As we know, a cloud forms when the air becomes saturated with water vapor, at which time, the air has a relative humidity of 100%. The relative humidity increases when either water vapor is added to the air or the air is cooled. When the relative humidity reaches 100%, the water vapor begins to condense and form tiny cloud droplets. We are all familiar with condensation. If I were to tell you to visualize condensation you might imagine water droplets forming onto a cold glass or drink can. In this example, the air next to the can cools, the relative humidity of the air increases. When the relative humidity reaches 100%, the water vapor begins to condense onto the can. But did you know that it is possible for air to have a relative humidity above 100%? This is called supersaturated air. In fact, if it were not for the presence of aerosols in our atmosphere, the air would have to reach a relative humidity of several hundred percent for condensation to occur and clouds to form. Now, lets see why.
The Role of Aerosols in Cloud Formation
If the definition of saturated air is air that contains all the water vapor it can hold, then how can it possibly hold anymore? How can air have a relative humidity of more than 100%? Well, when we speak of the relative humidity, it is always with respect to a plane (or flat) surface of water. Over a flat surface of water, as opposed to a curved surface of water or a surface of some other substance, water molecules are simultaneously condensing and evaporating onto the surface of the water. At a relative humidity of 100%, the number of molecules condensing and the number evaporating is equal. Air with a relative humidity of 100% cannot hold an additional water vapor molecule without losing one. In the atmosphere, there are no flat surfaces of water, but there are aerosols. These aerosols provide a surface onto which the water vapor molecules can condense.
Without the presence of aerosols or some surface to condense onto, the air would attain a relative humidity of several hundred percent (with respect to a plane surface of water) before the water vapor molecules would condense onto each other. This is due to the free energy barrier of water vapor molecules. The free energy barrier exists because of strong atomic bonds which hold the molecule together. This barrier inhibits two single water vapor molecules from easily bonding together. Interestingly, cloud formation normally occurs at a relative humidity of 100% even though the water vapor is not condensing onto a plane surface of water.
Water will condense onto some aerosols at a relative humidity as low as 70%, while large supersaturations are required for others. Because water vapor will condense onto different aerosols at different relative humidities, some are better for cloud formation than others. Those that readily participate in cloud formation are called cloud condensation nuclei or CCN, while those that require saturations higher than are normally observed in our atmosphere are referred to as condensation nuclei or CN. Relative humidities of more than 102% rarely occur in our atmosphere due to the abundance of aerosols present.
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