5.2 What are the requirements for forming a cloud drop?

5.2 What are the requirements for forming a cloud drop?

There are three requirements for forming a cloud drop:

1. Moisture
2. Aerosol
3. Cooling

If any one of these three is missing, a cloud cannot form. We have talked about moisture and aerosol and now need to consider ways that the air can be cooled. The air needs to be cooled so that the water vapor pressure initially equals and then exceeds the saturation water vapor pressure.

An easy way to remember these key ingredients is to think of a Big MAC.

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A Big MAC. Hungry for some more?

Credit: Big Mac Snip by Ian Burt is licensed under CC BY 2.0.

Saturation occurs when e = e_s, w = w_s, and condensation = evaporation. At saturation, RH = e/e_s ~ w/w_s = 1, or in terms of percent, 100%. When we find the lifting condensation level (LCL) on a skew-T, we are finding the pressure level at which T (as determined from the dry adiabat) = T_d (as determined from the constant water vapor mixing ratio), or when w = w_s.

Two variables that are useful in discussing the cloud drop formation are the saturation ratio and the supersaturation. In Lesson 3, we introduced the saturation ratio:

where e is the water vapor pressure and e_s is the saturation vapor pressure. S < 1 for a subsaturated environment, S = 1 for a saturated environment (condensation = evaporation), and S > 1 for a supersaturated environment. A second useful variable is the supersaturation:

s < 0 for a subsaturated environment, s = 0 for a saturated environment, and s > 0 for a supersaturated environment. Note that s and S are both unitless.

The above equations apply only for a flat surface of pure liquid water. When we get into situations where the water has a curved surface (as in a cloud drop), contains a solute, or is in solid form, we need to think about the saturation ratio and the supersaturation relative to the equilibrium value of e, e_eq, which can be different from e_s. So, depending on the circumstances, e_eq can be e_s (flat liquid water), e_sc (curved liquid water), e_sol (curved solution), e_i (flat ice), or some combination. We will see that a small supersaturation is actually needed to form clouds.

Note that it is possible to have the relative humidity be greater than 100%, which makes the supersaturation positive. This condition can't last long because condensation will exceed evaporation until they become equal. But how can supersaturation happen?