Why does the same air feel colder when it is moving (as opposed to being stagnant)?
The moving wind blows away the warm layer air that surrounds your skin. In addition, moving air is more efficient than still air at transporting heat (which is why computers have fans in them).
There is a layer of air around any solid object that doesn’t move, called the boundary layer. The faster the air moves the thinner that boundary layer gets. When the air is still you have a layer of a air several millimeters thick surrounding you body that is at the same temperature as you skin and so feels pretty warm. To lose heat in still conditions the heat needs to radiate away from the body through that boundary layer, and radiation is a fairly lousy way to lose heat.
As the wind speed picks up that layer becomes thinner and thinner and becomes less and less effective at keeping you warm. At really high wind speeds it disappears almost comepletly and the air moving past your skin absorbs the heat directly from the skin surface and carries it away. At that stage you’re no onger losing heat from radiation, you are losing it by direct transfer to the air, and that’s a far more efficient way to move heat and so feels much colder.
Just out of curiosity, do you live in a warm climate? Your question is the very basis of what us colder climate people know and love, wind chill.
Huh, I’ve just pondered this with a friend. What confuses me, is that this doesn’t seem to affect thermometers [as much as it effects humans].
It effects human beings because of the moisture on human skin. The wind causes the moisture to evaporate. Evaporation causes coolness.
Also humans, unlike thermometers, generate their own heat and usually have temperature higher than air ouside.
>Huh, I’ve just pondered this with a friend. What confuses me, is that this doesn’t seem to affect thermometers [as much as it effects humans].
Wind speed does affect thermometers. What it affects, though, is how quickly the thermometer responds to temperature changes, not what value the thermometer eventually approaches.
Once you’ve disturbed the thermometer equilibrium, for example by taking the thermometer from the warm indoors to the cold outdoors, the difference between the thermometer’s temperature and the temperature of its environment decays exponentially. The exponent in this decay has a multiplier in it called the relaxation time or the time constant. The relaxation time is equal to the heat content of the thermometer divided by the product of its surface area and the heat transfer coefficient. Wind increases the heat transfer coefficient.
Wind would still make you feel colder even if you did not sweat.
Here are a few examples that hammer the concept home:
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It is winter. The ambient temperature outside is 20 °F. You have a small rock in your house, which is maintained at 72 °F. Place the rock outside. The temperature of the rock will eventually reach 20 °F. But the rate at which it reaches 20 °F will depend on the speed of the wind. The windier it is, the faster the rock will cool.
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It is summer. The ambient temperature outside is 80 °F. After driving home from the mall, you park the car in the driveway. The engine is hot, but it will eventually become 80 °F in few hours. The rate at which the engine reaches 80 °F will depend on the wind. The windier it is, the faster the engine will cool.
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It is winter. The ambient temperature outside is 20 °F. Your house is maintained at 72 °F using a thermostat. The windier it is, the more energy that must be consumed (per unit time) in order to maintain your house at 72 °F. All else being equal, the windier it is, the higher your heating bill will be.
It should also be noted that wind doesn’t just cool things off faster. It can also warm them up faster, if the air is warmer than the object its blowing on. What it really does is increase the rate at which objects reach ambient air temperature.
I live in central NJ, so it’s cold in winter and hot in summer. Thanks for that link, but I checked it out before posting the question; it seemed a bit exhausting to grasp the basic concepts from that entry.
Thanks for the explanations. What I understand is that our bodies are naturally warmer than the surrounding air (in general), so MOVING air takes our body’s self-produced heat away from it. It makes us feel closer to the air around us. In stagnant air, we ‘bathe in the essence of our own heat’.
In the special case where the environmental temperature exceeds our body temperature (@ 98.6 f), I suppose that wind would make us feel warmer, using the same principles.
>Thanks for the explanations. What I understand is that our bodies are naturally warmer than the surrounding air (in general), so MOVING air takes our body’s self-produced heat away from it.
