Morning Thermals Rise, Evening Thermals Fall, and Both Decide Where You Can Sit
Cold air drains downhill overnight and keeps draining until the sun warms the slope, then reverses and runs uphill until that slope goes into shade. The flip time depends on aspect: an east face can turn 20 to 45 minutes after sunrise, a west face may not turn for two hours, and a shaded north face in late season may never turn at all. Hunt the slope that has already committed, not the one that is about to change its mind.
On any slope with sun on it, air warms, expands and runs uphill. When that slope goes into shade, the air cools, gets dense and drains back down. Those two flips happen every day, they happen at different times on every aspect, and they will override a 6 mph forecast wind completely.
The forecast on your phone describes the gradient wind a thousand feet above the terrain. What is happening against the ground where the elk are is a local convection cell, and on a clear September morning it can be running in the exact opposite direction to what the app is showing you. Hunters get busted by that difference constantly and blame their scent control.
The question worth answering is not which way is the wind blowing. It is what time does this particular slope flip, and how long is the transition, because during that transition the air swirls and there is no safe approach at all.
Why the forecast lies to you on a mountain
Overnight, every slope radiates heat to a clear sky, the air against it cools, and cold dense air slides downhill and pools in the drainage bottoms. This is why creek bottoms are colder than the benches above them at dawn, sometimes by 10 or 15F, and why an inversion can sit in a canyon until mid-morning.
After sunrise, the sun hits whichever face is angled toward it first, that ground warms, and the air above it begins to climb. The drainage does not flip all at once. The sunlit face can be flowing hard uphill while the shaded face fifty yards across the creek is still draining down, and the bottom between them is doing something unpredictable involving both.
In the evening the sequence runs backwards, and it runs faster. Shade arrives suddenly when the sun clears a ridge, and the downhill drift can establish within twenty minutes on a steep east-facing slope while the west face opposite is still pushing uphill in full sun.
The times below are stated relative to local sunrise and sunset because that is the only frame that travels between latitudes and dates. SR+30 means thirty minutes after sunrise. SS-60 means sixty minutes before sunset.
The timetable by aspect
These are clear-sky, moderate-canopy, September-to-October figures for mid-latitude western terrain, on slopes of roughly 20 to 35 degrees. Every one of them moves with cloud, canopy and snow, and the adjustments are in the next table.
Slope aspect Downhill drift ends Uphill flow steady Uphill starts to fail Downhill re-established Strength of daily swing East face SR+20 to SR+45 SR+60 onward SS-120 to SS-90 SS-60 Strong, and earliest both ways Southeast face SR+30 to SR+60 SR+75 onward SS-105 to SS-75 SS-45 Strong South face SR+30 to SR+60 SR+75 to SS-60 SS-60 to SS-30 SS-20 Strongest. Biggest swing of any aspect. Southwest face SR+60 to SR+100 SR+120 onward SS-45 to SS-15 SS+10 Strong but late West face SR+90 to SR+150 SR+150 to sunset SS-15 to SS+10 SS+30 Strong, latest flip in both directions North face SR+120 to never Weak and intermittent SS-150 onward SS-90 or earlier Weak. Treat as downhill-dominant all day. Canyon or valley bottom SR+60 to SR+180 Often never fully n/a SS-90, before the slopes above Cold air pools. Inversions hold for hours. Ridge top and above treeline SR+45 to SR+75 SR+75 onward, weak SS-60 SS-30 Weakest. Prevailing wind usually dominates. Find the aspect you plan to sit and read the two flip columns. Those are the minutes to be already in position or already gone, not the minutes to be walking in. Two rows in that table are worth memorising. The north face is the one that catches people out, because in late season with a low sun angle it can stay shaded all day and drain downhill from dawn to dark. Hunters assume it will flip because every other slope did, walk in from below at 9am, and push everything out ahead of them.
The canyon bottom is the other. Cold air sitting in a drainage does not obey the slopes around it. You can be standing in air moving steadily downstream at 10am while both walls above you are pushing uphill, and every elk on both slopes is downwind of the creek you just walked up.
Cloud, canopy and snow move all of those times
Everything in the table above depends on solar heating reaching the ground. Anything between the sun and the slope delays the morning flip and weakens the whole cycle. Apply these as minute adjustments to the aspect times.
Condition Morning flip Evening flip Strength of thermal What to do about it Clear, dry, light frost As tabled As tabled Full Trust the timetable to within about 15 minutes Thin high cloud +15 to +30 min -15 min earlier Slightly reduced Add a small margin at both crossovers Heavy overcast +45 to +120 min, may not flip Earlier and vague Weak to absent Hunt the prevailing forecast wind. It rules today. Dense conifer canopy +30 to +60 min -30 min earlier Damped and swirly Add a full half hour to your safe-approach window Open sage or burn scar -15 min earlier As tabled Amplified Flips fast and hard. Be in position before it happens. Fresh snow cover +30 to +90 min Earlier Much weaker Snow reflects the heat away. Slopes may never flip. Snow plus overcast May not flip at all n/a Effectively none Pure prevailing wind day. Thermals are off the board. Rain or wet ground +30 to +60 min Earlier Weak Evaporation eats the heating. Expect a sluggish day. Creek or standing water in the bottom As tabled on slopes Earlier in the bottom Bottom stays cold The drainage bottom holds downhill drift longer than the walls Prevailing wind above 12 mph Irrelevant on exposed faces Irrelevant on exposed faces Overridden See the next table Stack these on top of the aspect timetable. Two adjustments in the same direction, such as heavy canopy plus overcast, can push a morning flip past 11am or cancel it entirely. When the prevailing wind wins
Thermals are a gentle force. On a good clear morning a slope thermal runs somewhere in the region of 2 to 6 mph. Put a 15 mph gradient wind across the same slope and the thermal loses on any exposed face, though it survives in sheltered pockets, on the lee side of ridges and in timbered draws.
