Thermal Tunnel

[JoeRE]

I will say - in my opinion the "thermal tunnel" concept is a over-simplification of what is going on. Don't shoot me for saying that. Its a good starter to thinking about it.

Wind acts as a fluid across the hills. Yes there is always thermal activity, but there is also all sorts of things going on with "wind" itself. For instance, the wind stream blowing across a ridge has lower pressure because it is moving, so air below it on the back (leeward) side of the ridge is often drawn up to that lower pressure stream of air. Its a fluid mechanics thing, I won't get into the details its just how it works. Moving air has lower pressure, and calmer air will be drawn to that moving air stream due to the lower pressure...assuming similar temps etc. The air moving up the leeward slope (whether from thermals or just the pressure differential of the moving air current, or BOTH) mixes with the stream of air moving over the ridge, this does swirl around like is shown in the DVDs as the thermal tunnel concept. But often winds are blowing along ridges, not right across them, and bucks still like to cruise that military crest often 1/3 of the way down...they even prefer that most of the time if moving on completely windward slopes so I think it is just as much as a sight thing as a scent thing.

So - thermals on a warming slope do go upward as well. But if the slope is not warming then thermals are probably falling, such as is common in the early morning or evening. At these times there is no "thermal tunnel." If a stream of air is blowing over the ridge top in the early morning or evening, air movement on the leeward slope might be up in some areas if the pressure differential overcomes the falling thermals, or it might be down if the falling thermals are greater. Nobody guesses it right all the time in hill country. Its very complex stuff. Prepare to cuss out the wind regularly, but the more you understand the less that will happen.

You can take what you see in certain conditions, and expect similar things in the future though. That is the route I try to go...test things out as much as possible. I jot down what winds I tested during scouting and what I observed. Doing that I have been able to find good conditions to hunt some spots no one would have guessed was hunt-able due to wind & thermal swirl. I still get frustrated by wind and thermals quite often though. Anyway, just some thoughts. When I first heard about the thermal tunnel concept I spent a lot of time thinking about it, then have spent the last few years looking at all that really closely.

[DaveT1963]

I will say - in my opinion the "thermal tunnel" concept is a over-simplification of what is going on. Don't shoot me for saying that. Its a good starter to thinking about it.

Wind acts as a fluid across the hills. Yes there is always thermal activity, but there is also all sorts of things going on with "wind" itself. For instance, the wind stream blowing across a ridge has lower pressure because it is moving, so air below it on the back (leeward) side of the ridge is often drawn up to that lower pressure stream of air. Its a fluid mechanics thing, I won't get into the details its just how it works. Moving air has lower pressure, and calmer air will be drawn to that moving air stream due to the lower pressure...assuming similar temps etc. The air moving up the leeward slope (whether from thermals or just the pressure differential of the moving air current, or BOTH) mixes with the stream of air moving over the ridge, this does swirl around like is shown in the DVDs as the thermal tunnel concept. But often winds are blowing along ridges, not right across them, and bucks still like to cruise that military crest often 1/3 of the way down...they even prefer that most of the time if moving on completely windward slopes so I think it is just as much as a sight thing as a scent thing.

So - thermals on a warming slope do go upward as well. But if the slope is not warming then thermals are probably falling, such as is common in the early morning or evening. At these times there is no "thermal tunnel." If a stream of air is blowing over the ridge top in the early morning or evening, air movement on the leeward slope might be up in some areas if the pressure differential overcomes the falling thermals, or it might be down if the falling thermals are greater. Nobody guesses it right all the time in hill country. Its very complex stuff. Prepare to cuss out the wind regularly, but the more you understand the less that will happen.

You can take what you see in certain conditions, and expect similar things in the future though. That is the route I try to go...test things out as much as possible. I jot down what winds I tested during scouting and what I observed. Doing that I have been able to find good conditions to hunt some spots no one would have guessed was hunt-able due to wind & thermal swirl. I still get frustrated by wind and thermals quite often though. Anyway, just some thoughts. When I first heard about the thermal tunnel concept I spent a lot of time thinking about it, then have spent the last few years looking at all that really closely.

