Among the several suggestions in 2Poppa's post, this is the one I like best:
Many years ago I bought three hundred outdoor temperature gauges at a flea market, that are light weight and easy to hang. I have hung these gauges in many locations, such as different areas in hollows, on points, fields, creek beds, and ridges. __
Over the years after checking the temperature gauges, I've realized why certain particular areas hold deer, while other areas are void. I have noticed some hollows and ridges will have an eight to twelve degree difference especially on very hot days. Most of these areas that are cooler will have the ground holding moisture, water, and condensation, much longer, allowing a bunking deer to be much more comfortable than one bunking in a much similar, warmer area, marked by a higher reading on the temperature gauge.