Thermals on a typical mountain range go down in the morning and up from late morning through evening. As the evening approaches, the air starts to cool and settle, sinking down the mountain. It continues on this pattern until about midmorning when the sun warms air causing it to start to rise again. Think hot air balloon. Hot air rises, cool air sinks. Thermals are a very important thing to know in the high country. They can make or break a stalk and a strong thermal can push hard against wind direction, causing the dreaded swirl. Hope this helped a bit.
"For its in these mountains that I long to be, where I can hear God whisper down the creek and through the trees. Just one sunset and I know you'd think the same. Watch your worries fade 'neath a rocky mountain rain."
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my biggest weapon against thermals and wind change is cover scents. not scent elimenators, but cover scents. u allways produce ur scent no matter what u have on each time u breathe. use a scent from ur area ur hunting, pine, sage, oak, accorn, earth. pine and earth are my favs and works amazingly even surprisingly well even when the winds change. personaly i think sometimes the winds change toward ur stalked game and u atomatically freak out and i think the game sinces that before ur scent and spooks. may not be true but its worked for me.
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!
B&C / P&Y Official Measurer
Another thing to mention is where you live there is a thing called washoe zephyr foehn winds. Check it out, you can take a class up at the college. In the mean time, try finding s-190 and s-290 classes on YouTube. They are wildland fire weather classes and you can tailor them to hunting needs. Very informative and cover the basics.
Pay attention to the thermals in the area you are hunting, they can vary in time depending on weather patterns and the type of country you are hunting, desert or high country. Sometimes on a really hot day in the desert you only have an hour or so of downward thermals to make a move or you'll have to wait for a couple hours until the upward thermals take over. In the high country it can take till noon or later for it to warm up enough to have a consistent upward draft. There's always a couple of hours in the morning and evening where the thermals are fighting each other causing the wind to swirl. A strong prevailing wind will also mess up a good thermal wind. Knowing how the thermals work in your area and the weather can be important in knowing when (if at all) you should attempt a stalk.