CALCULATING RELATIVE HUMIDITY
 
METEOROLOGIST JEFF HABY
Calculating the
RH requires the correct
equation(s). The RH is the amount of moisture in the air (via moisture mass or vapor pressure) divided by the
maximum amount of moisture that could exist in the air at a specific temperature (via max moisture mass or
saturation vapor pressure). RH is expressed as a percentage and has no units since the units in
both the numerator and denominator are the same. The percentage is found by multiplying the ratio by 100%.
The RH is NOT the dewpoint divided by the temperature. For example, if the temperature was 60 F and
the dewpoint was 30 F, you would not simply take (30/60)*100% = 50% RH.
Method #1
When given temperature and
dewpoint, the vapor pressure (plugging Td in place of T into ClausiusClapeyron
equation) and the saturation vapor pressure (plugging T into ClausiusClapeyron equation) can be determined.
The RH = E/Es*100%.
ClausiusClapeyron equation
LN(Es/6.11) = (L/Rv )(1/273  1/T)
Es = Saturation vapor pressure
L = Latent heat of vaporization = 2.453 × 10^6 J/kg
Rv = Gas constant for moist air = 461 J/kg
T = Temperature in Kelvins
Method #2
The mixing ratio is defined as the mass of water vapor divided by the mass of dry air. In a lab setting,
the lab technician could measure both the mass of water vapor and mass of dry air in an air sample. The
mass of
water vapor in a sample of air divided by the mass of
dry air is W. The lab technician could then
saturated the air (making sure temperature remains the same) and recalculate the mass of water vapor
divided by the mass of dry air. This would be Ws. The RH = W/Ws*100%
To get W and Ws, use the equation:
W= (0.622*e) / (P  e) and Ws = (0.622*Es) / (P  Es)
This requires that E and Es are known. Therefore, without using the ClausiusClapeyron equation, calculating
RH outside of a lab setting is difficult.
operational methods of calculating RH
1. Mixing ratio can be determined using the SkewT logP diagram. For any pressure level, the mixing
ratio is read through the dewpoint and the saturation mixing ratio is read through the temperature. By
reading the mixing ratio values off the SkewT you can determine W and Ws for any temperature and dewpoint.
RH = W/Ws*100%
2. Take the temperature and dewpoint and plug them into the ClausiusClapeyron equation. There are computer
programs that will do this. The computer uses the graph of the ClausiusClapeyron equation for all temperature
and dewpoints to find RH.
3. Many textbooks have a graph or table data of saturation mixing ratio and/or saturation vapor pressure for
various temperatures. Using dewpoint will
either give the actual vapor pressure or actual mixing ratio while using temperature will either give the
saturation vapor pressure and saturation mixing ratio (depending on if graph is showing vapor pressure or
mixing ratio). RH is E/Es*100% or W/Ws*100%.


