40 30 30Temperature (\'C) 10 0 10 20 30 40 Figure 1.Saturation vapor pressure. d

Question

40 30 30Temperature ('C) 10 0 10 20 30 40 Figure 1.Saturation vapor pressure. depends on the equilibrium between phases. Since water molecules are more tightly bonded in solid than in liquid form, more energy is required to change the phase of water from solid to vapor (a process called sublimation) than from liquid to vapor (evaporation). Therefore, the saturation vapor pressure over liquid water is greater than the saturation vapor pressure over ice, and it is possible for an air sample to be unsaturated with respect to liquid water 5-T and supersaturated with respect to ice (Figure 2). Saturation with respect to water 1. Consider the air sample denoted by an asterisk () in Figure 2. Its temperature is -15 C and its vapor pressure is 1.72 mb. What is its relative humidity with respect to water if, at-15'C, the saturation vapor pressure over liquid water is 1.91 mb? (Your answer should be less than 100%, indicating an unsaturated sample.) Saturation with respect to ice 20 -19 18 -17 16 15 14-13 -12 1110 Temperature (°C) Figure 2. Saturation vapor pressure of water versus ice Copyright 02016 Pearson Education, I

Explanation / Answer

Relative humidity is a ratio of the air ’s actual water vapour content compared with the amount of water vapour required for saturation at that temperature (and pressure).

actual vapour pressure at -15 degrees is =1.72mb

required vapour pressure at -15 degrees for the air to be 100% saturate is =1.91mb

so relative humidity according to above definition is = (1.72/1.91)*100=90.02%

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