As we know, there is a water vapour in air which may range from 0% to 4% of air volume..
There is also so-called saturation limit i.e. the maximum amount of water vapor that can be contained in the air at a given temperature. The higher the temperature, the higher the absorption capacity of air.
The important characteristic of water vapor contained in the air is its pressure (elasticity).
Saturation pressure is the maximum possible water vapor pressure at a given temperature.
There is a chart describing the dependence of saturation pressure from temperature
The main method of measuring the humidity at a positive temperature is the psychrometric method. Moisture determination is carried out based on the readings of two thermometers with an accuracy of 0.1 degree Celsius. One thermometer measures the temperature of the air, and the second thermometer is wrapped in a damp cloth, so that it shows its own temperature, which depends on the intensity of the evaporation of water from the surface. The less water vapor is in the air, the stronger the evaporation from the surface of the second thermometer, and the lower the readings.
So, such system of two thermometers is called psychrometer.
According the difference of temperature readings the current pressure of water vapor in the air is determined by the formula:
where - the saturation pressure of the second thermometer
- psychrometer constant, taken to be equal 0.0007947,
- atmosphere pressure, taken to be equal 1000 hPa
- dry bulb temperature
- wet bulb temperature
And finally, the relative humidity is the ratio of current pressure to saturation pressure at a given air temperature
Dry thermometer temperature
Wet thermometer temperature
I just add that in the deserts the relative air humidity is 50% or less, and in the tropics it's 85% and above.