How thermals move your scent
Thermals are air moving because of temperature. When the sun warms a slope, the air touching it warms, gets lighter and climbs the slope. When the sun leaves, the ground cools, the air touching it cools, gets heavier and drains downhill like water. Prevailing wind can override all of this. On calm days, and in the first and last hours of light, thermals often decide where your scent goes.
Morning: the switch from down to up
Air is still draining downhill at first light, and it keeps falling after sunrise until the sun has had time to warm the ground. East and south faces switch first. North faces and deep shaded drainages switch last, and in late season some never switch at all. A stand that is safe at first light can be sending your scent uphill an hour later.
Evening: the switch from up to down
A slope starts cooling when it loses direct sun, not at sunset. East faces lose the sun first and turn down first. West faces hold rising air the longest. In a valley, cool air pools in the bottom before dark and starts moving down the drainage.
How terrain shapes the flow
Ridges and saddles. Rising air pulls up both sides of a ridge and lifts off the top. A saddle is the low point on the spine, so air crossing the ridge gets funneled through it. At night both sides drain away from the spine.
Drainage heads. A steep drainage head forces deer to walk around the top. At night falling air collects in the drainage and runs down it. In the morning the sunny wall can be rising while the shaded wall is still falling, so air in the cut swirls.
Sidehill benches. A bench interrupts the slope. Air moving up or down the hill slows and spreads across the flat before it continues, so scent lingers on a bench longer than on open slope.
Creek bottoms. Cold air settles into the bottom and flows down the creek in the evening and early morning. Moving water, especially when it is shaded, stays cooler than the air around it, so close to the creek your scent gets pulled down to the water and carried downstream for much of the day. Bottoms also warm late, so rising thermals start later there and falling thermals start earlier. Farther up the banks, strong midday sun still pulls air uphill.
Ridge to creek. Where a ridge runs down into a flat creek bottom, cold air pours off the slope at night, pools on the flat and drains down the creek. Near the water, the creek keeps pulling air toward it and downstream even after sunrise. By midday the ridge face pulls air uphill, but a stand close to the creek can stay in the creek's pull. A stand on the flat between the base of the ridge and the creek sits right where the two fight every morning and evening.
Ag field edges. Flat ground has weak thermals. Open fields heat faster than timber, so by midday air rises and swirls over the field. At night the field cools fast and air drifts toward the lowest ground.
Funnels and pinch points. A gap between two hills channels the air draining off both sides. At night falling air runs through the pinch and out the low side.
Special situations
Creeks and cold water. Moving water, especially shaded water, stays colder than the air around it. Close to a creek, that cold water pulls air down to it and carries your scent downstream, often well after the slopes above have turned up. The farther you set up from the water, the more the slope takes over.
Heavy canopy and mixed shade. Under thick timber, sunlight reaches the ground in patches. Sunny gaps warm and pull air up while shaded ground next to them stays cool and keeps air sinking. Where those meet, air swirls and your scent can go any direction. Thick canopy also shifts the timing: shaded ground switches up later in the morning and back down earlier in the evening than open ground nearby.
Season and region
The sun's angle changes everything. In September the sun is high, slopes heat fast and thermals turn up early. By December the sun is low, north faces can sit in shade all day and the rising window is short. The farther north you hunt, the stronger the effect.
Use it on your ground
Pick the terrain here that looks most like your stand location. Set your region, the date and the time you plan to be in the stand. Rotate the terrain until it faces the way your ground faces. Then confirm on the ground with a wind checker. Real wind, cloud cover and vegetation will change what you see.
Times are local solar time, which can differ from clock time by an hour or more depending on time zone and daylight saving. The model assumes clear sky and calm wind, and it simplifies how the ground heats and cools.
