Temperature - is that the right word?

[NewScientist]

I was thinking today about why air feels coller when you stick your head out of a moving vehicle and why it feels warmer when stationary.

I came to the conclusion the cooling properties of the air are greater when the car is moving as a larger volume of air is being passed over my per unit of time. Thus the air feels cooler as it is conducting more heat away from my skin unit of time.

My friend said - its because the temperature of the air has changed. And i was stuck because we differed over the definition of the word temperature - he argued that temperature meant rate of conduction of stored heat where I took it to mean the amount of stored heat. The issue was further clouided as we argued over the correct term to fit this sentence 'If we have some wood and metal at the same ________ the metal will feel hotter as it conducts faster'.

Help the argument be settled - please!

 

[inha]

Your friend is incorrect. Your friend is talking about heat conduction, not temperature.

 

[Claude Bile]

At room temperature metal feels colder than wood, because it is conducting heat away from the body at a greater rate (i.e. the word you are looking for is temperature).

Also, it should be pointed out that your hand feels colder in the wind because of convection, not conduction, so the argument regarding heat conduction is not really relevant as to why your hand feels cooler in the wind.

The reason your hand feels cooler in the wind is because you are heating the air around you and the heated air is continually being carried away, being replaced by colder (unheated) air. The net result is that heat is being carried away from your body at a faster rate. This is the same effect computer makers exploit to cool CPU's.

Claude.

 

[Renge Ishyo]

My understanding is that this phenomenon has nothing to do with the temperature of air per say, but is related to the rate of evaportation. When you blow air over a surface you lower the pressure above that surface. Heat is kept in a "body" by the vapor pressure above it. Release the pressure and more heat can escape the body. As an analogy, think of a bowl of soup. While it is stationary, the vapor pressure above the bowl of soup acts to prevent the heat in the soup from disappating into the air quickly. If you "blow" over the soup, you remove the vapor pressure which allows more heat to escape from the soup into the air (creating a new vapor pressure). If you continuously blow over the soup to remove the vapor pressure more and more heat can escape from the soup. This is why if you blow over soup it cools it down.

The same is true of your skin, if you are sitting in a room that is at 95 degrees F and you want to cool down you can turn a fan on yourself. Doing this will NOT change the temperature in the room appreciably. What it WILL do is remove the vapor pressure from around your skin allowing more and more heat to escape from your body at a faster rate (the process I think is termed "evaporation"). Since heat is escaping faster than it normally would, you feel your body cool down faster than it normally would even though the temperature in the room stays pretty much the same.

To consider an alternative analogy, think of how cold is feels when it is windy on a cold day. What do you do to keep warm? You put on a jacket. This helps keep the heat inside the jacket (you feel no wind inside the jacket), because the wind is unable to blow the vapor pressure away from your skin to increase the loss of heat from your body. The temperature of the cold day may be the same, but you feel warmer with the jacket on because the wind cannot bring about the same loss of heat that it can without you wearing one.

 

[zeta101]

What is your skin (or special cells in your skin etc, I'm not sure exactly) measuring the temperature of? that's the question you need to ask to solve the debate with your friend.

Your friend says it is measuring the temperature of the air (which seems very reasonable indeed!) and so he came to the conclusion that the air is colder if it is moving faster because it feels colder (a reasonable conclusion). The problem is that your skin does not measure the temperature of the air, its measuring the temperature of itself and you (or your brain) infers the temperature of the air from that. This 'technique' doesn't always work.

The process of measuring the temperature of air (like all methods I think) is by using thermal equilibrium. When you walk from a warm room to a cold one, your skin will start give heat to the cold room, so your skin drops in temperature and you 'feel the coldness of the room' through how the temperature of your skin has changed.

This technique of 'measuring the air temperaure' is not perfect though and can be tricked. It only works if you are allowed to actually reach thermal equilibrium (well that's not entirely true, you'd need to reach a point where the rate of heat movement in or out of the skin is low enough for you as a human not to notice, this would be a kind of 'effective equilibrium' I guess). It would seem that for air just 'sitting' in a room (for which the air molecules DO certainly move, think how fast a smell travels) and for a person who is not moving very fast, you can reach a kind of equilibrium (as far as your brain is concerned) and so the temperature measurement is reasonably accurate (which makes sense from an evolution point of view). In this case we can make the assumption that skin temperature is the same as air.

