Relative humidity is defined as follows:

RH=100*e/es

whe
e= partial pressure of vapour
es=saturation vapour pressure

es is tabled against dew point temperature.

Above 0°C no problem
Below 0°C (exactly 0.01°C is triple point of water) exist two tables
of saturation pressu
1) ews = saturation above water
2) eis = saturation above ice

Now my question a
1) eis or ews is used in RH formula?
2) real RH sensors measure 100*e/eis or 100*e/ews?

Thanks for any help!