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This morning I was biking to my university downhill in ~$40^{\circ}F$ temperature. I was wearing a hoodie that resembles the one below (click for larger).

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Before I pulled my hood over my head, my ears were getting cold from the wind. At this point, I tried pulling the hood over my head to see if it would keep my ears warmer. Even though none of the hood was touching my ears (there was empty space around them), the hood stopped some of the airflow around my ears (I could hear the difference, the wind noise was much less after the hood was pulled over my head) thus making my ears warmer. I could also feel more air flow on the back of my head than before I pulled the hood over my head, signifying that a portion of it was circulating in the hood.

I know that my ears became warmer due to the lessening in air flow directly by it, but my question is how does the hood help limit this air flow? Is it because there can only be so much air in the hood, thus more cannot come in until some has escaped, thus preventing some from entering it in the first place? I know the hood doesn't just make it more aerodynamic in the conventional sense because it catches more air. A diagram of the behavior and reason why I hear the wind less would be appreciated.

I tried looking up a bunch of similar searches but all gave me pages that are unrelated.

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    $\begingroup$ Air goes into the hood from around your face and wants to get out form where it came in (since the back of the hood is closed), which causes turmoil and reduces the speed. Also it sorta catches up in your hair and in the fur inside the hood, which again lessens the speed (it happens even if you're bald and the hood has no fur). Less speed means less amounts of air coming in per second, meaning your head and ears can keep more warmth to themselves. $\endgroup$ – Færd Dec 2 '16 at 19:02
  • $\begingroup$ The short answer to your question is: yes. You are correct. $\endgroup$ – Steeven Dec 6 '16 at 1:15
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Note that there are a lot of reasons "why" the air pocket gets trapped there, like that the air doesn't easily pass through the hood's fabric and that you are moving much slower than the speed of sound so that the air is not easily compressible, so that indeed, nothing can come in without something else going out. But you're sort of asking the question of "given that it gets trapped there, what flow are we talking about here?" So I wanted to answer that.

What likely happens to some extent is that the air passing over your face by the hood openings sets up some standing vortices inside of it. Here's what the turbulent flow looks like before and after: a stream of uniform lines comes from the left to approach first a "Before" case where there is just a person's head, leading to a turbulent stream behind the head, or the "After" case where a person's head is surrounded by a semicircle representing a hood. Inside the hood are two oval streamlines by the ears.

Note that the vortex stream behind you is depicted as getting larger due to the hood, this is the you becoming "less aerodynamic", but also notice that the flow inside your hood gets highly decoupled from the flow of the outside air (the separate streamlines by your ears). Zooming in on that process and using a different color to indicate direction, and focusing on two streamlines that go just barely to either side of your nose, you can see: two streamlines right next to each other come up to the nose of the person and along the cheeks, then skip over to the outside of the hood. inside the vortices are shown in greater magnification, flowing along with the air on the outside of the hood, coming around the hood until they both meet, flow towards the neck.

What do you see here? First off, a lot of the air that's hitting your ears is getting recycled, which means that it retains a lot more of its humidity (less wind chill!) and heat. Second, you see that indeed air is blowing onto your neck in a way it didn't before. Third, you see that the air that's blowing by your ears is not actually blowing directly into them, but passing over them "the wrong way". What you're not seeing directly is that the air that's inside the hood is only being induced to circulate by a momentum-diffusion tendency of air that we call its "viscosity": therefore the wind speeds inside the hood, while nontrivial, are generally going to be much lower than outside the hood.

(Disclaimer: I no longer remember my fluid mechanics course details well enough to know if these vorticial curlicues of the streamlines are right; in laminar flow I know that streamlines do not cross but in turbulent flow my intuitions generally suck a lot more. I'm mostly trying to focus on the front side of the flow.)

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  • $\begingroup$ Thanks for the info! Do you happen to also know why the air that passes by my ears is "passing over them the 'wrong way'"? The way your diagrams look it appears as if the incoming air is on the hood side, not the face side, and the possible air going out is on the face side. I'd expect the opposite because air wouldn't have to go through a new air stream to escape the hood. I.E. why isn't the flow within the hood reversed in direction? $\endgroup$ – Zach Saucier Dec 2 '16 at 22:00
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    $\begingroup$ Look at the position of closest approach of the two streamlines and understand that the air outside has to "drag" the air inside along the way it's going -- that's just how viscosity works. So because the air outside goes from cheek to hood, it drags the closest air inside from cheek-to-hood, this air at the hood side has to flow along the hood, pushing other air out of the way, until it eventually returns back to your neck. So the vortex needs to go forward along your head and backward along the hood if it's not going to contradict the airflow at the hood entrance. $\endgroup$ – CR Drost Dec 2 '16 at 22:07
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So as a short answer to your question:

there can only be so much air in the hood, thus more cannot come in until some has escaped, thus preventing some from entering it in the first place?

The answer is yes... That fact that there is air inside your hood that's just circulating and being refreshed slowly (relative to the air that was flowing past your ears when you weren't wearing the hood) means that your ears aren't acting as the effective radiator of heat that they were when you weren't wearing the hood. This is because when you weren't wearing the hood, heat energy was being transferred rather quickly from your ears to the flow of air particles passing your ear. Since heat is vibration of a molecule (an increased kinetic energy in a molecule) - when you wear your hood, there are less air particles of a lower state of energy passing your ears. So looking at the heat energy in your ear, and the heat energy of the cooler air, the air inside your hood has a closer heat energy to that of the heat energy in your ears, energy transfer is slower, meaning your ears lose less heat energy and stay warmer.

I'm sorry I wrote this answer in a rush (as you could probably tell) I hope you were able to extrapolate the reason for warmer ears when shrouded with your hood.

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