Understanding mountain thermals

[cnelk]

Im starting this topic for anyone that may be interested and hoping others will contribute and ask questions:
So here goes:

Thermals should not be confused with Wind.
Thermals are driven mostly by the heating/cooling of the earth surface

Thermals are very gentle movements of the air, sometimes almost imperceptible.
Cold air sinking below the rising thermal causes a downdraft.
That's why you can feel thermals in your face in one spot, then feel it on your neck in another.

Typically the thermals move DOWNSLOPE in the mornings until the air heats. When the air heats up, it mixes.
After the air heats up and stabilizes, the thermals will be UPSLOPE during the day.
Later in the day, as the air starts to cool, the thermals will mix again and then just before dark, they will be predominately
DOWNSLOPE again

Thermals are weather dependent. You wont have any consistent thermals if a weather front is moving thru.

Slope direction definitely has an impact. The sun or prevailing wind will warm one side or mix it before the other.

Here's some more info regarding thermals...
Downslope wind—

1. A wind directed down a slope, often used to describe winds produced by processes larger in scale than the slope. Because this flow produces subsidence, downslope winds experience warming, drying, increasing stability, and clearing if clouds are present.

2. Flow directed down a mountain slope and driven by cooling at the earth's surface: a component of the mountain–valley or mountain–plains wind systems; same as katabatic wind.

The many synonyms for downslope flow are sometimes used interchangeably, and this gives rise to ambiguity and confusion. Downslope can be used generically to denote any wind flow blowing down a slope, or it is used specifically for katabatic flows on any scale, such as the nocturnal slope-wind component of mountain–valley wind systems or mountain–plains wind systems.

See katabatic wind, gravity wind, drainage wind, fall wind, bora, foehn, chinook.

[msd1228]

Great post, cnelk. I learned a few things about thermals from it! I primarily hunt elk either in the prairie or small foothills so I haven't had the opportunity to learn about them too much (since I haven't really had to). Good info!

[Swede]

Thermals can drive you nuts. The sun heats the earth at different rates due a multitude of factors. Aspect, placement on a mountain, vegetation, soil or rock color, water and more all can influence thermals. Thanks Brad for pointing this out.

[Fridaythe13th]

Do thermals change around creeks or wallows?
2013 was the first year of archery hunting we had a straight west wind down the valley perfect for the bull we were hunting, we went down to the creek and started a 100 yds climb to get to the same level, we cross the creek and the wind went to a south east game over. Was that thermals or just bad luck? time was 8-830 AM.

[mtnmutt]

Thank you that helped a lot.

One morning in September, it was overcast, not raining and no real wind. The thermals were blowing uphill. It baffled me because the day before they were most definitely flowing downhill at the same time/at the same location.

Does Brad have wind on his mind because we are having some 80 mph Chinook wind gusts today along the Front Range?

[Swede]

Friday: Thermals can change in draw bottoms. I don't think creeks and wallows are large enough or exposed enough to make much difference in themselves. What happens is when you go from one side of a creek to another, the aspect, exposure to the sun, and sometimes vegetation changes. I too have had to be careful to stay on one side of a draw where the wind was in my favor, knowing that if I went to the other side I would have the wind taking my scent right to the animal. Sometimes the trick to use there, is to go well above the animal if the wind is going uphill on his side of the drainage. If the thermals are going down on his side, as it could be in the morning when his side of the drainage is still in the shadows, then come in from below him. Sometimes the thermals are shifting as the earth starts to heat or cool and it is best to pull back, or wait.

[>>>---WW---->]

Good info Brad. Here are a few more little bits and pieces.
1) Think of morning and evening thermals as if they were water. Thermals flow almost like water does.
2) Low pressure can hold scent closer to the ground while Hi pressure can let it rise much the same as thermals.
3) Sometimes, in deep/steep gullies with water flowing in them, it is possible for the thermal to go down hill all day long. If you find one of these with a wallow in it, you can sneak in from the bottom side just about any time you want.
4) On north/south drainages, the thermal on the east facing slope can be rising while on the west facing side it is still flowing down.

