1. Saturation = filled to capacity
HUMIDITY
To understand water vapor in the atmosphere, you need to understand saturation.
Saturation = filled to capacity
There are several measures of humidity in the atmosphere:
Absolute Humidity
: The absolute amount of water in the air
Weight of Water Vapor in Air
Absolute Humidity =
Weight of Unit Volume of Air

2. : The amount of water in a unit MASS of Air
HUMIDITY
Specific Humidity
: The amount of water in a unit MASS of Air
Mass of Water Vapor in Air
Specific Humidity =
Mass of Air
Specific Humidity is useful because the humidity is constant even as the Volume of the Air in question changes (due to Pressure).

3. Weight of Water Vapor in Air Relative Humidity =
: The ratio of the air’s actual water content to its potential water vapor content at a given temperature.
Relative Humidity
Weight of Water Vapor in Air
Relative Humidity =
Weight of Potential Water Vapor in Air at Saturation
Basically, relative humidity tells you how saturated the air is with water.

4. HUMIDITY
Dewpoint Temperature: The temperature the air would have to be cooled to saturate the air (causes water to condense)

5. A psychrometer is used to measure relative humidity.
The dry bulb measures the air temperature
The wet bulb gives a measure of how unsaturated the air is—the evaporation of water from the wet bulb lowers the temperature reading (and the evaporation depends on the saturation of the air)