Have you ever wondered why you feel so uncomfortable on those dog days of August when the weatherman says that it is very hot and humid? what has humidity got to do with your being comfortable? What is humidity in the first place?
To understand the concept of humidity, we must first understand the concept of evaporation. consider the two bowls. Both are filled with water. bowl 1 is open to the environment, whereas a glass plate is placed over bowl 2. If we leave the two bowls overnight, on returning the next day we would find bowl 1 empty while bowl 2 would still be filled with water. What happened to the water in bowl 1? The water in bowl 1 has evaporated into the air and is gone.
Evaporation is a process by which water goes from the liquid state to the gaseous state at any temperature.
Boiling, as you recall, is the process by which water goes from the liquid state to the gaseous state at the boiling point of 100 Celsius. that is, it is possible for liquid water to go to the gaseous state at any temperature.
As latent heat of vaporization for boiling water is Lv = 540 kcal / Kg and water at 0oC is Lv = 600 kcal/ kg. The latent heat at any in-between temperature can be found by interpolation. Thus, in order to evaporate 1 kg of water into the air at 0oC, One would have to supply 600 kcal of thermal energy to the water.
The molecules in the water in bowl1 are moving about in a random order. But their attractive molecular forces still keep them together. These molecules can now absorb heat from the surroundings. This absorbed energy shows up as an increase in the kinetic energy of the molecules, and hence an increase in the velocity of the molecule. When the liquid molecule has absorbed enough energy it moves right out of the liquid water into the air above as a molecule of water vapor. (Water molecule is same either it is solid, liquid or gas)
Since the most energetic of the water molecules escape from the liquid, the molecules left behind have lower energy, hence the temperature of the remaining liquid decreases. Hence, evaporation is a cooling process. The water molecule that evaporated took the thermal energy with it, and the water left behind is just that much cooler.
The remaining water in bow 1 now absorbs energy from the environment, thereby increasing the temperature of the water in the bowl. This increased thermal energy is used by more liquid water molecules to escape into the air as more water vapor. The process continues until all the water in bowl 1 is evaporated.
Now when we look at bowl 2, the water is still there. Why didn’t all that water evaporate into the air? To explain this we do the following experiment.
We place water in a container and place a plate over the water. Then we allow dry air, air that does not contain water vapor, to fill the top portion of the closed container. Using a thermometer, we measure the temperature of the air as t = 20oC, and using a pressure gauge we measure the pressure of the air po, in the container. Now we remove the plate separating the dry air from the water by sliding it out of the closed container. As time goes by, we observe that the pressure recorded by the pressure gauge increases. This occurs because some of the liquid water molecules evaporate into the air as water vapor. Water vapor is a gas like any other gas and it exerts a pressure. It is this water vapor pressure that is being recorded as the increased pressure on the gauge. The gauge is reading the air pressure of the dry air plus the actual water vapor pressure of the gas, po + pwp. Subtracting po from po + pwp, gives the actual water vapor pressure, pwp. As time goes on, the water vapor pressure increases as more and more water molecules evaporate into the air. However, after a while, the pressure indicated by the gauge becomes a constant. At this point the air contains the maximum amount of water vapor that it can hold at that temperature. As new molecules evaporate into the air, some of the water vapor molecules condense back into the liquid. An equilibrium condition is established, whereby just as many water vapor molecules are condensed as liquid water molecules are evaporating. At this point, the air is said to be saturated. That is, the air contains the maximum amount of water vapor that it can hold at that temperature. The vapor pressure read by the gauge is now called the saturation vapor pressure pwp.
The amount of water vapor in the air is called humidity.A measure of the amount of water vapor in the air is given by the relative humidity, RH, and is defined as:
“The ratio of the amount of water vapor actually present in the air to the amount of water vapor that the air can hold at a given temperature and pressure, times 100%”
The amount of water vapor in the air is directly proportional to the water vapor pressure. Therefore, we can determine the relative humidity, RH, of the air as
RH = [(Actual vapor pressure) / (Saturation vapor pressure) ]x 100%
When the air is saturated, the actual vapor pressure recorded by the gauge is equal to the saturation vapor pressure and hence, the relative humidity is 100%.
If the air in the container is heated, we notice that the pressure indicated by the pressure gauge increases. Part of the increased pressure is caused by the increase of the pressure of the air. This increase can be calculated by the ideal gas equation and subtracted from the gauge reading, so that we can determine any increase in pressure that would come from an increasing the air temperature to 25oC, the water vapor pressure also increases. After a while, however, the water vapor pressure again becomes a constant. The air is again saturated. We see from this experiment that the ‘maximum amount of water vapor that the air can hold is a function of temperature’. At low temperatures the air can hold only a little water vapor, while at high temperatures the air can hold much more water vapor.
