Question

Purpose: To gain deeper understanding of the processes involved in the formation of clouds and precipitation.

Part 1: Cloud Droplet Growth

1. On Figure 8-2, consider a sample of air denoted by the * with a tem- perature of −15◦C and ea = 1.72mb. If, at −15◦C, es = 1.91mb with respect to water, what is the RH?
2. What will likely happen to liquid water when the surrounding envi- ronment is unsaturated?
3. What is the RH with respect to ice if, at −15◦C, es = 1.65mb with respect to ice?
4. What happens to water vapour if, with respect to the surrounding ice crystals, the environment is supersaturated?

Answer 1

1. The relative humidity is

$$RH=\frac{P_a}{P_s}·100\%=90\%.$$

2. When the surrounding environment is unsaturated, liquid water will likely evaporate.

3. The relative humidity relative to ice is

$$RH=\frac{P_a}{P_s}·100\%=104\%.$$

4. If, with respect to the surrounding ice crystals, the environment is supersaturated, the water vapour will condense.