You use a Hygrometer.
-9c
for air water system wet bulb equals to dry bulb at 100 % relative humidity for that given temperature of air.
That means that there is 100% humidity. Normally evaporation from the wet bulb keeps its temperature lower than the dry bulb. At 100% humidity, there would be no evaporation, so they would show the same temperature.
-17
at 17% relative humidity the dew point is -5.8C at a dry bulb of 20C
A wet bulb measures the temperature after water evaporation allows to cool and a dry bulb measures air temperature.
-9c
A dry bulb thermometer is an ordinary one. It's given this name when it's used to measure air temperature. A wet bulb thermometer takes into account humidity.
for air water system wet bulb equals to dry bulb at 100 % relative humidity for that given temperature of air.
the "current" temperature, ie, the temperature at which wet bulb and dry bulb are the same. when the wet bulb and dry bulb temperaturs equalized the dew point emperature equals them, because the air is saturated now.
it is difference between wet bulb temperature and dry bulb temperature.
An instrument that uses the difference in readings between two thermometers, one having a wet bulb and the other having a dry bulb, to measure the moisture content or relative humidity of air.
Dry bulb Temperature - Dew Point Temperature
the evaporation off the moisture on the wet bulb absorbs the heat.
A sling psychrometer (or hygrometer) uses the difference in readings between a wet bulb thermometer and another with a dry bulb to measure the relative humidity. The bulb that is wet will cool by evaporation to the "dew point", which is the temperature at which the current humidity would be the maximum possible. Comparing that to the dry bulb (the general air temperature) on a comparative chart will yield the approximate relative humidity.
Hygrometer
50 percent