And that's where the thermal starts. Spots where there's a lot of surface heating is usually the most common location for thermals. So places like asphalt parking lots, junk yards, and rock outcroppings are great places for thermals to form.
As the air above these spots starts to heat, small plumes of warm air begin to rise. Think of it like bubbles rising up from the bottom of a champagne glass cheers!
As the small plumes rise, they spin around randomly, and eventually, they start bumping into each other. The plumes continue to bump into each other as they rise, turning into larger and larger blobs. Eventually, as they leave the surface layer meters AGL, or roughly feet , they're a full-blown thermal. As the thermals rise, they twist and flow with the wind.
They typically rise at meters per second - which computes to about feet per minute. But they don't keep rising forever. Some thermals only last for a matter of seconds, while others can last up to minutes. As thermals continue to rise, they cool, and eventually, they reach the same or slightly cooler temperature as the surrounding air. And this usually happens at a place called the boundary layer.
The boundary layer is the part of the atmosphere that's affected by the earth's surface. It's typically very shallow at night meters , and grows throughout the day meters , as the day progresses and the ground is warmed by the sun. When a thermal hits the boundary layer, it typically flattens and spread out. As thermals columns are rising, the air outside the columns is cooler, and more dense, which means it's sinking. But since the sinking air usually covers more area and has more mass , it sinks at a slower rate than the rising thermal columns.
The best area to find thermals are dark areas that absorb energy from the sun, and rapidly heat the air directly above them. Dark fields and parking lots are perfect places for thermals to form. Because they heat up quickly, they rapidly heat the air directly above them, creating strong thermals. But there's another place that's great for creating thermals, especially early in the day. And that's mountain ridges. So to first order, thermals are columns of warm and therefore buoyant air that rise from the surface layer to the inversion.
The spacing between thermals is about 1. The thermals themselves are somewhat less than half that, say m in diameter. Most thermals span the boundary layer vertically. There is, of course, a distribution of sizes. Between thermals are broad areas of sink. The sink is weaker than the lift because it covers a larger area. The opposite is true at the top of the boundary layer, but we rarely fly that high. There are, as always, complications. Sometimes we fly in the surface layer and sometimes in the lower part of the boundary layer.
Rising air in the surface layer the lowest m is in the form of small plumes, themselves a few tens of meters in diameter.
These plumes converge near the top of the surface layer to form thermals. The surface layer to boundary layer transition is not sharp, so we often find ourselves flying in either well-organized thermals or disorganized plumes, or some of both. Thermals evolve over time, are influenced by terrain, and are shaped by and move with the wind. Boundary layer thermals form and dissipate with time scales of minutes, surface layer plumes faster.
This can lead to the apparent phenomenon of "bubbles" or detached thermals or plumes. Plumes and thermals respond to irregularities in the surface different amounts of vegetation, houses, and so on by forming more often in some places than others. Dark ground if it's not wet! If the wind is light, thermals may stay attached to the hot spot. If not, thermals may form repeatedly over the hot spot and drift downwind.
Thermals drift with the average wind over their height, so they may travel at a higher speed and in a somewhat different direction than the surface wind. Thermals also tilt if the wind is stronger at higher altitude, th usual case.
Thermals are not uniform, nor do they have sharp edges. The edges interact with the surrounding air, so thermals have a warm, usually fairly smooth core surrounded by turbulent edges. Learning to work your offense to beat this incredible defense can be done by understanding and utilizing wind and thermal currents. Here are five items to consider as you prepare to hunt this fall:. Wind direction is the primary aspect that affects the success of our hunts each fall.
Wind direction can be affected by many factors such as terrain, vegetation types, vegetation density, temperatures and the jet stream and so on. Air masses moving through an area can cause the wind direction to change degrees in a relatively short time.
The prevailing wind direction for a region is the single most important wind direction to focus on when setting up a property for effective hunting, including the placement of habitat work such as clear cuts food plots and tree stands. For most whitetail hunters, the prevailing wind is a west wind of some sort, from south southwest to north northwest. Thermals are the movement of air due to temperature — warm air rises and cool air sinks.
Thermals associated with local terrain are somewhat predictable in that they are pronounced in the evening and morning when the wind is not blowing, especially in hilly or mountainous terrain. Places where thermals have the greatest impact on a hunt are when trying to hunt the bottom of valleys. Valley bottoms are great places to find deer crossings as deer generally funnel into worn muddy trails as they cross creeks.
Hunting ridge tops is generally the easiest way to avoid swirling air currents, but sometimes that is not possible or practical. The rugged terrain and deep valleys meant that if you wanted to hunt across a property at least some stands needed to be in or near the bottom of the valleys. This is where learning local thermal air movement is critical. This is because each day as the air temperature began to warm, thermals would rise up the valleys. Late in the day when the shadows lengthened the temperature would begin dropping and they would switch and go down the valleys.
In the early morning, they would always be flowing down the valley and then sometime before 11 A. This generally meant a quality one to four-hour hunt could be had before the wind switched and ruined the stand site each morning.
Evenings worked the opposite direction but the wind switch often occurred closer to sunset. Not all trees are great stand sites. Narrow down your potential stand sites by learning deer patterns and monitoring the wind and thermal. The homemade stand was in disarray by the time I came along and on the second year I hunted from it, a strong storm split the tree in two.
The stand had trails passing by in several directions and I planned to bow hunt it the entire season. Nobody ever let on, but years later I learned that I had simply been wearing out the stand site with repeated hunts during all weather conditions and wind directions. So whether it is from scent or physical disturbance, the stand site was well burned out. This is when I started investing money in more treestands.
Now I have treestands positioned for all wind directions, approach scenarios and thermal patterns scattered throughout my hunting properties. No longer do I have to sit at home fuming that I have nowhere to hunt or worry I may spook deer by slipping into a stand with the wrong wind.
Having stand choices is especially important when working around an inflexible schedule. This has occasionally been the case throughout the years as work or family obligations caused me to slip out when I had time instead of when the time was right. Being prepared with ample stand sites ensures at least one or two stand sites have the right wind for a quality hunt.
This is where a climbing stand can be effective. An important aspect in hanging stands for varying wind directions is to have a good handle on exactly what wind directions work well for the site.
Placing treestands often means traveling down winding logging roads and busting brush with the stand, steps, and saw in tow. There are smartphone apps that can pinpoint your location on a map while you are at the stand site, but cellular service may not always be available.
This is where I continue to utilize my old-school compass. Taking a moment to determine the optimum wind directions for a stand as I hang it saves me time and disturbance during my valuable hunting season. Time spent on a property and in each stand location is the only way to truly learn how air flows through the property whether by thermals or wind.
This is also true for how exactly deer move around in relation to air currents, terrain, and vegetation. There is no doubt studying terrain and aerial maps can unveil a lot of information about a property but especially the first season or two, time in the field is critical to successful hunting. It can happen the opposite also, as it did for me hunting a new property last fall. It was early November and I was hunting an area with quite a bit of topography. My stand was toward the top of a ridge overlooking a well-worn trail.
From behind me and directly downwind five bucks were chasing a doe on the next ridge. I blew a series of tending grunt calls and the largest of the bucks peeled off and came my direction.
For one hundred and fifty yards he walked through a seriously deep ravine from my downwind direction. The only possible explanation was that it was a sunny morning and the thermals had switched in time to carry my scent up and out of the ravine instead of down into it, providing the buck a path free of my scent.
As you can imagine, I learned things about that stand site that morning.
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