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Answers
Answer:
The expression “it’s not the heat, it’s the humidity” makes a valid point. We keep cool in hot weather by evaporating sweat from our skin and water from our breathing passages. Because evaporation is inhibited by high humidity, we feel hotter at a given temperature when the humidity is high. Low humidity, on the other hand, can cause discomfort from excessive drying of mucous membranes and can lead to an increased risk of respiratory infections.
When we say humidity, we really mean relative humidity. Relative humidity tells us how much water vapor is in the air compared with the maximum possible. At its maximum, denoted as saturation, the relative humidity is 100%, and evaporation is inhibited. The amount of water vapor the air can hold depends on its temperature. For example, relative humidity rises in the evening, as air temperature declines, sometimes reaching the dew point. At the dew point temperature, relative humidity is 100%, and fog may result from the condensation of water droplets if they are small enough to stay in suspension. Conversely, if you wish to dry something (perhaps your hair), it is more effective to blow hot air over it rather than cold air, because, among other things, hot air can hold more water vapor.
The capacity of air to hold water vapor is based on vapor pressure of water. The liquid and solid phases are continuously giving off vapor because some of the molecules have high enough speeds to enter the gas phase; see Figure 2a. If a lid is placed over the container, as in Figure 2b, evaporation continues, increasing the pressure, until sufficient vapor has built up for condensation to balance evaporation. Then equilibrium has been achieved, and the vapor pressure is equal to the partial pressure of water in the container. Vapor pressure increases with temperature because molecular speeds are higher as temperature increases. Table 1 gives representative values of water vapor pressure over a range of temperatures.
Relative humidity is related to the partial pressure of water vapor in the air. At 100% humidity, the partial pressure is equal to the vapor pressure, and no more water can enter the vapor phase. If the partial pressure is less than the vapor pressure, then evaporation will take place, as humidity is less than 100%. If the partial pressure is greater than the vapor pressure, condensation takes place. The capacity of air to “hold” water vapor is determined by the vapor pressure of water and has nothing to do with the properties of air.
Table 1. Saturation Vapor Density of Water
Temperature (ºC) Vapor pressure (Pa) Saturation vapor density (g/m3)
50 4.0 0.039
20 1.04 × 102 0.89
10 2.60 × 102 2.36
0 6.10 × 102 4.84
5 8.68 × 102 6.80
10 1.19 × 103 9.40
15 1.69 × 103 12.8
20 2.33 × 103 17.2
25 3.17 × 103 23.0
30 4.24 × 103 30.4
37 6.31 × 103 44.0
40 7.34 × 103 51.1
50 1.23 × 104 82.4
60 1.99 × 104 130
70 3.12 × 104 197
80 4.73 × 104 294
90 7.01 × 104 418
95 8.59 × 104 505
100 1.01 × 105 598
120 1.99 × 105 1095
150 4.76 × 105 2430
200 1.55 × 106 7090
220 2.32 × 106 10,200