Sweat: Newsletter Challenge (01/10/06)

Sweat, is mostly water. For water to evaporate is must change state from a liquid to a gas. At STP water will boil off as a gas at 100 degrees C or 212 F. While the ambient temperature in the desert is below 212 degrees (thankfully), water can still change state to a gas, but it needs to borrow from the "heat bank" to draw the extra energy required to change state from liquid to gas.

As water evaporates from your skin, the skin temperature is lowered as heat energy is transferred from your body to the sweat to allow it to change state. The breeze improves the evaporation rate by keeping the relative humidity low at the site of evaporation. In other words, as sweat evaporates the immediate volume of air at the spot of evaporation becomes momentarily saturated with water vapor. The breeze accelerates the dissipation of the water vapor. Localized saturated air about the site of evaporation is substituted with drier air. Since the desert normally is vary arid, the evaporation process is usually very efficient.

Nice answer, I agree.

Anonymous coward #1 is correct. It's evaporation that cools you, not the sweat itself. That's why it's harder to stay comfortable in a very humid climate

That's entirely right. In humid conditions the evaporative effect of sweat is lost. This is the reason it's so important to maintain fluid levels. As the body tries to maintain its core temperature at a nominal 37.5C, it sends more and more fluid to the skin surface in an attempt to cool the blood. This causes the body to lose vital electrolytes which must be replaced. If these fluids are'nt replaced (which is easy to do even though you think you're drinking enough), the brain ends up having overheated blood sent to it. Which it doesn't like! The result?....Heatstroke

I understand about the evaporation of the sweat off off the body, but doesn't the fact that the water (or sweat) create a different thermal coefficient then just air over your skin, and it's this increased amount of heat being sucked from the body that we feel and that is what makes us feel cooler? Much like when you step from the carpet with bare feet to the kitchen floor and that floor feels cold. The 2 surfaces are the same temperature, it's just how fast your body is loosing the heat that actually makes your feet feel cold.

Not in this case. The sweat is at body temperature, too. However, in order for the water to evaporate it must source heat energy from the neighboring tissue to reach the point of evaporation. The sweat has a temperature close to 100 degrees F, but needs additional energy to evaporate. Essentially, the sweat molecules draw that energy, in the form of heat, from the body tissue.

This is called the enthalpy of vaporization. In order for a liquid to change state to a gas it must overcome the intermolecular chemical bonds that maintain water as liquid; hydrogen bonding in this case. Hydrogen bonding is weak, but still requires energy to break the bonds. The amount of energy required is dependent on the current energy state (i.e., 100 degrees F) and the energy required for vaporization (212 degrees F). The energy needs to come from somewhere in the form of heat and the thermal conductivity of human tissue is much better than surrounding air, so the bulk of it comes from the human body as it is transferred from tissue to the water molecules.

This is different than walking on carpet and ceramic tile. In this example carpet is a poor conductor of thermal energy, whereas ceramic tile is better. Additionally, tile in your home (and carpet) is almost always at a lower temperature than the human body. If tile was at the same temperature as the tissue in your feet it would not feel cold. You can get the same effect if you slip into bath water that is at the exact same temperature as the skin. You don't feel the water cooling you off. In fact it is a very neutral feeling. You are aware of the water, but there is no temperature sensation. When you exit the tub, enthalpy of vaporization begins and you feel a chill.

Interesting enough, some environmental cooling systems use liquid water and the enthalpy of vaporization to provide air conditioning. I saw such a machine in Singapore. Sort of looks like an industrial water fountain. ;-)

That cooling system exists in a similar function but different form in some homes in airid climates. It's called a swamp cooler, and it humidifies the air, cooling the air by the aforementioned phase change energy concept, creating a net effect of sweating for you.

