Overlooking thermals in flat terrain could be your biggest mistake you make all year.
For those of us that bow hunt the flatlands of the midwest or even down south some of us may think that thermals won’t have any effect on our set up. When we hear the discussion of thermals we may think of the mountainous regions of the Virginias or the hill country of western Wisconsin, etc. While these regions can be extremely difficult in regards to getting busted by whitetails it’s the flatlander thermals that tend to get overlooked.
First, what are thermals and how do they work? Thermals are air currents that move depending upon the heating and cooling effect of the earth’s surface. When a surface area that leads upwards is being heated, this causes air to move upward or uphill. When a surface area is cooling it causes the air to move downward or downhill. This is very prominent in hill country and mountainous regions and more subtle in these flat lands. The easiest way I remember thermals is when the sun rises the thermals rise uphill and when the sun goes down the thermals go down hill.
Second, for the purpose of this article, flat land refers primarily to the “flatter” farm country of the midwest. In reality no land is ever truly flat. Ditches, knolls, knobs, ponds, rivers, dips, etc. all provide some type of region for air to rise and fall during these thermal transitions. Matter of fact, any type of gradient, even if its only 5% will cause a thermal effect. No region, even in flatlander land, is ever truly flat over a myriad of geographical spaces.
Even in flatlander country, there are always dips and slopes that affect your thermals.
Several years ago I started to learn about thermals as I turned my attention to hunting hill country in western Wisconsin. I read everything I could on thermals and how they worked. While there is a plethora of info on hill country and mountainous terrain thermals, we will keep this topic on how the thermals relate to flat land.
My first encounter with how I noticed the effect of flat land thermals came when I was doing an observation sit for whitetails a few years ago. One week prior to the bow season opener I was sitting approximately 250 yards with binos in hand overlooking an alfalfa field for opening day. My goal was to observe what came out of the corner of this 15 acre swampy wood lot. I positioned myself on a dead end fence line that had some dead trees along with a myriad of brush. I was hidden perfectly as I could scan this whole wood lot from afar to see if there were any good bucks worth harvesting for the Wisconsin bow opener.
Everything was spot on, my approach, the wind was in my face at 10 -12 mph and I could view any deer that stepped out onto the field that night. Approx 40 minutes before dusk a wary doe and 2 fawns stepped out. She knew something was up. About 5 minutes later another doe and her fawn stepped out. The two older doe started to place their noses in the air as their olfactory system was kicking in big time. And just like that they bailed back into the swamp.
So what in the world just happened? It made no sense. When I set up, the wind was in my face and I was 250 yards away! Thermals...that’s what happened.
In the pic below you can see where I was set up on the red dot overlooking that field. You can also see the contours lines that slope downward to the east. This is a very gradual slope.
How I got busted was that when I set up for the observation sit I had a pretty good breeze that afternoon in my face coming from the east. The easterly winds died down and then the temps started to drop rapidly. This caused a greater cooling effect with the rapid temperature drop creating a pronounced air movement for the thermals to move down that slope faster. Matter of fact, how I knew for sure that it was the thermals is that I had just a few stray pieces of milkweed in one of my pockets. I placed a few in the air and watched those little white floaters head right to those deer.
Another factor that played a role in that scenario is water. When warm water cools from its previous temperature it creates air above the water to go straight up. This creates a vacum effect that pulls the surrounding air around a pond or lake and any scent molecules from that area. The water in the swamp that the doe came from I’m sure played a role in the deer busting me. Once again from that thermal effect.
The best product to help determine your thermals is milkweed. I like to get a few pods and then open them up and let them air dry. I then take the dry, fluffy silk and place them in a larger pill bottle to keep them dry. Where I am located here in Wisconsin there is a large supply of milkweed usually found in road ditches or mixed in with open natural grasses. I then take the milkweed strands with just a few at a time and let the air current take them where the air current wants to go.
