If you've ever heard a deer blow and thought it was because of something besides you, you're wrong. Assume it was you. Do that and you're going to learn thermals very quickly. Once you start asking the question, "how did they wind me," you're going to draw a mental map of the thermals and the wind. This will make you a better hunter every time you go out.
Warm air rises. Cool air sinks. In the morning the sun heats the ground and your scent climbs uphill. In the evening the ground cools and your scent pours downhill.
That is thermals in four sentences.
The part that costs hunters deer is not the concept. It is that thermals usually override the wind, and most stand choices are made off the wind alone.
Wind is what the sky is doing. Thermals are what the ground is doing.
Your weather app tells you the prevailing wind. That reading comes from an open area at a set height, and it describes air moving across the region.
Thermals are local. They are driven by the ground under your feet heating and cooling.
And it is the ground doing the work, not the air. Sunlight passes through air without warming it much. It heats the dirt, the rock, and the leaf litter, and that surface heats the thin layer of air touching it. Air rises because the ground under it got warm.
That is why shade matters as much as sun angle. A north face under closed canopy, or the bottom of a deep drainage that never takes direct light, can hold cool dense air and keep draining downhill all day. Two hundred yards away an open ridge in full sun is running the other direction.
When the prevailing wind is light, thermals win.
Here is the threshold that matters. Above roughly 4 to 5 mph, wind overrides thermals. Below that, thermals rule. Most hunters know that much. What they miss is what happens at the boundary, and we will get to that.
The daily cycle
Overnight and early morning: falling. Ground has been radiating heat all night. The air layer against it is cold and dense, and it slides downhill and pools in low ground. Creek bottoms, drainages, and hollows collect it.
Mid morning: the switch. Once the sun gets on a slope, the ground warms, the air above it warms, and it starts to climb. This does not happen everywhere at once. East facing slopes light up first. North facing timber can stay in falling mode for hours after south facing slopes have flipped.
Midday through afternoon: rising. Air moves upslope. On a warm day this is the strongest and most predictable thermal of the whole cycle.
Last hour of light: the switch back. Sun comes off the slope, the ground gives up heat faster than the air above it, and everything reverses. Your scent starts running downhill into the low ground.
Two of those four are transitions, and transitions are the problem.
The transition windows are where hunts die
During the switch, the air is not doing one thing. Warm pockets are still lifting while cooled air has already started to slide. The result is swirl.
That is the swirling wind you have felt at last light with a perfect forecast. Nothing is wrong with your read. The ground is changing states, and for twenty to forty minutes there is no reliable direction.
If your only sit window is the last thirty minutes of light, elevation relative to the deer is doing more work than wind direction. Which brings up the thing most hunters never account for.
Elevation relative to the deer, not just wind direction
A stand can have perfect wind on paper and be unhuntable in practice.
Picture a bench halfway up a hillside with bedding below you. Wind is quartering across the face. Looks fine. Then the sun drops, the thermal turns, and your scent runs straight downhill into the beds you were waiting on.
The wind was never the problem. Your position above them was.
Two rules come out of that.
In the evening, be below the deer or level with them. Falling air carries your scent down. If they are downhill of you, you get winded.
In the morning, be above the deer or level with them. Rising air carries your scent up. If they are uphill of you, they get it.
You will not always get both. Most spots are a morning setup or an evening setup, not one that works twice. Deciding which one it is before you hang anything is most of the work.
The gusting problem
This is the one nobody plans for and it is worth its own section.
Wind above the threshold (4 to 5 mph) overrides thermals. Wind below it does not. When the wind is gusting above and settling below, you get both in cycles.
During the lulls your scent pools. It sits in the low ground or hangs in the timber around you and accumulates. Then a gust comes through and moves all of it at once.
You are not putting out a cone of scent in that condition. You are pushing a wall of it across the terrain in pulses.
An average wind speed hides this completely. A day that reads 5 mph average can be a day of 9 mph gusts and dead lulls, and those two conditions do not average into anything useful.
Terrain steers thermals
Air moving up and down a slope does not spread out. It follows the shape of the ground the same way water does.
Draws and drainages are chimneys. Rising thermals run up the adjacent ridges. In the evening, falling air pours down them and runs "downstream" at the lowest point of the draw.
Benches hold air. A flat shelf on a hillside slows the vertical movement and creates a pocket where scent hangs before it moves on.
Points and spurs split it. Air moving up a face divides around a point, which is one reason a buck beds there. He gets two directions of information at once.
