A familiar deer-hunting rule points hunters toward south-facing slopes when the weather turns cold. Like most rules of thumb, it gets the answer right for a reason no one puts a number to. Winter sunlight comes from low in the southern sky. The hours of direct sun reaching any patch of ground depend strongly on the direction its slope faces and the surrounding terrain that blocks the light. Across Illinois terrain, the modeled cold-season range is wide: direct sun reaches some ground for almost no time and other ground for most of the short day. That heat supply affects where snow remains, where deer can still reach forage, and where they can bed in warmth while using the fewest calories. This paper calculates what the familiar advice leaves vague, treating the result as an input to a hunting decision rather than a prediction.
KEY FINDINGS
- Terrain changes winter sunlight. The sun's low southern position during cold months means a hillside's direct exposure depends on aspect, its facing direction, plus shadows from nearby landforms. Two neighboring slopes can receive daily sunshine durations that differ by hours.
- The range is wide. Our model puts cold-season direct sunlight across Illinois at roughly zero to nearly ten hours per day for individual grid cells. An open slope facing south collects a full winter day of sun, while a neighboring shaded north-facing slope receives only a fraction.
- Cold-weather deer cut their energy costs. Snow tends to clear earlier from a slope that gets sunlight, which can make food more accessible. One Maryland study linked winter bed sites with an open exposure toward the southeast. That association helps explain the rule favoring south-facing slopes and why exceptions, such as southern slopes shaded from the sun or battered by wind, matter.
- Use sunlight as information, not a marked bedding spot. During cold weather, sunshine increases cover's appeal without establishing a bed's location. Combining the sunlight budget with relief and thermal information helps focus the search. It makes no claim about any particular deer's whereabouts.
ITHE RULE EVERYONE KNOWS, HALF-EXPLAINED
Which southern slopes offer winter warmth, and how much
Where should you look for deer in December? South-facing slopes are a ready answer among cold-weather deer hunters. The advice is sound, but hunters usually give it without a figure to explain it. Earth's orbit provides the reason. Deep in winter, the sun follows a low path through the southern sky, and its light strikes the land at a shallow angle. A slope tipped toward that path, facing south or southeast, receives the rays almost head-on and stores substantial warmth. A north-facing slope tips away, leaving the rays to strike at a glancing angle or allowing the ridge above to block them altogether. The familiar advice packs these effects into a compass heading, but the details of the ground matter. A south-facing slope beneath a taller hill's shadow can be just as cold as a north-facing slope. An open southeast bench can collect more heat than a due-south slope that loses its sun behind a ridge by mid-afternoon. To calculate how much sun reaches a south-facing slope, you have to account for both its direction and the terrain that shades it.
IIMEASURING THE BUDGET
Turning slope direction into daily sunlight duration
A compass supplies a heading. For a deer, the relevant measure is time: the hours a location spends under direct winter sunlight once nearby landforms have cast their shadows. That amount can be calculated. A detailed ground-elevation model provides the starting point. The low seasonal solar path can then be followed hourly, checking the terrain surrounding each location to determine when sunlight rises above its horizon and when a ridge interrupts it. Adding those intervals within each day, then averaging them over the cold months, produces a sunlight budget. Each grid cell receives a modeled daily duration of direct sunshine under clear skies. This analysis performs that calculation for a cold-season window running roughly from mid-October to mid-January. Across Illinois, modeled daily exposure reaches nearly ten hours at the upper end and about zero at the lower end. Maximum exposure occurs on an unobstructed hillside turned toward the seasonal sun. Minimum exposure occurs on a shaded northern hillside or ground kept dark by a ridge. This terrain-wide spread puts a quantity on the difference hunters recognize through their familiar guideline.
Swipe table to see every column
| Slope situation | Cold-season sun budget | What it means on the ground |
|---|---|---|
| Open south / southeast face | High: much of the short winter day | Tends to shed snow sooner and can leave food easier to reach |
| South face shaded by a higher ridge | Lower than its aspect implies | The compass says 'good,' the terrain says otherwise: the rule-of-thumb trap |
| East / west face | Moderate: a partial day | Morning or afternoon sun only; warms on a schedule, not all day |
| North face | Low: a fraction of the day | Tends to hold snow and cold longer |
| Valley bottom / deep draw | Variable: often shaded | May bank little direct sun even facing south, if walls block the low arc |
A compass shows a slope's direction. The sunlight budget also accounts for whether the ridge behind that slope has cut off its sunlight by two o'clock.
