"Hunt high in the morning and low in the evening" is the rule most hunters learn about thermals. It's a good start. This page explains why the rule works, where it fails, and how to figure out when the air actually switches at a given stand. For the bigger picture on wind direction, access routes and checking wind from the tree, see our hunting the wind guide.
What a thermal is
A thermal is air moving because of temperature, not because of the weather system's wind. Meteorologists have names for it:
- Anabatic wind is upslope air driven by the sun heating the slope's surface. The American Meteorological Society's glossary describes it as a daytime, fair-weather flow.
- Katabatic wind is downslope air driven by the slope surface cooling, usually at night, when larger-scale winds are light.
- Drainage wind is the cold, dense air that flows down a valley at night. The glossary lists typical depths of 10 to 400 meters.
In hunting terms: warm ground pushes air up, cool ground lets air sink, and the sinking air collects and runs down low ground.
The basic daily pattern
| Time | What the air does | Why |
|---|---|---|
| Before sunrise | Sinks and drains downhill | Ground cooled overnight |
| After the sun hits the ground | Starts rising | Sun warms the slope |
| Midday | Rising, often mixed by wind | Heating at its strongest |
| Evening, as slopes shade | Sinks back toward low ground | Ground cools |
The NDA's Anthony Harris describes the same pattern: in the morning, as the sun heats the ground, air rises, and as the earth cools in the evening, air settles back down, pulling your scent with it. He writes that 85 percent of his own hunting is done mornings through midday because of the evening fall.
Where the rule breaks
The switch follows the sun on the ground, not the clock. That's where most mistakes come from.
- North-facing slopes warm late. Harris describes sitting partway down a north-facing slope in late fall while the sun lit the ridge above. The air where he sat was still cool and falling, like evening. Once the sun warmed his part of the hill, the thermals rose.
- East-facing slopes warm first. North American Whitetail notes thermals can start as soon as the sun peaks over the horizon on east-facing ground that gets early direct sun. The AMS glossary says the same: east-facing slopes get sun first, so upslope flow starts there earlier.
- Sunny slopes hold out in the evening. NDA writer Mark Turner, writing about lessons from elk hunting, found that slopes in the sun right up to sunset keep rising until just before dark.
- Low ground holds cold air. The AMS glossary describes cold, dense air draining down valleys at night. Turner notes the air doesn't rise until the sun hits the ground in that spot, so a shaded bottom can keep sinking air after the ridges above have switched. He also moved one stand to use afternoon thermals pulling air down a creek drainage.
- Wind can override it. The AMS glossary notes these systems are clearest when larger-scale winds are weak. North American Whitetail adds that weather can cancel out thermals, at least in the short term.
Time the switch at each stand
A stand's slope and direction don't change, so the timing is learnable. NDA's Turner tracks when the sun reaches the ground at each stand in the morning and when it goes into shade in the evening. That gives a rough time for each switch.
A simple way to do it:
- Note which way the stand's slope faces and whether it's in a bottom.
- On scouting trips, write down when direct sun reaches the ground there and when it leaves.
- Plan to be settled before the morning switch and check the air around it.
- In the evening, expect the fall to start once the stand goes into shade, even if the ridge is still lit.
That timing matters because, as Turner writes, deer are most active right around the times thermals usually switch.
Read it in the tree
You can't see thermals, so watch something that floats. I check mine with Windfloaters. In the morning, a floater that drifts downhill or sinks means the air hasn't switched yet. For a comparison of wind checkers, see wind checkers compared.
What thermals mean for your setup
- Mornings: hunt above where deer will be once the air starts rising, or wait for the switch before you move. Harris describes waiting for the sun to warm the hillside, then looping around to come back into his original stand on the hillside.
- Evenings: expect your scent to sink toward low ground. Hunt low, or keep low ground downwind and out of the deer's path.
- Creek bottoms and valleys: plan for sinking air there in the early morning and late evening, even when the forecast wind says otherwise.
For access routes, stand choice and how scent travels in general, the hunting the wind guide goes deeper.
Other good options
- Milkweed. NDA's Turner checks the air with a piece of milkweed. It's free if you collect it.
- Podcast deep dives. Southern Outdoorsmen episode 385, per its show notes, covers thermal switches and thermal dead zones. Episode 282 with Bill Vale covers buck movement based on wind and thermals.
Questions hunters ask
What time do thermals switch? There's no set time. It depends on when the sun reaches or leaves the ground where you sit, which changes with slope direction, season and clouds.
Do thermals matter in flat country? The AMS definitions describe slope and valley flows, so expect them most where there's relief. Watch your wind checker either way.
Which is stronger, wind or thermals? The AMS glossary says these flows are clearest in fair weather with weak larger-scale winds, and North American Whitetail notes weather can cancel thermals, at least short term.
Sources
- American Meteorological Society Glossary, anabatic wind: https://glossary.ametsoc.org/wiki/anabatic-wind/
- AMS Glossary, katabatic wind: https://glossary.ametsoc.org/wiki/katabatic-wind/
- AMS Glossary, along-valley winds: https://glossary.ametsoc.org/wiki/along-valley-winds/
- AMS Glossary, mountain-valley wind systems: https://glossary.ametsoc.org/wiki/mountain-valley-wind-systems/
- National Deer Association, "To Beat a Buck's Nose, Remember Thermals," Anthony Harris, Aug 11, 2014: https://deerassociation.com/beat-bucks-nose-remember-thermals/
- National Deer Association, "Hunt Elk to Kill More Whitetails," Mark Turner, Oct 14, 2020: https://deerassociation.com/hunt-elk-to-kill-more-whitetails/
- North American Whitetail, "The Thermal Factor": https://www.northamericanwhitetail.com/editorial/thermal-factor/481157
- Southern Outdoorsmen, Ep. 385 show notes: https://www.tapesearch.com/episode/385-catch-the-bucks-where-they-ve-been-busting-you/23wkwpVm7jUCZrzv3gGv34
- Southern Outdoorsmen, Ep. 282, The Wind Rule with Bill Vale: https://www.podbean.com/media/share/dir-vr52e-1019d0ce