Evaporation cools you indeed but it also has other often undesriable effects (particularly in the desert). As a system, the human machinery for Homeostasis works fairly well, but if you're in the desert or say, forming concrete and you start to feel "cool" it is often the first sign of heat stroke. Hard physical labour (or walking in the desert) increases body temperature and triggering the production of perspiration. While perspiring, the body is cooled by the evaporation. It is simultaneously robbed of significant amounts of NaCl. That is why a bag of salt is almost as important as a bottle of water in the desert. Neither are as imporant as a bottle of rubbing alcohol or vodka though.

nice answer . Water changes from rivers and ponds below hundred degrees of celsius temperature. Air contains some humidity init .Which cools down our body temperature.For cooling our body water goes in a swept form through the porous part of the skin due to difference of temperature.out side temperature is greater than the body temperature. but our body consists of water 90 percent in the liquid form .so our body temperature is less than the surroundings. Due to air water drops on body is sucked by air and it look like cool. thanking you

The reason you keep cool in the 100 deg F heat with a slight breeze is because the breeze causes the sweat to evaporate and as doing so cools the skin temperature. this is known as the wind chill effect and will only work up to temperatures of 32 deg C.

Ok...enlighten us. Why does it stop working at temperatures above 32degC?

I think you might be dead.

The Arabs and Australians seem to survive temperatures of over 40degC quite happily. I assumed we were talking air temperature here.

In the event that we're talking body temperature, I went hunting:

Having checked the web I find that, with a fever, temperatures of 38degC can be measured on the forehead. If you've got a fever, you're not dead. Normal body temperature is quoted as 37degC internally, so skin temperature is 1 - 2 degC cooler, which is still above the 32degC quoted.

So please, why does wind chill effect not work above 32degC??? And is that air temp or body temp?

Okay, I suffer from Male Answer Syndrome, so here is where a little knowledge is dangerous.

First, wind chill is more arbitrary than absolute. A number of different standards exist and they all attempt to describe the apparent temperature a person feels. You can't put a sensor probe on a feeling, so it is not an absolute thing that can be measured, just derived.

Wind chill is a combination of two effects. First, is the displacement of warmer boundary air that surrounds the human skin or lies under layers of clothing with cooler ambient air. The second is the displacement of localized air which has a higher humidity percentage over areas of sweat evaporation with air that has a lower humidity content and thus a faster evaporation rate occurs and enhanced cooling. I described this in an earlier post.

According to formulas used by the US and Canada, wind chill has a negligible effect above 20 degrees C. Again, this is an arbitrary number because it depends on the formula used to calculate wind chill. However, wind chill does diminish in effect as temperature rises.

Here is where I knowingly skate on thin ice and will postulate my thinking. In plain vernacular; beyond here lie dragons.

As ambient temperature rises, wind chill becomes null because:
1) The ambient air temperature is close to the skin temperature or warmer than the skin, so the air does not feel much different that the boundary air temperature.
2) As temperature of air rises, its capacity to hold water vapor increases. At some point localized evaporation occurs just as fast in still air as it would with moving air because the diffusion rate of water vapor is faster in warm air and the capacity to hold water vapor is higher.

Now, I know what you are going to say, "What about when the humidity is 100%?" Well, sweat vaporization will not occur and the ambient air temperature is close or higher than skin temperature. Of course you will feel miserable!

So, 32C is not an absolute number, just a point on somebody's wind chill chart. I don't know whose.

Thank you.

It may seem odd to think of wind chill at high tempuratures but that is the key ! Plain old sweat at a windless setting cools the body but the wind adds to the evaporation. The important lesson for the lads answer is to replenish the fluids to avoid dehydration. Bonus Point:; If the humidity were high and the air windless a fire would help to make the skin feel cooled by drying the air and increacing evaporation.

Ah Ha! That as a particularly witty conundrum you presented there with the fire drying the especially humid air and multiplying the evaporative cooling effect. That would be compounded by the fact that the fire will draw in oxygen from the surroundings, and creating a mild breeze. Perhaps this theoretical cooling effect would be compromised, however, by the radiant heat being expelled in all directions by the fire. You would have to be standing further from the fire to feel the cooling effect in your example.

Actually, the fire creates a "breeze" by generating heat, which causes the warmed air to rise and is replaced by cooler air descending toward the base of the fire. Essentially, you create a thermal.

I would agree, the radiant heat would swamp the wind chill effect. That is why you can get warm with an outside bonfire.