The great thing about milkweed is that it’s white and easy to see as it floats away from you. Essentially, you’re watching your scent in real time. You can see how the milkweed goes around trees, near slopes, near water, etc. I always keep some within an arm's reach away especially if I am hunting a mature buck with a quartering wind. Where that milkweed/scent is traveling may determine where you will release an arrow dependent upon the route he is headed. If milkweed isn’t available in your area, cattail fluff will work too. However, milkweed floats a little longer in the air and is the preferred choice
Let’s take a look at some principles and examples for flatlander thermals and how to “work” with them.
The mounds. The mounds in my neck of the woods are sometimes the size of a pickup truck. And yes, they can have thermal effects. Especially when a mound may be steeper on one side compared to its other side. I have seen many times milkweed dropped in one particular direction due to the predominant wind and suddenly turn directions and move slowly down the steeper drop off of the mound due to the dropping thermals. Some hunters prefer these mounds as they can give a hunter a height advantage in comparison to the surrounding landscape. However, the cooling process for evening hunts can bring your scent down this terrain feature right into a deer’s path. Consider all the options and set up accordingly.
Ponds or standing water - Once again, as the air cools right before dark, in turn this draws the air upward and pulls scent with it. Some hunters like to use standing water as part of a funnel. Trying to get the deer between them and the pond. Towards dark when deer tend to be most active, the thermals from the pond will draw your scent towards the deer, even if your wind was blowing the other direction for most of the day. In this case, anticipate a whitetail’s movement and have yourself between the deer and the water. Even swamps that are laden with water can have this thermal effect.
Old river rims - Where rivers have receded over time make great bedding areas for deer in these woody, thick basins. Where the rivers ran hard and sharp years ago can create rims or old river banks where the water once was. These rims can sometimes be hundreds of yards from the current river. Deer traveling from these basins in the evenings like to travel to the base of the rim and then travel along that base. This allows them to utilize the dropping thermals in the evenings from one direction and any winds that are traveling from the other direction. If the rims are steep and long enough this can then revert to hunting deer along the military crest like one would hunt in hill country. In the mornings as thermals rise, deer can be found traveling along the top portion of the rim.
Rims can be tricky as those basins below can have bucks bedded in them sometimes less than 100 yards from the base of the rim and sometimes up top. Morning hunts can be effective for this approach as you can slip in somewhere up top and not reveal your location. Personally I like to set up where a rim has a deep cut or where it flattens out to a certain degree. Technically, any change of the rim.
Wood lot basins - Similar to my observation sit story, small wood lots tend to sit lower in elevation at times in comparison to the fields around them. Most woodlots are there because they were not suitable for farming. Personally, I prefer to be close to a bucks bed putting me in the confines of the wood lot which can negate some of the thermal effect. However, there are some instances when you have to stay outside of the wood lot basin due to the woodlot possibly being too small and you have to ambush a large whitetail as he comes down a brushy fence line. If the fence line slopes down gently to the woodlot this would create a thermal effect.
Matter of fact I have one such spot. It happens to be an apple tree that is located on a fence line that slopes down gently to a nasty, thick wood lot. If I sit between the apple tree and the woodlot where the slope is I get busted due to the thermals. However, if I sit 25 yards on the other side of the apple tree that is going away from the woodlot I am fine as the fence line is on level ground at that point.
Any terrain feature that has elevation change can create a thermal effect in flatlander country. Honorable mention can go to ditches, gravel pits, rock piles, burms, kettles, base of railroad beds, etc. Also from a H2O standpoint of standing water in a field, drainage ditches, swamps, lakes or literally anywhere there is water.
Hopefully this gets you thinking outside the box a bit and maybe think why you have been getting busted in some spots when it seemed like the wind was just right. Bring your milkweed on your scouting trips and when you are in these questionable locations like I previously mentioned release a little milkweed and test those thermals. Especially early morning and late day. Good luck to all my fellow hunters this year!
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I want to give recognition to Dan Infalt who because of his knowledge, opened my eyes to how thermals work several years ago. I have brought my milkweed with me ever since and it has given me a different perspective to what really goes on in the woods from the deers perspective and not of my own. So Dan…. Thank You!