Field notes · Hunting thermals

Hunting thermals: scent follows the sun, not the clock.

On a calm, clear day the forecast wind hardly reaches the ground. What carries your scent is heat and cold: air draining downhill as the ground cools, and rising up a slope once the sun warms it. Get the timing wrong and you sit exactly where your scent is going.

Groundwind · 5 October 2026 · 7 min read

When thermals take over

A wind forecast is made for 10 m up, over open ground. Under the trees the air at your head moves at a fraction of that: in dense spruce, about 5 to 15% of it (Cionco 1965; Finnigan 2000). Thermal flows are gentle too. Cold air draining down a gentle slope runs at about 1 to 5 km/h (Mahrt 1982; Whiteman 2000). So they take over when the wind above is light, under about 8 km/h, and the sky is clear. A breeze of 7 to 11 km/h is already enough to push them around.

Clear nights are when it matters most. Cloud keeps the ground warm. A clear sky lets it cool fast, and the air at the ground comes loose from the wind above (Stull 1988). One clear evening in late September at our camp near White River, the forecast had 13 km/h at 80 m and 5 at 10 m. By midnight the air at head height was 2 °C colder than the air above it, and in the bog at dusk the breeze had died and was drifting off the forecast direction altogether.

Draining air: evenings, nights and most mornings

As a slope loses the sun it cools the air lying on it, and that cold air slides downhill like water. It runs down slopes and gullies, along creek bottoms, into hollows, bogs and the low corners of cutovers, and out over the lake. Your scent goes with it.

• It starts when the slope loses direct sun, not at sunset. Under tall conifer it can start in the late afternoon (Pypker 2007).
• It runs all night and on into the morning, until the sun reaches that slope.
• It pools. In a bowl, a saddle or a frost pocket the air goes still. Your scent sits there and creeps slowly toward the outlet.
• Gullies are scent highways. In the evening, scent from the high ground flows down every draw to the shore, and the shore is where moose feed in the fall.

Rising air: only once the sun is on the slope

Air rises up a slope when the sun warms it, and each slope gets the sun at its own time.

• On an open, east-facing slope, within about 30 minutes of sunrise (Farina 2023).
• On a valley floor, about two hours later (Farina 2023). On a steep west face it can start 40 to 50 minutes after sunrise and take five hours to set in fully (Nadeau 2020).
• Under tall, dense conifer the air below the crowns can keep draining all day, until the sun reaches the forest floor (Pypker 2007).

So "thermals rise in the morning" is about the sun, not the clock. At first light nearly everything is still draining. A rule from hunters in the mountains gets it about right: the air turns 90° at first light and 180° by the time the sun hits the slope (Hunt Talk).

Where to sit

You'll still read "in the evening, sit above them" in hunting advice. Follow the air instead. Evening air drains, so a hunter on the hill above a moose feeding along a creek sends their scent straight down onto it.

Lakes: a breeze that switches sides

On a sunny afternoon the land warms faster than the lake. Air rises over the warm land, and cooler air off the lake flows in to take its place: the lake breeze. It blows onshore, fades over about 500 m inland, and is stronger off bigger water, from about 4 km/h off a pond to about 9 km/h off a 10 km² lake (Crosman and Horel 2010). At night it turns round. The land cools below the water, and a weaker land breeze drifts off the shore.

So a morning call from a point sends your scent out over the water, where nothing is listening with its nose. Sit the same point on a sunny afternoon and the lake breeze carries your scent inland, into the bush behind you.

Why the powder swirls

Swirling wind usually has a cause you can name.

• The switch. When a slope turns from draining to rising there's a lull of anything up to 40 minutes with almost no wind (Nadeau 2020). The powder goes every way.
• Two flows meeting. In the evening, slopes draining into a bottom run against the day's wind still coming up it.
• The lee. Wind over a ridge or a bank rolls back on the far side. In wind over about 20 km/h the lee below a crest eddies scent every way, and animals bed there on windy days.
• Midday heat. On a sunlit slope with a breeze above, warm air lifts off in bubbles and the wind gusts and turns.
• Still, clear nights. Cold air pooled in a basin sloshes slowly back and forth.

Check it where you sit

Every rule here, and every model, is a guess until you check it. Puff your powder when you get to your spot, again when the light changes, and again when the sun reaches your slope. The switch is the moment to be watching.