Saddles funnel it. The same low crossing that concentrates deer movement concentrates air movement.
If you can read those features on a map, you can predict the thermal before you walk in. The four that matter most for whitetail are broken down in how to read a topo map for deer hunting.
What changes the strength of a thermal
Not every day produces the same effect.
Sun exposure. Clear and warm produces strong thermals. Overcast suppresses them, sometimes enough that a weak prevailing wind takes over all day.
Slope aspect. East faces warm early and flip early. West faces stay in falling mode longer in the morning and hold rising air later into the evening. North faces in heavy timber are the slowest to switch, and some never do.
Canopy. Closed timber shades the ground and delays the whole cycle. An open hardwood ridge and a dark hemlock bottom on the same hillside can be running opposite directions at the same hour.
Season. A 70 degree September afternoon builds a much stronger thermal than a 25 degree November one. In late season with snow on the ground and low sun angle, thermals get weak enough that the prevailing wind is usually your real problem.
Flat country still has thermals
They are weaker, and most hunters write them off. That is a mistake.
Even three or four feet of relief will move air on a still morning. Ditches, creek bottoms, and old field drainages collect cold air overnight and hold it. Standing in a low spot at first light in flat country, you are standing in a pool.
Two adjustments for flat ground.
Treat drainages as your terrain. Where there are no ridges, the low lines are the feature, and they behave like miniature versions of everything above.
Weight the prevailing wind heavier. With less relief there is less vertical drive, so the sky wins more often than it would in hill country.
When the sun heats up the ground and the air rises, cool air gets pulled in from the sides, creating a slight drifting effect. So even on a calm sunny day your scent is drifting due to thermals.
Reading it in the field
Milkweed is your best friend for understanding how thermals work. And if you don't like thinking about thermals you can just keep dropping it all day long to watch what they are doing.
Check it on the walk in and again once you are set. The cycle moves. A read taken at 0600 is stale by 0800.
Watch where deer put their noses. Deer solve this problem for a living. If bedded deer are consistently facing one way on a given hillside at a given hour, they have told you what the air is doing there. It is likely coming over their back at ground level.
Three mistakes thermals punish
1. Picking the tree off wind direction alone. Wind gets you half the answer. Your elevation relative to the deer gets you the other half.
2. Treating the whole property as one condition. Two slopes a quarter mile apart, facing different directions, can run opposite thermals at the same time.
3. Ignoring the shade. Sun angle tells you part of it. What is actually getting light on the ground tells you the rest.
Frequently asked questions
What are thermals in hunting?
Thermals are vertical air currents created by the ground heating and cooling. Sunlight heats the ground, the ground heats the thin layer of air touching it, and that air rises. When the ground cools, the air against it becomes dense and sinks. Thermals carry scent uphill or downhill independent of the prevailing wind.
Which way do thermals go in the morning?
Up. Once the sun warms a slope, air climbs it. Before sunrise and during the first part of the morning, air is still falling, so there is a switch window in between.
Which way do thermals go in the evening?
Down. As the ground loses heat in the last hour of light, air becomes dense and slides downhill into low ground.
At what wind speed do thermals stop mattering?
Roughly 4 to 5 mph. Above that the prevailing wind generally overrides the thermal. Below it, thermals control where your scent goes.
Should I hunt above or below deer?
Above them in the morning and below them in the evening, or level with them either way. Rising air carries your scent uphill and falling air carries it downhill, so your elevation relative to the deer decides who smells whom.
Do thermals go down all day in the shade?
They can. Thermals are driven by ground temperature, so ground that never takes direct sun does not warm enough to reverse. A shaded north face or a deep drainage under closed canopy can keep draining cold air downhill all day while a sunlit ridge nearby runs uphill.
Do thermals matter in flat country?
Yes, but weaker. Cold air still collects in ditches, creek bottoms, and drainages overnight. Rising air also pulls cool air in from the sides, so your scent drifts even on a calm sunny day.
Does cloud cover affect thermals?
Yes. Overcast reduces the heat the ground absorbs, which weakens or delays the rising thermal. On a heavily overcast day the prevailing wind may control conditions all day.
Why does the wind swirl at last light?
Because the ground is switching states. Warm pockets are still lifting while cooled air has begun to sink, and for twenty to forty minutes there is no single direction.
Thermals tell you where your scent goes. Terrain tells you where the thermal goes. The two are the same skill.
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Written by Jake Roberts, Green Beret and founder of Wayward Ridge.