[WVARCHER]

I need to take notes....so much good info on this site that I can't remember it all...thanks

[Indian Summer]

Man... WW that was a damn good answer dude. Usually when I hear people talk about thermals they say they drop at night or when air is cool or cooling, and rise during the day when it's hot or heating. But there are places where it drops all day. What goes up must come down.

Like WW said they can be getting pushed up a sunny SW slope and dropping down the opposite side al day long. But sometimes there are steep narrow draws on the sunny slopes that are a downdraft chute where all of the surrounding air is rising. You can use them to go against the grain when you're trying to work your way up in the day.

Other than that the water scenario is the way to get a mental picture. Keep in mind streams have eddies: Places where the water rolls off of a point or rock, and flows up stream and things settle in the middle behind it in the slackwater area. So.. a prevailing or thermal wind can push up to the top of a hill and curl over the top and spin up under itself like a wave creating an area of very unstable air in that spot.

You can never be 100% certain of a wind direction on the next hill over. A lot of stories go "everything was perfect, and all the sudden I felt the wind hit the back of my neck and it was all over" A lot of times that's because there can be a constant struggle between thermals and a prevailing wind which, as they overcome each other can keep switching the wind back and forth on us at the drop of a hat.

Like Cnelk stated, thermals can be so subtle that they are imperceptible. The best way to check them is with a wind indicator like Smoke In A Bottle. I always have one in an easy access pocket in my bino pouch and use it every day.

Wind: Friend or foe no doubt.

[cnelk]

Here are some visual aids

[msd1228]

Great addition, WW. Those are things I never would have known, but will jot down for later use. Thanks to everyone who has contributed here.

[dreamhunter]

Now this is going to be of interest to me.I need to learn more about these thermals for sure. As you all post , I will be seeing what I can learn from you.
Thanks

[BuckyHunter13]

Thanks for this. I had just asked for someone to spell it out in another thread yesterday. This makes sense.

[bnsafe]

im not sure im smart enough to hunt elk. thanks for the pics cnelk, I learn from pics an that helped me a ton. if I can see it I can remember it. reading stuff usually just confuses me more. everybody learns differently so I really appreciate it. im a kiss principle kinda guy. lol

[Trophyhill]

great thread. i'll come away with a little more cautious approach regarding thermals.

[Daved270]

Thanks for the visual aids cnelk. It's always nice to see pictures. I learn more and more every day that I look at this forum! One day I might be lucky enough to kill my first elk!

[Kevrod3]

Two thumbs up guys! Thanks!

[ferris bugler]

I have also noticed that thermals on the same exposure side of a mountain can have two directions. This usually happens when there is a very small gully. The colder air will filter down the gully while the warmer air will work up the top of the ridge.

I think as asked earlier that streams can cause a change in thermal direction. Cold water will hold onto the cooler air and create a direction that flows down stream.

[txhunter58]

What makes thermals so confounding, is that the most productive time to hunt is the first hour in the morning and the last hour in the evening. This is the time that thermals can swirl and turn on a dime. And you can have thermals turn on you later in the day. Example, hot sun shine on the ridge you are hunting so thermals are going up hill. But as you hunt, a big cloud comes between you and the sun and the ridge is suddenly cooler, so the thermal switches and goes down hill.

If at all possible, I stillhunt sidehill to minimize the effects of thermals. However even going sidehill there are ridges up and down. Can be very frustrating.

Basic rule to remember: heat rises, cool sinks. That is just restating what others have said. But to put it another way, if where you are AT THE MOMENT is heating up, the thermal will rise, but if AT THE MOMENT, the air is cooling where you are, the thermal will go down hill!

You can plan, but if you ever figure out how to beat it all the time let me know!

[Mikeha33]

Bump for a little refresher in the evil ways of mountain thermals!!!! Leaving in 27.5 hours! Not getting any "work" done, so I'm brushing up on some major fundamental points!!!!

[Waygoner]

A quick way to remember this, sun up and sun down = thermal up and thermal down (as a general rule).

[crazyaboutelk]

So are thermals convection currents that stay close to the ground?

