Humidity

• There are several ways to measure amount of water vapor in the air.  One is to relate the mass of water vapor to the mass of air.  This is called the mixing ratio and it has units of grams of water vapor per kilograms of air.
• The saturation mixing ratio is the mass of water vapor at saturation divided by the mass of air.

This depends only upon the temperature.  At saturation as the temperature increases, so does the number of water vapor molecules and thus the saturation mixing ratio also increases.
• The actual mixing ratio is the actual mass of water vapor in the air divided by the mass of air.
• Another way to measure the actual amount of water vapor in the air is to cool the air until it is saturated and dew starts to form.  The temperature at which dew starts to form is called the dew point.  This depends upon the actual amount of water vapor in the air.
• The relative humidity is a measure how close the actual mixing ratio is to saturation.

Relative humidity = actual mixing ratio/saturation mixing ratio * 100%
 
• Typically the actual mixing ratio changes little over the course of a day (unless a front passes), but since the air temperature changes significantly (lowest in the morning and highest in the afternoon) the saturation mixing ratio changes also.  This in turn causes the relative humidity to change also over the course of the day (largest in the morning and smallest in the afternoon).