Thermal air flow has been described fairly accurate in Darktimber's post above, IMO. I'll only add two things: In steep, broken terrain, thermals can be re-directed as different air temperatures are encountered as it rises or sinks. This is what creates "swirling"........unfortunately, most good mule deer terrain is in broken and steep terrain. With that in mind, the most success I've had stalking bedded high-country bucks has been between 11:30am to 2:30 PM, as the thermals are generally the most consistent. However, partly cloudy skies, and shadowed cuts and canyons (which create cooler air) can still cause inconsistent air flow, causing havoc on an approaching bowhunter. It's truly what makes stalking high-country mule deer with a bow a supreme challenge. Good luck and I hope you stick a good one!