[ctdad]

Thermals are definitely convection currents. As the air heats up, it goes up the mtn. This occurs sometime in the am after sunrise and varies by location and when the sun hits that part of the mtn. The thermals often rise all day (but are often not consistent due to wind). As the sun sets, the air cools and goes down hill. This continues until the air warms again. Thermals are easy to learn to use but the unpredictable wind has saved many elk lives

[foxvalley]

I have good good luck sitting downslope on a wallow reguardless of the thermals by sitting right next to, or even straddling the creek, getting as low as possible. The cold moving water seems to suck my scent down and keep it flowing downstream just like the water itself.
There is one drainage that I hunt that the themals seem to defy logic by flowing down after the sun hits it,and the elk hang out in the bottom untill this happens,around 8:00am. Then they head uphill with the strong downflow all the way up.

[Indian Summer]

I hear you Foxvalley. I know of steep little draws, on the sunny SW slope even, where the chute on the backside drops all day. If I'm hunting the area and I want to move uphill during the day I use them. Not only does it put the wind in your favor but they are the coolest spots around to do your climbing in.

Another good way to picture the tricky aspect of thermals is think of a stream flowing. The eddies that swirl around when the flow hits obstacles and the little side sloughs where the water turns and even flows upstream are exactly what the wind does. When it crests a ridge it can go up over top and roll backwards like a wave breaking so the tops can be tricky.

[mjblowers]

If I'm understanding this correctly, then...

It's about 80 during the day here and 30 at night. I hike on a .5 mile slopes that ascend about 1k-1500 feet. At 5-6 AM those thermals are going to be very gently moving down, prob a little quicker around 8-9, ceasing about 11-12 at which point they'll start moving uphill until about 7pm when it starts to cool off again?

EDIT: I feel like wind/thermals are thee thing I think too little about and truly is my last piece of my puzzle before I become a consistent harvester.

[ctdad]

Times vary a lot between locations. Most places I've hunted, the thermals will turn up hill about 1-2 hours after first shooting light. As the sun gets high enough to hit the part of the mtn you are on, the air warms and rises.

[mjblowers]

So... calling from below where you are expecting the elk to come from will more than likely not yield an elk if it's 10 am......

[Thunderfoot]

Mike,

As stated before, ther is no set time for when the thermal movement of air will change from downslope to upslope. There are too many variables. The only advice I can offer is that if your unsure which way the air is moving, check it. I constantly check the wind/thermals. I always have a full bottle of wind checker in my front shirt pocket. I also like to try and get onto the same level as the elk and attack from the side as opposed to from below or above. Hope this helps.

[BRazz]

Mike,

As stated before, ther is no set time for when the thermal movement of air will change from downslope to upslope. There are too many variables. The only advice I can offer is that if your unsure which way the air is moving, check it. I constantly check the wind/thermals. I always have a full bottle of wind checker in my front shirt pocket. I also like to try and get onto the same level as the elk and attack from the side as opposed to from below or above. Hope this helps.

Agreed. Just check the wind at your setup before you begin calling.

[mjblowers]

I'm going to Cabela's to buy a shower tent............... and a couple of those bottles here in a bit. I always thought I could tell which way it was moving... didn't know thermals was not the same as the wind until now.

[djchere]

I work as a wild land fire fighter. So how wind moves in the mountains is a critical subject.
Think of air like water. It flows up and down drainages if it is hot or cold. Hot air rises cool air sinks. The sun is huge. If it is hitting the surface then the air touching it is heating and rising. The more perpendicular the sun is to that land surface the greater the heating. Even a small cloud can disrupt this process temporarily. Like the tide it builds in speed in that daily ebb and flow, but in this case it is from afternoon to predawn. Atmospheric winds also respond to the topography of the land and can either enhance or retract form the daily diurnal heating and cooling patterns that exist. At the bottoms of drainages the overall relative up or down canyon winds are dominant over mid slope air movement and often more significant. Ridges perpendicular to the main drainage can cause a large degree of eddying which is relatively unpredictable on the leeward side. Trust ultimately that it is the Sun that influences these "thermal" daily cycles. However larger scale atmospheric weather can fully override the surface influences.