More importantly the fire evaporates the cooling sweat to give a temporary cooling effect. The skin has a new higher and drier heat sensation that is less objectional to the heavy moisture laden air. And yes the radient heat would "swamp" the cooling effect if you stayed in the heat. I learned this by experience while burning wood scraps from my shop out doors on an incredibly hot and humid day. The drying effect of the fire was quite refreshing.... though temporarily. Drink hot coffee on a hot day to cool off. This works too.I don't prefess to know the science but it works. Something about the body cooling the core.as well as the skin.

Hot coffee on a hot day to cool off seems odd. Thermodynamically it doesn't make sense to me. You say you don't know how the science of it works and neither do I. Basic reasoning would make the point in the contrary. Remember the 1st law of thermodynamics: Energy in = Energy out. Your body as a certain amount of energy that it generates in the form of heat through the process of burning calories. Your body will continue to burn calories to maintain a certain core temperature (98.6 degF). When you add a hot coffee to your system, you are adding energy to the system. Remember also the second law of Thermodynamics: Energy has quality. A hot coffee in a cool room will always expel energy to the room until an equilibrium is reached. It will never absorb engergy from the room because of the 2nd law. Likewise, a cold drink will never expel energy to the room and become colder, unless the room is colder than the drink. More to the point, the hot coffee (temp is greater than 98.6 degF) will add heat energy to your system. This will theoreticly raise your body temperature as the coffee transfers heat energy to the "lower quality" of your body. Perhaps then it is similar to drinking brandy on a cold day, where you feel warmer but your body temp is not so. Perhaps when you drink hot coffee on a hot day, you feel cooler for some reason when in fact your body temperature is rising.

You may be right, but the human body's heat regulation is not a passive system.

A wild guess would be if the core temperature elevates, then the body increases sensible perspiration to compensate?

One key to the answer is the breeze. Sweat cools the skin as it evaporates and the greater the wind velocity the greater the rate of evaporation. The rate of evaporation is also dependent upon the relative humidity. The lower the relative humidity the greater the rate of evaporation. In the desert during the day a person would sweat profusely but never know it because the sweat would evaporate so quickly that the skin would always seem dry even though there might not be any noticeable wind. Where I live the humidity seldom drops below 75%. Just simply taking a walk on a hot day can saturate one's clothes even if the wind is blowing. When the humidity soars up above 90% a breeze makes it feel as though you are in front of a heater vent.

You need to consider dry-bulb temperature and wet-bulb temperature. Dry-bulb is easy, it is what a standard thermometer reads. Wet-bulb is a little more difficult to understand. It is the temperature a thermometer can be depressed to by putting a pourous sock on the sensing element, keeping it soaked with distilled water and blowing air across it. Wet-bulb is important in that it more correctly describes thermal conditions for water cooled machines, like the human body or cooling towers. Ambient wet-bulb temperatures rarely exceed 80 deg F (26.7 C)anywhere in the world. The reason the breeze cools is that it reduces the air boundry layer around the person. With no breeze, the boundry layer of air around a person is thicker reducing the rate that water can evaporate -- its like a blanket holding the moisture in. When the wind blows its like going from a heavy blanket to a thin sheet. About this evaporative thermal effect not working above a certain temperature, I am reminded of a story about a "magician" who would enter a large oven with a piece of raw meat and exit after a time with the meat cooked and himself unhurt. This was around the turn of the 19th Century, and he understood how evaporation cools the human body.

If I understand your point on evaporative thermal effect not working above a certain temperature, are you talking about the wind chill not "working" above a certain point?

If so, that is a different thing altogether. The wind chill's effect does diminish as the ambient temperature rises. Sweat still evaporates, but the breeze has less and less impact as temperature rises.

Oh, it's a dry heat! ;-)

Thermodynamics,its the exchange of sensible and latent heat.

Just doing a search to find the reason drinking coffee makes you feel cooler on a hot day. All your reasoning makes sense to this layperson. I see though, that nobody came up with a valid reason for it. Just thought I would throw in my reason I came up with after reading all the intelligent conversations here. Could it be that by drinking the hot coffee, it heats the body and therefore creates more sweat. If there is more sweat, there is more sweat to evaporate, thus, more cooking of the skin? Just a though. I am not enineer nor do I know anything about thermodynamics, I just know it works. Thanks for the education. Nothing learned is ever wasted.