In order for water vapor to condense out of the air it will need something to condense onto. Moisture will typically condense out of the air when the relative humidity is 100%. It is possible though the relative humidity could go well over 100% before moisture condenses out of the air. The reason for this is that moisture needs a condensation nuclei to condense on. Imagine that the air is perfectly clean of any condensation nuclei. With no condensation nuclei present, adding moisture to this air will not produce condensation. Eventually spontaneously condensation will occur when the moisture amount is extremely high with the relative humidity much higher than 100%. This does not occur in the atmosphere though since some condensation nuclei are always present. Thus, in the atmosphere cloud formation and condensation occur when the relative humidity hits 100%. Condensation can occur when the relative humidity is less than 100%. Some condensation nuclei, such as salt nuclei, draw in moisture so easily that condensation will occur before the relative humidity reaches 100%. This can produce haze and cloud formation at relative humidities less than 100%. Condensation is moisture going from the vapor state to the liquid state. Deposition is moisture going from the vapor state to the solid state. When the relative humidity is 100% and the temperature is below freezing, then deposition can occur. This can produce the growth of ice within clouds and frost on the ground surface. Objects on the ground surface act as condensation and deposition nuclei when the relative humidity reaches 100%. This is how dew and frost develop on objects on the ground. |