Great post Joe. I need to get up to some hill country one day... Maybe one of the beast hunters can mentor me a little :) as I'm sure there is much i need to learn. One of the reasons I've always wanted to hunt Wisconsin is because personally i find the hill country beautiful.... Just sitting in a tree in some of that country would be therapeutic :)

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[Twenty Up]

IMO you also need to take into consideration time of year (leaf coverage) that can reduce wind speed and sunlight penetration. This will not eliminate a thermal tunnel but it's something to take into consideration.

Once you get the grasp of playing thermals & the wind it can actually become very productive. There have been several times that I have hunted a thermal tunnel and had my scent reduced to a limited distance. I haven't confirmed this with milk weed, I need to get a hold of some but rather deer, which is good enough for me.

[mheichelbech]

How often or how much difference does it make a difference being in a tree stand versus being on the ground?

[dan]

How often or how much difference does it make a difference being in a tree stand versus being on the ground?

On a Leeward ridge its really hard to get away with ground hunting.

[JoeRE]

How often or how much difference does it make a difference being in a tree stand versus being on the ground?

On a Leeward ridge its really hard to get away with ground hunting.

Agreed, its challenging. If I really want to hunt from the ground what I look for is an obstruction like brush pile, blow down, or ditch crossing on the leeward slope that force deer to walk around just ABOVE the thermal tunnel line and then I should be able to set up on the ground with the consistent breeze blowing across the ridge above the tunnel next to that obstruction.

Its still tricky though...most of the time best to look for a tree.Sometimes your scent can travel right along the ridge through that tunnel zone and deer can bust you in what seems to be a cross wind. That was an ah-ha moment for me when I realized that after thinking about the thermal tunnel/mixing zone concept a while....that explained a lot of strange encounters I have had in past hunts where I had no idea how a deer busted me! I see that in hunting videos quite a bit....guys point down the ridge a ways, in what seems to be a cross wind, and say "there is no way that deer is snorting at me!" while it clearly is snorting at them. Your scent can move up and down the ridge along that mixing zone/thermal tunnel unless you set up enough above it and the wind blowing over the ridge will then carry your scent out over the valley - as is explained in the vids.

[JoeRE]

I need to get up to some hill country one day... Maybe one of the beast hunters can mentor me a little :) as I'm sure there is much i need to learn. One of the reasons I've always wanted to hunt Wisconsin is because personally i find the hill country beautiful.... Just sitting in a tree in some of that country would be therapeutic :)

[ Post made via Android ]

Dave you would love bluff country or as the hippy dippy crunchy granola types now call it, the driftless area of the midwest. I left for a few years but had to come back, the siren call was too strong. You find yourself up here for any reason drop me a line. Sitting on a bluff about mid-October is seeing mother nature in her finest glory.

[Stanley]

Lot of great stuff. Joe another great post. I have had the wind whip my but in the hills more than any other terrain. I got caught up in the "I got this figured out syndrome". You gotta use that milk weed indicator in my opinion you can't hunt hills effectively unless you know what the wind/thermals are doing[glow=red]at the moment[/glow].

[checkerfred]

How often or how much difference does it make a difference being in a tree stand versus being on the ground?

On a Leeward ridge its really hard to get away with ground hunting.

Even with a gun?

[ Post made via iPhone ]

[dan]

How often or how much difference does it make a difference being in a tree stand versus being on the ground?

On a Leeward ridge its really hard to get away with ground hunting.

Even with a gun?

[ Post made via iPhone ]

Easier with a gun... But still a lot better off in a tree. Gun or bow your going to get busted a lot. Wind and air currents tends to go every direction at ground level, and a buck is likely going to smell you before you see it. As Joe said, if you get real good at understanding the air flow you can find obstacles to hunt around, or with a gun you might be able to hit a ridge from a distance... But, I would rather be in a tree with a gun where I can see farther, and beat the currents...

[DaveT1963]

I need to get up to some hill country one day... Maybe one of the beast hunters can mentor me a little :) as I'm sure there is much i need to learn. One of the reasons I've always wanted to hunt Wisconsin is because personally i find the hill country beautiful.... Just sitting in a tree in some of that country would be therapeutic :)

[ Post made via Android ]

Dave you would love bluff country or as the hippy dippy crunchy granola types now call it, the driftless area of the midwest. I left for a few years but had to come back, the siren call was too strong. You find yourself up here for any reason drop me a line. Sitting on a bluff about mid-October is seeing mother nature in her finest glory.

