Water vapor in the atmosphere

Air in the troposphere (surface to 10 km): approximated by 2 components
"dry air" (mixture of nitrogen, oxygen etc. at constant ratio)
water vapor (ratio changes according to time and place)
Expressions of the quantity of water vapor in air
Specific humidity [non-dimensional (kg/kg); g/kg also often used]
= mass of water vapor / mass of air (including water vapor)
Vapor pressure [SI unit is Pa; hPa is often used]
= number of water molecules / total number of molecules × air pressure [assuming ideal gas]
Relative humidity [non-dimensional; % is often used]
= vapor pressure/ saturation vapor pressure
= specific humidity / saturation specific humidity
What is saturation vapor pressure?
Pressure of water vapor in equilibrium with liquid water
A function of temperature but not of air pressure (ideal gas approx.)
(Saturation specific humidity depends also on pressure.)
Mass balance of the atmosphere
As the global total, only water vapor is changeable.
Mass of local air columns mainly change by horizontal transports.