Vapor pressure  measures the water vapor content of the air using the partial pressure of the water vapor in the air. (Pressure may be expressed using a variety of units: in pascals, in millibars, in pounds per square inch, among others).  The gases in the atmosphere exert a certain amount of pressure (about 1013 millibars at sea level).  Since water vapor is one of the gases in air, it contributes to the total air pressure.  The contribution by water vapor is rather small, since water vapor only makes up a few percent of the total mass of a parcel of air. The vapor pressure of the water in the air at sea level, at a temperature of 20 degrees C, is 24 mb at saturation.

Most of these measures of humidity are not easily determined directly.  It is actually easier to measure relative humidity.

Relative Humidity: we can compare how much water vapor is present in the air to how much water vapor would be in the air if the air were saturated.  For this we use relative humidity.  Relative humidity is a ratio that compares the amount of water vapor in the air with the amount of water vapor that would be present in the air at saturation.   One way it can be stated would be as the ratio of the actual mixing ratio to the saturation mixing ratio.  Relative humidity is given as a percentage: the amount of water vapor is expressed as a percent of saturation.  If 10 grams of water vapor were present in each kilogram of dry air, and the air would be saturated with 30 grams of water vapor per kilogram of dry air, the relative humidity would be 10/30=33.3%.

For example, a parcel of air at sea level, at a temperature of 25 degrees C, would be completely saturated if there were 20 grams of water vapor in every kilogram of dry air. (Question: which measure of humidity are we using here?  Answer 1M).  If this air actually contained 20 grams of water vapor per kilogram of dry air, we would say that the relative humidity is 100%. 
If the parcel of air (at sea level at 25 deg C) actually had 10 grams of water vapor per kilogram of dry air, what is its relative humidity?   Answer 2M. 
If a parcel of air (at sea level at 25 deg C) had 18 grams of water vapor per kilogram of dry air, what is its relative humidity?  Answer 3M.

Humidity and Temperature

Consider a bowl of water. We've already seen this bowl, and we made water evaporate from it.  As you recall, to make the water evaporate, we added heat, which was absorbed by the individual molecules of water.  As each molecule absorbs heat, it gets more energetic, and eventually, has so much energy that it breaks the hydrogen bonds holding it to the other water molecules, leaves the liquid water, and floats off on its own, as a molecule of water vapor.  In other words, it evaporates.

Even while some water molecules are evaporating, others are condensing, changing from the vapor state to the liquid state, and joining the liquid water in the bowl.  At any given temperature, there will eventually be an equilibrium between the number of molecules evaporating, and the number of molecules condensing.  When the number of molecules evaporating balances the number of molecules condensing, we say that the air above the liquid water is saturated.  The term saturation refers to the maximum amount of water that can be present as a vapor in the atmosphere.  If the air above the water bowl is saturated, then for every molecule of water that evaporates, another molecule condenses.

But we know that to make water evaporate, you add heat.  So what happens if we raise the temperature of the bowl and the surrounding air?  More water will evaporate.  For a while, the rate of evaporation will exceed the rate of condensation.

Eventually, though, the balance between evaporation and condensation will stabilize at the new temperature.  Once again, evaporation will balance condensation.  The air will again be saturated, but there will be more molecules of water vapor present in the air above the bowl.  At the higher temperature, more water vapor will be present in the air at saturation.  This is a good general rule to remember: the higher the air temperature, the more water vapor will be present in the air at saturation.

What happens if we lower the temperature?  As the temperature is lowered, more water molecules return to the liquid state (condense) than evaporate.   Eventually, at the new lower temperature, there will again be a balance between the number of molecules evaporating and the number condensing.  But there will be fewer molecules of water vapor in the air at the new cooler temperature than were present at higher temperatures.  This rule to remember is just a restatement of the previous one: the lower the air temperature, the less water vapor will be present in the air at saturation.

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