IIIWHY THE SUN MATTERS TO A DEER
Saving calories through the cold
A hunter has reason to pay attention to the sunlight budget because it affects deer. The reason is simple. In cold weather, a whitetail burns more calories than it eats and draws on stored reserves. Every joule saved on keeping warm stays available to the deer. Sunshine gives a slope heat without costing the deer energy. Sunny slopes tend to lose snow earlier, which can make acorns, waste grain and browse easier to reach while shaded slopes still carry snow. The sun heats the ground and the air touching it, so a deer resting in sunshine on a cold, windless afternoon keeps its body heat with less energy instead of shivering it away. Those benefits arrive on the very days when sunny and shaded slopes differ most: clear, cold stretches under high pressure. The preference for south-facing slopes follows the way deer gain and lose heat. That same heat balance explains the exceptions. A south-facing slope swept by cold wind can offer a worse bed than a sheltered pocket facing north, because wind can remove more heat than sunlight adds. Sunlight is one part of the deer's heat balance; wind and cover supply the other parts.
IVAN INPUT, NOT AN X
Using sunlight to focus scouting without claiming a deer's location
Turning a sunlight map into a bedding prediction would be simple, but mistaken: mark the warm slopes and claim that deer sleep there. We do not make that claim, for the same reason we will not identify a bed from relief alone. Bedding choices depend on cover structure, meaning the screening vegetation and thick growth that conceal a deer. Available sunlight influences that choice; it cannot take the place of cover. A sun layer helps hunters choose which places to scout first. In one study in western Maryland, winter nighttime beds had dense conifer cover, hemlock to the north and west, and an open exposure toward the southeast. In our work, combining sunlight, cover and relief reduces the possible bedding areas on a hillside to pockets that meet all three conditions. Reading the three terrain layers together gives hunters direction for scouting. Relief describes the ground's shape and shows how deer travel through it. The thermal layer shows how warming and cooling air carry a hunter's scent over that ground. The sunlight budget shows where a cold deer may spend less energy keeping warm. None of the three identifies a particular animal's location. Together, they reduce a broad stretch of country that looks much the same throughout to a short list of places worth visiting on foot. Our goal is to help hunters select better scouting spots.
The sunlight a deer receives changes the energy it needs to keep warm in cold weather. Knowing that helps narrow the search without locating the deer.
NOTES AND SOURCES
- Cold-season sunlight budget: we model direct-beam solar insolation over a period with approximate endpoints of October 15 and January 15. The model uses a USGS 3DEP elevation base and accounts for surrounding terrain that shades the ground from the low winter sun. We report the result as measured daily hours of direct sunlight mapped to each cell. Measured Illinois values range roughly from 0 to ~9.9 hours/day.
- The winter sun's low angle follows basic solar geometry. Slope direction, or aspect, works together with shadows cast by terrain to determine insolation; a compass heading therefore provides only part of the guidance. Wildlife literature documents well both nutritional deficits during cold weather and body-temperature regulation in white-tailed deer.
- Bedding is still fundamentally a matter of cover structure; see the companion terrain paper. The sunlight budget influences the appeal of cover during cold weather without predicting bedding. We present it as a layer that supplies context and input, never as a claim about a deer's location.
- Limits: the model calculates direct sunlight under clear skies without accounting for a particular day's cloud cover; cloud effects are added separately when the layer is used. It also does not model light reaching beneath thick evergreen canopy. Wind and cover can influence a deer's choice of site more than sunlight does. This information does not predict where any particular animal will be.
Fully Drawn Research is an independent data analysis desk, not affiliated with any state wildlife agency, transportation agency, mapping provider or outfitter. Fully Drawn Research is an independent data and analysis desk. This paper is educational: it explains cold-season solar insolation as a terrain input and how it influences cold-weather deer behavior. FDR expresses the sun budget as a measured context layer, not a prediction of deer location or a guarantee of hunting outcome; bedding is treated as cover-driven, never a terrain or sun pin. Solar figures are modeled from public elevation data and are current to the 2026 cold-season build.
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