Deer live and die by their senses, so choosing stand locations that enable you to stay upwind of predicted deer movement is vital to success. Unfortunately, all too often the wind direction in a treestand is not what the forecast predicted. This can be caused by a variety of reasons such as unpredicted weather shifts, wind eddies caused by forest opening/patches, topography, and thermals. Many of these factors are property specific and require experimentation to figure out these unique wind variations. However, thermals operate by several general rules that once understood can lead to increased success and less blown hunts.
While we are not physicists, we all probably understand the general rule that warm air rises and cool air sinks. This drives thermals by causing updrafts when the air is warming and rising up from the earth. Likewise, when air is cooling it will fall and run downhill or toward areas of lower elevation.
The general rule of thermals is that they rise in the morning (when the earth’s surface is cool and warming) and fall in the evenings (when the earth’s surface is warm and beginning to cool). Obviously the intensity of thermals will depend on the speed with which the earth’s surface temperature is changing.
Topographical features such as ridges and valleys offer a range of elevations along the ground causing updrafts and downdrafts to be smelled at ground level. These features can be a detriment to your hunting or a benefit depending on how you adapt to them. To avoid being winded in the morning you should try to hunt high on ridges and avoid bottoms where your rising scent can run up the adjacent hills and spook deer. During evening hunts setting up in bottoms will allow you to avoid spooking deer below you. Realize that these are generalizations and every situation is different and requires critical thinking and analysis to see how these affects alter windflow.
Water also interacts with atmospheric temperatures to create exaggerated thermals. I notice the thermal effects of nearby water much more during evening hunts than mornings. If you can setup your stand adjacent to or over flowing water during evenings with the wind pulling toward the water, often the thermal pull of the quickly cooling water over the stream will suck in your scent and keep it suspended on the water and not over land where deer may smell it.
Applying these basic thermal strategies to your hunting will get you on the right track to take advantage of thermals. The longer you watch wind currents in relation to water, topography, forest edges, and wind direction, the more you will understand and be able to predict micro-wind currents on your hunting properties.