The dangerous case is neither of those. It is when the two are roughly matched and pointing at each other, because then the air does not go anywhere at all. It stalls, rolls over, and moves in unpredictable pulses. Deer and elk in that zone will get you eventually.
Prevailing wind at the slope Thermal state What the air actually does Huntable? 0-4 mph Any Thermal rules completely. Use the timetable. Yes, if you respect the crossovers 5-10 mph, same direction as thermal Established Reinforced, steady, predictable Yes. The best conditions on the mountain. 5-10 mph, opposing the thermal Established Stalls, swirls, pulses both ways No. Move to a different aspect or wait it out. 11-18 mph, any direction Established Prevailing wind wins on exposed faces, thermal survives in timbered draws Yes on open faces. Distrust the draws. 11-18 mph, any direction Weak or absent Prevailing wind rules everywhere Yes. Hunt it as flat-ground wind. 19-30 mph Any Prevailing wind rules, with heavy mechanical turbulence behind ridges Yes on the windward side. Lee slopes are a rolling mess. Below 4 mph during a crossover window Reversing Dead air, then reversal, then dead air again No. This is the worst twenty minutes of the day. Check wind speed against thermal state before you commit to an approach. Two of these rows say no, and both of them are days hunters get away with because they never learn they were smelled. Decision tree: which side of the drainage
Run this before you leave the truck, with sunrise time, aspect and forecast wind in hand.
- Morning hunt, clear sky, animals above you: approach from below only before the flip, which on an east face means before roughly SR+20. After that, your scent is going straight up to them. Get in early or come in from the top.
- Morning hunt, clear sky, animals below you: approach from above and stay above until the flip, then be settled before it happens. On a west face you have until roughly SR+90, which is the most generous window on the mountain.
- Morning hunt on a north face in late season: assume it never flips. Treat the whole slope as draining downhill all day and never approach from below at any hour.
- Evening hunt, animals on a south or west face: those slopes hold uphill flow until nearly sunset, so you can sit below them for most of the sit. Get out before SS+10 or your scent goes back down the mountain to them.
- Evening hunt, animals on an east face: the slope shades early and can be draining by SS-60, which is inside your best hour. Sit above them, not below.
- Heavy overcast on any aspect: ignore the timetable entirely and hunt the forecast wind as if the terrain were flat.
- You must cross the drainage bottom to get in: cross it during the middle of the day if there is any way to do so, because the bottom is the least predictable air on the mountain and it feeds both slopes.
- Prevailing wind is 5 to 10 mph and opposing the thermal on your target slope: pick a different slope. There is no clever approach that beats stalled air.
The general principle underneath all eight branches is simple. Hunt the slope that has already committed to a direction, and be in position before the one you are hunting changes its mind.
The two crossover windows are where hunts die
Each flip takes roughly 15 to 40 minutes to complete, and during it the air near the ground reverses in patches rather than all at once. You will feel the wind on your face, then on your neck, then on your face again. Whatever is downwind of you during that window is going to get a lungful.
Both crossovers land badly. The morning one arrives during the first hour of shooting light on east-facing ground. The evening one arrives inside the last hour on the same aspect. Those are exactly the hours you are trying to hunt.
The only reliable answer is to be settled and still before the window opens, in a spot chosen so that both wind directions put your scent somewhere harmless. That is a saddle, a spur nose, or a bench where the slope falls away on the side you do not care about. Picking a stand location that survives a reversal is worth more than any amount of scent control.
Reading it in the field rather than trusting the table
Carry something that shows drift, not just direction. Unscented powder in a squeeze bottle is the standard and it works down to almost dead calm. Milkweed floss is better still, because it hangs in the air for thirty or forty seconds and shows you what the air is doing over a hundred yards of slope rather than at your fingertips.
Check it every ten minutes during the hour either side of a predicted crossover, and check it at head height and at knee height, because on a shallow slope the two can disagree. If your powder falls and your floss climbs, you are in the transition and you should not be moving.
One useful habit: puff powder at your entry point before you walk in, watch where it goes, and only then decide your route. Thirty seconds of that beats an hour of guessing from an app.
Where these times stop holding
Latitude and date move the whole table. Late-season hunts at high latitude have a low sun angle that never gets above about 25 degrees, so slopes receive far less energy, the flips get later and weaker, and north faces stay in permanent shade. A November hunt in Montana is not running the September timetable.
Big open basins behave differently from steep drainages. In wide country with gentle slopes, the thermal cell is broader and slower, the flip is vague and can take an hour, and the prevailing wind takes over at lower speeds. The times above are for defined drainages with real relief.
Coastal and lake-adjacent terrain has a second circulation stacked on top. Onshore and offshore breezes run on their own daily cycle and can dominate the slope thermals within a few miles of large water.
Deep canyon inversions break the model outright. When cold air is trapped under a warm layer, the bottom stays cold and drains downstream regardless of what the sun is doing on the walls above it, and that can persist until early afternoon. If the bottom is holding fog at 10am, you are looking at an inversion and the timetable does not apply to anything below the fog line.