At a temperature between -10 and -20 "C, a cloud will often contain water in all three forms - vapour, supercooled liquid droplets, and crystals. The tendency for water to move from one phase to another depends on the equilibrium between forms. Since water molecules are more tightly bonded in solid form than in liquid form, more energy is required to change the phase of water from solid to vapour (le, sublimation) than from liquid to vapour (le, evaporation). Therefore, the saturation vapour pressure over liquid water is greater than the saturation vapour pressure over ice, and it is possible for air to be unsaturated with respect to liquid water and supersaturated with respect to ice (Fig. 2). 5 4.5 A 3.5 3 saturation with respect to water Saturation Vapour Pressure (mb) 2.5 N 1.5 saturation with respect to ice 0.5 -20 -18 -16 -14 -12 -10 Air Temperature (*c) Fig. 2. The saturation vapour pressure curve is different depending on whether the H, O is in liquid or solid form. 1. Consider the air sample denoted by the star in Figure 2. Its temperature is -12.8 :C and its vapour pressure is 2.15 mb. What is its relative humidity with respect to water if, at -12.8 *C, the saturation vapour pressure for the liquid phase is 2.29 mb. Is the air unsaturated, saturated, or supersaturated with respect to liquid water? [2 marks] 2. What will likely happen to any water droplets contained in the air in Question 1? Will the droplets grow or shrink? [2 marks] 3. What is the relative humidity of the air sample in Figure 2 with respect to ice if, at -12.8 *C the saturation vapour pressure for ice is 202 mb? Is the air unsaturated, saturated, or supersaturated with respect to ice? [2 marks] 4. What will likely happen to any water vapour contained in the air in Question 3? Will the crystals grow or shrink? [2 marks]