There are three requirements for forming a cloud drop:

**M**oisture**A**erosol**C**ooling

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 water saturation vapor pressure.

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

Saturation occurs when* e=e _{s}, w=w_{s}*, and condensation = evaporation. At saturation,

*RH/100 = e/e*, 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

_{e}~ w/w_{s}= 1*T*(as determined from the dry adiabat) =

*T*(as determined from the constant water vapor mixing ratio), or when

_{d}*w = w*.

_{s}Let’s define two new variables that are useful in discussing the cloud drop formation.

Often we talk about the **saturation ratio**:

$$S=\raisebox{1ex}{$e$}\!\left/ \!\raisebox{-1ex}{${e}_{s}$}\right.$$

where *e* is the water vapor pressure in hPa or Pa and *e _{s}* is the saturation vapor pressure.

*S*< 1 for a subsaturated environment,

*S*= 1 for saturation (condensation = evaporation), and

*S*> 1 for a supersaturated environment.

We also talk about **supersaturation**:

$$s=S-1=\raisebox{1ex}{$e$}\!\left/ \!\raisebox{-1ex}{${e}_{s}$}\right.-1$$

*s *= 0 at saturation; *s* < 0 for a subsaturated environment; *s* > 0 for a supersaturated environment. Note that *s* and *S* are both unitless.

This equation applies only for a flat surface of pure water. When we get into situations where we have curvature or a solute, we need to think about the supersaturation relative to the equilibrium value of *e, e _{eq}*, which can be different from

*e*. So, depending on the circumstances,

_{s}*e*can be

_{eq}*e*, or some combination. We will see that a small supersaturation is actually needed to form clouds.

_{s}, e_{i}, e_{sc}, e_{sol}#### Check Your Understanding

The relative humidity is 85%. What is the saturation ratio? What is the supersaturation?

**Click for answer.**

ANSWER: S = 0.85 and s = 0.85-1 = -0.15

The relative humidity is 102%. What is the saturation ratio? What is the supersaturation?

**Click for answer.**

ANSWER: S = 1.02 s = 0.02

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?