We can now see why the water in bowl 2 did not disappear. Water evaporated from the liquid into the air above, increasing the relative humidity of the air. However, once the air became saturated, the relative humidity was equal to 100%. And no more water vapor could evaporate into it. This is why you can still see the water in bowl 2, there is no place for it to go.
Because of the temperature dependence of water vapor in the air, when the temperature of the air is increased, the capacity of the air to hold water increases. Therefore, if no additional water is added to the air, the relative humidity will decrease because the capacity of the air to hold water vapor has increased. Conversely, when the air temperature is decreased, its capacity to hold water vapor decreases, and therefore the relative humidity of the air increases. This temperature dependence causes a decrease in the relative humidity during the day light hours, and an increase in the relative humidity during the night time hours, with the maximum relative humidity occurring in the early morning hours just before sunrise.
The amount of evaporation depends on the following factors:
- The vapor pressure. Whenever the actual vapor pressure is less than the maximum vapor pressure allowable at that temperature, the saturation vapor pressure, then evaporation will readily occurs. Greater evaporation occurs whenever the air is dry, that is, at low relative humidities. Less evaporation occurs when the air is moist, that is, at high relative humidities.
- Wind movement and turbulence. Air movement and turbulence replaces air near the water surface with less moist air and increases the rate of evaporation.
Cooling of human body
Now by keeping in mind the concept relative humidity we can understand how the body cools itself. Through the process of perspiration, the body secrets microscopic droplets of water onto the surface of the skin of the body. As these tiny droplets of water evaporate into the air, they cool the body. As long as the relative humidity of the air is low, evaporation occurs readily, and the body cools itself. However whenever the relative humidity becomes high, it is more difficult for the microscopic droplets of water to evaporate into the air. The body can not cool itself, and the person feels very uncomfortable.
We are all aware of the discomfort caused by the hot and humid days of June to August. The high relative humidity prevents the normal evaporation and cooling of the body. As some evaporation occurs from the body, the air next to the skin becomes saturated, and no further cooling can occur. If a fan is used, we feel more comfortable because the fan blows the saturated air next to our skin away and replaces it with air that is slightly less saturated. Hence, the evaporation process can continue while the fan is in operation and the body cools itself. Another way to cool the human body in the summer is to use an air conditioner. The air conditioner not only cools the air to a lower temperature, but it also removes a great deal of water vapor from the air, thereby decreasing the relative humidity of the air and permitting the normal evaporation of moisture from the skin.
In the hot summertime, people enjoy swimming as a cooling experience. Not only the immersion of the body in the cool water is so satisfying, but when the person comes out of the water, evaporation of the sea or pool water from the person adds to the cooling. It is also customary to wear loose clothing the summertime. The reason for this is to facilitate the flow of air over the body and hence assist in the evaporation process. Tight fitting clothing prevents this evaporation process and the person feels hotter.
What many people do not realize is that you can also feel quite uncomfortable even the wintertime, because of the humidity of the air. If the relative humidity is very low in your home then evaporation occurs very rapidly, cooling the body perhaps more than desirable. As an example, the air temperature might be 70 oF but if the relative humidity is low, say 30%, then evaporation readily occurs from the skin of the body, and the person feels cold even though the air temperature is 70 oF. In this case the person can feel more comfortable if he or she uses a humidifier. A humidifier is a device that adds water vapor the air. By increasing the water vapor in the air and hence increasing the relative humidity, the rate of evaporation from the body decreases. The person no longer feels cold at 70 oF, but feels quite comfortable. If too much water vapor is added to the air, increasing the relative humidity to near a 100%, then evaporation from the body is hampered, the body is not able to cool itself, and the person feels too hot even though the temperature is only 70 oF. Thus too high or too low a relative humidity makes the human body uncomfortable.
Medical point of view
We should also note that the evaporation is also used to cool the human body for medical purposes. If a person is running a high fever, then an alcohol rub down helps cool the body down to normal temperature. The principle of evaporation as a cooling device is the same, only alcohol is very volatile and evaporates very rapidly. This is because the saturation vapor pressure of alcohol at 20 oC is much higher than the saturation vapor pressure of water. At 20 oC, water has a saturation vapor pressure of 17.4 mm of Hg, whereas ethyl alcohol has a saturation vapor pressure of 44 mm of Hg. The larger the saturation vapor pressure of a liquid, the greater is the amount of its vapor that the air can hold and hence the greater is the rate of evaporation. Because the alcohol evaporates much more rapidly than water, much greater cooling occurs than when water evaporates. Ethyl ether and ethyl chloride have saturation vapor pressures of 442 mm and 988 mm of Hg, respectively. Ethyl chloride with its very high saturation vapor pressure, evaporate so rapidly that it freezes the skin, and is often used as a local anesthetic for minor surgery.
