Sep 9, 2017
Blowing in the Wind
By: Doug Thalacker
The wind sock showed a southeast wind as I stood by the cabin door. So, I decided to use the stand in the corner of a cornfield. There were woods behind me and to my right, with the now harvested cornfield in front and to the left. Several times that morning I had heard deer moving through the woods behind me. They would stick their noses out of the woods and then silently melt back into the trees. It became obvious that they were sensing me. I could see the stiff wind pushing the old corn stalks away from me although I felt little wind sitting in the corner. I had walked in along the far edge of the field so I had not crossed the trail the deer were using. The next day the wind was from the same direction but not as strong. Same situation, same results. I was baffled. I had showered, kept my hunting clothes outside, wore rubber boots. I had done everything to keep them from detecting me by smell, yet they had very definitely smelled me.
On the third day, after being frustrated again, I decided that it was time for a little scientific investigation. This investigation however wasn’t about deer, it was about wind. The field research consisted of going to the open field on a windy day with a ten foot step ladder and an atomizer spray bottle of flour to detect wind direction and going to the river and watch the water flow.
To most of us air is that hopefully invisible medium that we live in every day and pretty much ignore until it blows us over. To a physical scientist, however, it is classified as a fluid, that's right a fluid. It acts and reacts just like water. It flows, it meanders, it swirls. It creates waves and currents and backwaters....just like water.
A river is a good place to begin to understand wind because it is predictable in the direction of flow and it is visible. Water in the middle of a river flows nice and smooth. When it encounters an object, an object such as a rock or log, the river bed or bank, however, its flow changes. In general, this is called turbulance. In the case of a rock in the middle of the river the water has several paths it can follow (Fig 1). The obvious ones are that it can go over or around on either side. There is also a less obvious path on the upstream side in that it can flow back against the current for a short ways along the river bed. The stronger the water, the shorter this back flow will be because of all the pressure from the main current. On the downstream side of the rock, there are two types of turbulance. One is not really a current but rather a breakline, seen by a line of bubbles and debris, created as the water flows past the “edge” of the rock. This current break shows the divide between the main river current and the calmer water behind the rock. These current break lines converge downstream of the rock; the stronger the current the further downstream the convergance.The second motion is a backcurrent, sometimes called a gyre (defined as a whirl, a spiral or a vortex), that flows upstream toward the center of the rock. You can often see debris moving in a circle in this area.
The banks and bed of a river also change the flow by slowing the water through friction. Also just as with the rock there can be gyres formed behind obstructions along the bank. If the river widens suddenly gyres can form on each side of the main current. If the river bends (called a meander) a gyre can form on the inside corner.
Air does the same thing. As it moves over an open field it flows straight and steady. When it encounters a woods it does much the same as water in the river - it can flow over, around, and of couse, into the woods since it is not solid like a rock AND it can move back along the ground opposite the direction of the main flow. This distance may not be far but it may be enough to get your smell to an approaching deer. On the leeward side of the woods, as the wind moves up and over, it can create a back flow opposite direction of the main wind. This is a result of the Bernoulli Effect. As a fluid moves over an object it creates a slight vacuum on the downwind side of the object. This is what pulls leaves and such into the front of the box of a pickup truck. It is also what lifts airplanes and allows NASCAR drivers to draft behind other cars.
Using this knowledge about water flow, I needed to quantify this research with some field observations. I walked a grid pattern around the field. About every twenty feet I squirted some flour. I recorded the wind dierection about 3 feet off the ground and using the step ladder at nine or ten feet. The next day I repeated the same observations. Fig. 2 shows a rough summary of my results. The wind out in the middle of the field was steady, as predicted by the windsock on the cabin. The surprise was the direction of the wind near my stand; right at ground level it had changed from a steady wind to many variable eddies. One thing that did stand out and explained why I was getting busted was the backcurrent that blew from the north for about 20 yards right toward the deer trail I was hunting.
Because wind is a physical substance it will obey physical laws. The means for any given set of conditions it will behave roughly the same way every time. This also makes predicting the subtleties of wind direction easier to predict in a new spot. While I have not tested any other locations and wind conditions, I believe the following points are going to hold under most conditions:
1) On the windward side of a woods (the wind blowing across the field toward the woods) there will be a slight upstream wind, away from the woods, close to the ground right where a deer's nose is. It won’t be a great distance and the stronger the wind, the shorter the distance. For gun season, this might not be a big factor but ten yards is as good as a mile when bowhunting.
2) On the leeward side of a woods (the wind blowing over the woods and into a field) there will be a backflow, toward the woods. This is very predictable and is dependent on the strength of the main wind. The stronger the wind the further this back flow extends from the edge of the woods. Also, if a gyre forms it can keep your scent moving around and around for some time.
3) The stronger and steadier the wind, the more consistent the results. On the third day of my opservations, the wind had dropped to only one to two miles an hour and the results were completely unpredictable.
4) Wind can be reflected, deflected and funneled producing meanders and eddies. Although wind is not bounded by banks like a river, it will blow horizontal if it can and follow the path of least resistance. Instead of a straight flow up and over woods, it will meander following the path of least resistance, often contrary to the main flow higher up. This can create an increased speed as the air moves through a narrow access between two fields. This can create back flows and eddies on both sides of the opening that are very predictable.
5) Wind only moves smooth and straight when it is high up. At ground level any obstacle will produce a change in the flow.
So what is the upshot of this for deer hunters? An obvious problem with this simple experiment is that each location has a different field topography and arrangement of woods and open areas. Each change of these variables will produce different results from mine. However, it seems that for any given spot the pattern will be the same under the same conditions.
I would have three pieces of advice. First, keep a journal of wind patterns whenever you are out in the area that you hunt. Over time you can establish some basic wind patterns for a given area. Secondly, don’t trust the windsock hanging from your cabin or shown on your phone app. As you walk to your stand use a wind direction indicator such as the Code Blue or Moccasin Joe Smoke Wind Detector to determine any local wind patterns around the immediate area. A hand-held anenometer such as those made by La Crosse or HoldPeak are inexpensive ($25+/-) and not only show you wind speed but also temperature and windchill. Thirdly, be aware that deer have an inherent sense of the wind, it's part of their survival package.
It would probably be safe to say that deer and most wild animals have a better feeling for the physics of wind than all our scientists. You will never come close to having such an innate sense but you can still play the wind game and win.