Understanding Thermals

[Harvestor]

I am trying to get a better handle on how morning thermals work on a north facing slope. Obviously, the sun rising in the southeast will hit the top of the south facing slope first in the morning and the "sun line" will progress down the hill. According to the Hill Country DVD, once the sun gets to the valley floor, the thermals should be in full effect taking a draft up the south facing slope where it hits a prevailing north wind and creates the wind tunnel effect approximately 1/3 down. My questions is, as the sun moves across the valley floor, warming it and causing the warm air to rise, does this create a vacuum effect and draw the cool air laying along the shaded north facing slope to fall into the valley to fill the void left by the rising warmer air in the valley? In the event that this is true, would a prevailing north wind coming across the valley hit the dropping cool air along the bottom of the north face and create a mixing effect similar to what happens 1/3 down on the south face? It seems that the lighter, rising warm air in the valley at the base of the north slope, would not be able to "get under" the heavier cool air laying on the still shaded north side and push it up the hill...? Could a vacuum effect as I've described lead to dropping thermals on the north slope in the morning and create a situation where it is necessary to hunt lower on the hillside for longer into the morning on a North face? And, if a prevailing north wind is in fact hitting a dropping thermal on the north side, could a "wind tunnel" be created around the sun/shade line at the bottom of the hill and present another lower morning travel corridor like the 1/3 wind tunnel travel corridor on the south slope? I am trying to understand how to effectively use the thermals to my best advantage in all situations and any information from those who are much more experienced than myself is welcome and very appreciated. Hopefully the link to the basic photo I drew helps explain my question...I am hoping the link works for everyone.

https://imageshack.com/i/ipEMTCaIj

[Harvestor]

Here is the photo again...

[Zap]

that's deep......

[Harvestor]

Here is the photo link again from photobucket...

http://s1148.photobucket.com/user/Reubi ... sort=3&o=0

[Liberty-Hunt]

Here is the photo link again from photobucket...

http://s1148.photobucket.com/user/Reubi ... sort=3&o=0

Clear info! Nice.

[dan]

Sounds good in theorie... But I can't recall it working that way. Thermals don't occur instantly when the sun hits the ground, it takes some amount of time for the ground to warm, and that does not occur at an even rate. Factors such as soil type, color, and tree growth can effect how fast the thermal takes effect.
Before you get consistent thermals there is a period of back and forth and swirling...

If anybody else has any experience with this, I would like to hear from them...

[BigHunt]

Sounds good in theorie... But I can't recall it working that way. Thermals don't occur instantly when the sun hits the ground, it takes some amount of time for the ground to warm, and that does not occur at an even rate. Factors such as soil type, color, and tree growth can effect how fast the thermal takes effect.
Before you get consistent thermals there is a period of back and forth and swirling...

If anybody else has any experience with this, I would like to hear from them...

AGREED

[Harvestor]

Thanks for the reply. Just to make sure I understand, the swirling effect will take place as the sun is warming the ground below, or will it also swirl further up the north facing slope in the morning until the sun is high enough to more directly hit or warm the north side? If the thermals are swirling, how do you hunt these spots in the morning or would you avoid them and hunt the north side on an evening hunt? In the scenario that I'm AM rut hunting on a north slope, should I be still adherring to the school of thought that lower stands are better early in the morning and then move up the hill as the morning progresses and ground temp rises?

[PK_]

I believe, in theory, the North wind would pull the air up and over the North face, depending on the steepness(angle of attack), height/position relative to surrounding topography, windspeed, etc…

I think there is a better chance of what you are describing happening on a day with very, very little wind. In that case the cooler air from the North slope should continue to fall until it hits the sun/shade line where it would rapidly warm and then rise. I don't think there would be much of a 'tunnel' effect, but perhaps hunting right on that shade line would keep the wind in your favor from each direction on a calm day. But the deer you are after would still need a reason to be in that spot in order for it to be a good setup(obviously).

Also, understand that a rising thermal is not necessary for the wind tunnel to form on the top 3rd of a leeward slope. This is a natural occurrence when air (or water) passes over an object.

If the prevailing wind was strong enough it could turn the entire valley into a wind tunnel and would in fact pull the air down the North slope and up the South slope. In this scenario you could end up with smaller wind tunnels throughout the valley and slopes, but predicting their location and the chances of them remaining consistent would be next to zero and the sun/shade lines would have little to no impact. Just remember that 99% of the time, wind trumps thermals.

[dan]

Yes, I would hunt lower in the early morning if I was planning only hunting the spot for an hour or two, but I would likely not hunt down in the valley unless it was a very large valley or had a cross wind cause of the swirling... My lower morning sets are usually at the bottom 1/3

[Harvestor]

Thank you for the information PK and Dan. I'm thinking milk weed or smoke bombs could help me unravel the conditions in my exact area during these morning scenarios.

[dan]

Thank you for the information PK and Dan. I'm thinking milk weed or smoke bombs could help me unravel the conditions in my exact area during these morning scenarios.

Stick with the milkweed... I tried smoke bombs and cause of the heat they give off the warm smoke does not flow naturally.

[semperfidelis]

Yes, I would hunt lower in the early morning if I was planning only hunting the spot for an hour or two, but I would likely not hunt down in the valley unless it was a very large valley or had a cross wind cause of the swirling... My lower morning sets are usually at the bottom 1/3

Dan, why r your sets at the bottom 1/3? I would have assumed if u were bed hunting your set would have been in close proximity to the bed. I would have assumed u would be higher, even in the morning.

Can u elaborate?

[ Post made via iPhone ]

[remmag]

I would collect some milkweed and test your theory out one morning before season. I didn't fully understand thermals or how wind worked in relation to terrain features and temperature until I spent a full season of hunting and constantly checking the wind with my seeds throughout the hunt. Even now I still keep a bag of seeds in my pocket during scouting to check wind and thermal tendencies in beds and travel routes where I intend to hunt and access my trees

[ Post made via Android ]

[Stanley]

I added the picture to your post. I think the question is a great one. I think in answer to your question I would say no. The vacuum effect is very slight plus you have some radiant heat going on. Like sitting in front of a fire place. The heat does move up but also radiates back. This is a more pronounced movement in a fire place, compared to a big area, so a fire place would have more draw or vacuum.