Our office forecasters are used to analyze humidity values as relative humidity, while IAPP algorithm retrieves specific humidity. Therefore conversion is needed.
Simple and quick way to do that:
Assume that we know temperature T (Celsiums), air pressure P (hPa) and specific humidity S (g/kg). These are actually the variables retrieved by IAPP. To calculate relative humidity f (%) we should know actual water vapor pressure e (hPa) and saturation water vapor pressure E (hPa).
Actual water vapor pressure e can be calculated as following:
e = S*P/622
E = 6.107E(7.632*T/(241.9+T))
and finally relative humidity:
f = e/E*100
The equations were taken from A. Hrgian, "Physics of Atmosphere", 1978.