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.