as a makeshift smoke bomb, or I’ll pop a balloon in an area where I want to learn how thermals are behaving.
It sounds silly, I know, but it helps paint a picture of exactly how my scent will be dispersed within my chosen hunting area. Even starting a very small fire, if it is legal, and creating smoke by tossing some green leaves on top can suffice. In very similar hunting locations, it is worth the time. It will help create an almost tangible visual.
You can take smoke canisters or balloons to other areas during the off-season to learn the skill of reading thermals, wind and the effect of topography on each. Be sure to check with your game warden to make sure your chosen method of creating smoke is legal. I have not used all of the commercially available wind indicators on the market, and I’m by no means an expert in reading thermals, but I find milkweed to be about as good as it gets when hunting season arrives. I always try to have milkweed on hand, regardless of whether I am actively hunting or just entering or exiting the woods. Plus, it’s free, and it allows us to monitor its behavior long after it is released.
Elk hunters understand thermals as well as any faction of hunters. In fact, western big-game hunters often live and die, so to speak, by studying thermals. While Midwest hunters do not generally encounter the same thermal effects as those chasing high country mule deer, anyone can benefit from understanding these updrafts of warm air and their effects on deer behavior. Even flat-land whitetail hunters would be prudent to study what causes thermals, how topography effects them and how to use them to get within bow-range of an animal whose primary job is to survive predators.
Hunters benefit from learning to read thermals and then capitalizing on that knowledge. When I first heard about thermals, it meant nothing to me. Now, the thermal factor is one of my top considerations when trying to “seal the deal” on a mature buck equipped with nearly 300 million olfactory receptors.