The Vapor pressure is a measure of how much water vapor is present in the atmosphere. It’s actually a partial pressure of water vapor, for those of you familiar with Chemistry. Consider a contained filled with water, but not all the way – that is, there’s dry air space above the water. Cover this container. Water vapor evaporating from the water will fill the airspace. As soon as water vapor molecules are in the air space, they will also start to condense back into the water, but at the start, there are so few water vapor molecules that evaporation greatly exceeds condensation. As more and more molecules evaporate, the number of molecules above the water surface increases, the rate of condensation increases. At some point, the number of water molecules reaches a maximum – for every molecule that evaporates, one condenses. This condition is called saturation. The number of water vapor molecules above surface can be computed, and a partial pressure of those molecules, that is the saturation vapor pressure, derived. How much vapor can evaporate before the atmosphere above is saturated increases as the temperature increases, because the warmer temperature of the water promotes evaporation. What if the water is frozen in the container? Can you accept that it is harder for any molecule to transition from solid to gas (that is, sublimate) than it is for a molecule to transition from liquid to gas (that is, evaporate)? When equilibrium is reached, therefore, there are fewer vapor molecules over the ice than there would be over water at the same temperature. Saturation vapor pressure over ice is lower (smaller) than the saturation vapor pressure over water. What happens, then, if the atmosphere is saturated (with water vapor) with respect to ice? At such a point, it is unsaturated with respect to water, meaning evaporation will exceed condensation. That evaporation will put water vapor into the atmosphere, and if the water vapor moves over ice, it will be supersaturated with respect to ice. In a supersaturated environment, the rate of condensation exceeds that of evaporation. The figure below shows the difference in saturation vapor pressures over ice and over water. The maximum difference between the lines occurs at about -12C. The red line in the figure traces out the maximum amount of vapor for any particular temperature. Note the temperature is along the x-axis and the vapor pressure in mb is along the y-axis. Combinations of temperature and vapor pressure below the red line correspond to an unsaturated atmosphere. Combinations of temperature and vapor pressure above the red line correspond to a super-saturated atmosphere. For example, if the temperature is 20 C and the vapor pressure observed in the atmosphere is 10 mb, the atmosphere is unsaturated. How could you achieve saturation in such an atmosphere? Two ways: First, you could cool the temperature, in which case your point will move to the left until it reaches the red saturation line. Second, you could add moisture to the atmosphere, in which case the point will move up vertically on the graph. In Nature, a common way to saturate is to cool to the wetbulb, in which case evaporation adds moisture to the atmosphere as it cools.