 Clouds will not form unless they become supersaturated, meaning that its relative humidity is slightly greater than 100%, or to put it another way, its supersaturation is greater than zero. Let's look at the three ways that supersaturation can be achieved. Radiative cooling, mixing, and adiabatic ascent. We can use the water vapor phase diagram, which is water vapor pressure on the y-axis versus temperature on the x-axis, to examine these processes. Super saturation means that the air parcel moves from the all-vapor part of the phase diagram, reaches the equilibrium line, which is given by the Clausius-Clapeyron equation, and then crosses it. I will mention only the essentials of each process, what changes and what stays the same. The radiative cooling, the water vapor pressure stays the same, but the temperature drops. Because the saturation vapor pressure drops as the temperature decreases, remember saturation vapor pressure depends only on temperature, then the saturation vapor pressure decreases until it gets equal to, and then a little less than the vapor pressure, and then the supersaturation goes above zero. The next process is mixing. Mixing clouds usually form when unsaturated warm, moist air from a source is mixed into the unsaturated colder, drier environmental air. As the warm, moist air mixes with the colder, drier air, the temperature and vapor pressure of the mixed air parcel take on the values that lie on the line between those between the two initial air parcels. As the air parcel mixes with more environmental air, the mixed air parcel's temperature and vapor pressure move along the mixing line. If this line crosses the equilibrium line and goes into the liquid phase of the phase diagram, supersaturation becomes greater than zero in the cloud forms. If the mixed air parcel continues to entrain cold, dry air, it may eventually cross the equilibrium line back into the vapor region and the cloud will evaporate. Contrails are one example of a mixing cloud. The contrail length tells you something about the temperature and vapor pressure of the environmental air. The third process is adiabatic ascent. As the air parcel ascends, its pressure and temperature drop. Because the water vapor mixing ratio is constant until a cloud forms, the drop in the pressure means a drop in the water vapor pressure. At the same time, the drop in the temperature means a drop in the saturation vapor pressure, which depends only on temperature. So vapor pressure and saturation vapor pressure are both dropping. However, in adiabatic ascent, the saturation vapor pressure drops faster than the vapor pressure and eventually they become equal and then supersaturation becomes greater than zero and the cloud forms.