I am going to start trying to build some points for Iowa this year. I actually studied under a pretty good taxidermist in Solon -Joe Meder - I will NEVER forget the racks that he was working on - unbelievable. This was about 15 years back. If I ever do draw I would love to come meet you in person and talk strategy - figure I could learn a lot from you.

[BassBoysLLP]

Great thread and great questions! So, is there a point where say high winds will overide a rising thermal?

No... Thermal rise occurs on the Lee side and there is a collision even on small hills... There is a hill on a farm I hunt that is about 15 feet in rolling elevation. Still has thermal rise. And the fast the wind, the more you can see it... A lot of the time I can drop milk weed there in the valley head level and it blows down wind, drop it by my feet and it follows the vacuum up the hill 180 degrees different.

Yes and No. Many times the perceived thermal is stronger on the leeward side during stronger winds due to the higher pressure drop (i.e. vacuum) at the ridge top. Its a large scale example of the venturi effect. The falling thermal can be offset on the leeward side during high winds. A lot depends on the topography of the land and the canopy. A small pocket in the canopy on a ridge top can have a huge vacuum pull. You can often pick out these pockets on an aerial laid over a top by evaluating the canopy count and stem density.

Common examples of deer cruising on the windward size involve open ridge tops (clear cut, roads, etc.) There is a pressure drop generated on the transition on the windward side that pulls scent from the windward valley and leeward side of the ridge.

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[JoeRE]

Now you did it Bassboys. I was trying to avoid dropping the "V" word in my explanation earlier so folks' eyes didn't glaze over.

[mheichelbech]

How often or how much difference does it make a difference being in a tree stand versus being on the ground?

On a Leeward ridge its really hard to get away with ground hunting.

Agreed, its challenging. If I really want to hunt from the ground what I look for is an obstruction like brush pile, blow down, or ditch crossing on the leeward slope that force deer to walk around just ABOVE the thermal tunnel line and then I should be able to set up on the ground with the consistent breeze blowing across the ridge above the tunnel next to that obstruction.

Its still tricky though...most of the time best to look for a tree.Sometimes your scent can travel right along the ridge through that tunnel zone and deer can bust you in what seems to be a cross wind. That was an ah-ha moment for me when I realized that after thinking about the thermal tunnel/mixing zone concept a while....that explained a lot of strange encounters I have had in past hunts where I had no idea how a deer busted me! I see that in hunting videos quite a bit....guys point down the ridge a ways, in what seems to be a cross wind, and say "there is no way that deer is snorting at me!" while it clearly is snorting at them. Your scent can move up and down the ridge along that mixing zone/thermal tunnel unless you set up enough above it and the wind blowing over the ridge will then carry your scent out over the valley - as is explained in the vids.

I wasn't clear enough...what I meant was if I am considering hunting an area where I thought a thermal tunnel should be, what would the difference be between me dropping some milkweed while standing in the ground versus being up in the air? I've seen Dan talk about the air movement differences between just feet and shoulders...also, how could I test out how high I need to be so my scent is blowing over instead of getting sucked into a thermal tunnel? Drop some weed and see if swirls?

[ Post made via iPhone ]

[JoeRE]

I wasn't clear enough...what I meant was if I am considering hunting an area where I thought a thermal tunnel should be, what would the difference be between me dropping some milkweed while standing in the ground versus being up in the air? I've seen Dan talk about the air movement differences between just feet and shoulders...also, how could I test out how high I need to be so my scent is blowing over instead of getting sucked into a thermal tunnel? Drop some weed and see if swirls?

[ Post made via iPhone ]

Yea only testing it in the tree will tell you if you are high enough to be in a consistent air current carrying out over the valley IMO. The combination of air currents varies so much. I will say in my experience this often has to be pretty high...as in 20+ feet but it depends a lot just where the base of your tree is in relation to the ridge crest too, if you start out lower obviously you have to climb more. Maybe its because I hunt pretty big bluffs in the 150-250 foot range, so dropping down to the military crest puts me well below the ridge top, forcing me to climb higher to find a really consistent current. In small hills I bet it doesn't require as much height on average.

Another thing to keep in mind I learned to let the milkweed go down by my feet because thats the lowest part of my body that needs to be out of the scent stream. I have screwed up in the past, felt like I had a consistent current around my head and chest but deer were busting me and it was because 5' lower around my feet was not high enough.