 Clouds will not form unless the air becomes super saturated, meaning that its relative humidity is slightly greater than 100%, or put it another way, its super saturation is greater than 0%. Let's look at the three ways that super saturation can be achieved. Radiative cooling, mixing, and adiabatic ascent. We can use the water phase diagram of water vapor on the y-axis versus temperature on the x-axis to examine these processes. Super saturation means that the environment moves from the all vapor part of the phase diagram into the all liquid part by crossing the equilibrium line, which is given by the Clausius-Clapeyron equation. I will mention only the essentials for each process, what changes and what stays the same. For radiative cooling, the water vapor pressure stays the same, but the temperature drops. And because the saturation vapor pressure depends only on temperature, the saturation vapor pressure also drops. The saturation vapor pressure decreases until it gets equal to and then a little less than the vapor pressure, and then the super saturation 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 dryer environmental air. As the warm moist air mixes with the colder dryer air, the temperature and vapor pressure of the moist air parcel becomes the average of the temperature and vapor pressure of the moist warm air parcel multiplied by the number of moles, and the temperature and vapor pressure of the cold dry environmental air multiplied by the number of moles. All this divided by the total number of moles. As the air parcel mixes with more environmental air, the parcel's temperature and vapor pressure move along the mixing line between the two initial air parcel states. If this line crosses the equilibrium line and goes into the liquid part of the phase diagram, super saturation becomes greater than zero in the cloud forms. If the air parcel continues to entrain the dry air, it continues along the mixing line and 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 what the temperature and environmental pressure of the environmental air must be. The third process is adiabatic ascent. As the air parcel ascends, his 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 saturation becomes greater than zero in the cloud forms.