What if we force the air to move around more than normal (eg. with a fan). We might not be able to reach this 'equilibrium' and then heat would be moving at a rate above which our brains notice and so we cannot make the same assumption anymore about the skin and air temperature being the same.

This is just what I thought by myself so it may be wrong I guess, anyone agree/disagree?

 

[zeta101]

...to clarify a bit on my last post. Thinking about it, really it's your brain that 'does the measuring' of the temperature, and it measures the temperature of your skin. Since your skins temperature is linked to the temperature of the air around it, you (or your brain) can infer the temperature of the air from that of the skin, but only in certain circumstances is this accurate (as previously mentioned).

 

[Redfox]

And i was stuck because we differed over the definition of the word temperature - he argued that temperature meant rate of conduction of stored heat where I took it to mean the amount of stored heat.

Two factors of temperature:

Movement of atoms within their cage - transfer motion impulses on contact
and therefore transfer heat.

Radiaton: Infra red or any other that can leave the heater and conduct impulses to anything so that atoms within the object being heated move faster, i.e. it becomes hotter.

Object temperature itself is the degree of stored heat, both motion impulses and IR radiation within that object. Temperature is linked with thermoconductivity because conditions them an object will store and not conduct or absorb heat (i.e. "emit coldness") can be achieved but exist rarely in the natural world.

Degree of ability to conduct heat is known as thermoconductivity, it only describes the objects ability to conduct heat (main reason for differences in different materials - quantity of heat and IR radiation conducting particles per square inch of contact surface or amount of IR radiation per square inch of heated surface, say in vacuum ) Therefore some ceramic materials known for their poor ability to conduct heat can in fact store a significant amount of heat because processes of thermo-exchange within such object are slower than thermo exchange between that object and the air or other contact material. So they create a cool outside layer and loose a lot less of their own heat.

The air feels cooler when in motion because your body (usually hotter than the air) meets more heat absorbing air particles than when you are standing still. In other words you virtually increase thermo-absorbtion or the volume of air and your own body's thermoconductivity by moving around.

 

[quark]

This is technically called as Wind Chill Effect and the reasons are well explained above. The apparent temperature can be calculated by the following formula,

Apparent temperature (F) = 35.74 + 0.6215T - 35.75 (V^0.16) + 0.4275T(V^0.16)

Where T is actual air temperature (DBT) in F and V is wind velocity in mph(incase of still air and you are moving, it is your vehicle velocity).

Google on wind chill effect.

 

[Art]

.
The reason your hand feels cooler in the wind is because you are heating the air around you and the heated air is continually being carried away, being replaced by colder (unheated) air. The net result is that heat is being carried away from your body at a faster rate. This is the same effect computer makers exploit to cool CPU's.

Claude.

If the air temperature is greater than your body temperature presumably moving air will heat you rather than cool you?

 

[FredGarvin]

If the air temperature is greater than your body temperature presumably moving air will heat you rather than cool you?

You just described a convection oven.

 

[Art]

You just described a convection oven.

I wondered if in the case of humans does the evaporation of water secreted from the sweat glands nullify the effect of the warm air? As an example I remember hearing many years ago that when camping you can cool a container of water by wrapping it in wet newspaper. As the water in the newspaper evaporates it lowers the temperature of the water in the container to several degrees below the ambient temperature. Presumably a flow of warm air would achieve this effect quicker??

 

[FredGarvin]

I wondered if in the case of humans does the evaporation of water secreted from the sweat glands nullify the effect of the warm air? As an example I remember hearing many years ago that when camping you can cool a container of water by wrapping it in wet newspaper. As the water in the newspaper evaporates it lowers the temperature of the water in the container to several degrees below the ambient temperature. Presumably a flow of warm air would achieve this effect quicker??

That is absolutely true. Convective (forced) heat transfer does work quite well. I was in the first gulf war. We would use socks soaked in water to help cool bottles of water. What is happening is that the water in the bottle is supplying the energy for the heat of vaporization of the water in the sock/newspaper, thus lowering it's internal energy. That is why your skin feels cooler after you sweat and then stand in front of a fan. The same thing is happening. Of course, that is happening on the very near surface of the skin. I would think that if there is a higher temperature breeze, then there is another source for the evaporating water to get it's energy from, thus reducing the amount of energy pulled from the water you are trying to cool thus making it less effective. Of course, this is all predicated on the heat transfer coefficient in $$q = h a \Delta T$$. H is going to be a function of Reynolds number and Prandtl number. The temperature might go out in the wash depending on air velocity and density.