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11 Nov 2019
Clouds are a dispersion of liquid water droplets suspended in air. Liquid water is ~ 1000 times as dense as air. Here is the puzzle: if there is enough liquid water to make clouds appear white, why aren't clouds so dense that they settle to the ground? The answer to this puzzle lies in the enormous light-scattering efficiency of small droplets. In this exercise, you are to compare the amount of liquid water in a cloud with the amount of gas-phase water within the cloud to learn that the density of a cloud isn't significantly different than the density of clear (cloud-free) air. Consider a cloud that has these conditions, which are based on data for continental stratocumulus clouds (NL Miles et al., Journal of the Atmospheric Sciences 57, 295-311, 2000). H = 800 m altitude T = 10 degree C temperature (= 283 K) D = 10 mu m diameter of cloud droplets N = 350 cm6-3, number concentration of liquid water droplets in the cloud P = 0.91 atm air pressure (a) Determine the mass concentration of liquid water associated with droplets in the cloud. (b) Assume that the air in the cloud is saturated with water vapor, i.e., relative humidity = 100%. Determine the mass concentration of water vapor in the cloud. (c) What is the total density of the cloud, accounting for all three components: (i) dry air + ii) water vapor + (iii) liquid water? (d) What fraction of the total cloud mass is associated with the liquid water droplets?
Clouds are a dispersion of liquid water droplets suspended in air. Liquid water is ~ 1000 times as dense as air. Here is the puzzle: if there is enough liquid water to make clouds appear white, why aren't clouds so dense that they settle to the ground? The answer to this puzzle lies in the enormous light-scattering efficiency of small droplets. In this exercise, you are to compare the amount of liquid water in a cloud with the amount of gas-phase water within the cloud to learn that the density of a cloud isn't significantly different than the density of clear (cloud-free) air. Consider a cloud that has these conditions, which are based on data for continental stratocumulus clouds (NL Miles et al., Journal of the Atmospheric Sciences 57, 295-311, 2000). H = 800 m altitude T = 10 degree C temperature (= 283 K) D = 10 mu m diameter of cloud droplets N = 350 cm6-3, number concentration of liquid water droplets in the cloud P = 0.91 atm air pressure (a) Determine the mass concentration of liquid water associated with droplets in the cloud. (b) Assume that the air in the cloud is saturated with water vapor, i.e., relative humidity = 100%. Determine the mass concentration of water vapor in the cloud. (c) What is the total density of the cloud, accounting for all three components: (i) dry air + ii) water vapor + (iii) liquid water? (d) What fraction of the total cloud mass is associated with the liquid water droplets?
Irving HeathcoteLv2
22 Sep 2019