Right. Then, there’s also the evaporation effect, which takes extra body heat, if you are sweating or otherwise wet. Which one is more important depends on conditions.
>In the special case where the environmental temperature exceeds our body temperature (@ 98.6 f), I suppose that wind would make us feel warmer, using the same principles.
Right.
Do you have a cite for the thickness of the boundary layer having htis effect on heat transfer?
There is a lot of confusion about boundary layers, and several different meanings that people use. In my opinion the most proper meaning is relevant in systems where there is turbulence. Because the viscous effect on air flow approaches infinity as the distance to a stationary surface goes to zero, at points close enough to the surface there is no longer any turbulent flow, only viscous flow, which is laminar. The layer next to a surface where there is no turbulence is the boundary layer, in this sense.
There is no layer of completely stationary air next to a surface in a system where the core air flow (away from the surfaces) is moving.
However, in a boundary layer where the flow is no longer turbulent, the movement in air is all tangental to the surface and does not transport heat any better than still air.
It is common in computational fluid dynamics to exploit the Bousinesque hypothesys, namely, that turbulent air transports momentum and heat the same way that a more viscous fluid that is not turbulent would do it. Thus, there are turbulence models, such as the Kappa Epsilon and the Kappa Omega model, in which a turbulent kinetic energy becomes one of the field variables along with the 2 or 3 velocities and the temperature and pressure. Then, a dissipation rate or relaxation time for the kinetic energy becomes yet another field variable. In applying one of these models, for example the Kappa Omega (which is valid all the way up to the surface, while the Kappa Epsilon is only valid for core flows), it is important to calculate the likely thickness of the boundary layer so that you can place several layers of mesh nodes within that thickness. A couple of years ago I wrote a program that calculates this, from within the scripting language of a mesher (a program that creates meshes over which to apply the Finite Element or Finite Volume method).
>the movement in air is all tangental to the surface and does not transport heat any better than still air
Sorry, forgot where I was headed with this point.
In applying the Bousinesque hypothesys, one gets much bigger momentum conductivities (in other words, viscosities) and thermal conductivities close to the surface. Thus the models predict that force and heat are transported very well close to the surface, just as the models would if they could do direct numerical simulation (DNS) of all the turbulence (which today and for some time remains impossible because it’s far too compute intensive). It’s been a major goal of models to reproduce this heightened conductivity near surfaces. Yet, the last little bit of the way, the region adjacent to the surface for a very small distance, still has the same dumb old 0.025 W/(m K) conductivity of air, and the same old viscosity (I think around 1e-5 Pa s). This is the boundary layer, as it is in real life, and as Bousinesque turbulence models generally capture it too (though through the artificial high viscosity and conductivity nearby).
Not the whole story:
Most meteorologists and engineers are familiar with wet-bulb temperature. To measure this, you take a piece of absorbent cloth, wet it, then wrap it around the thermometer bulb. If there is any wind AND the relative humidity is less than 100% the wet bulb temperature will be lower than the dry bulb temperature.
This is because evaporation of water (even though it occurs spontaneously at relative humidity > 100%, temperature above freezing ((more or less))) requires input of energy. This energy comes in the form of heat from whatever body the water is covering, and causes a decrease in temperature as it occurs. The faster evaporation occurs (low humidity, higher temp, greater mass transfer of water vapor away from the boundary layer (because of high wind speed)) the greater difference between dry and wet bulb temperature.
Since humans tend to be “moist” by nature the evaporation effect occurs and wind will in fact make humans colder than the dry bulb temperature.
I dispute this.
Not that your knowledge of fluid dynamics doesn’t vastly outweigh mine; It is just that at temperatures exceeding 98.6F the human body tends to sweat a lot. This tends to make us cooler, especially in the wind. Although I’d be interested in learning at what temperature, if any, one effect begins to outweigh the other and the wind begins to warm us up.