If alcoholic disinfection is carried out before injecting, when alcohol evaporates, he will have a cold feeling to because heat is taken.

In order for a liquid to turn into gas, energy is required and this energy is called evaporation heat.
The thing which enabled it to measure humidity using this is a Psychrometer

The Psychrometer is a hygrometer which measures humidity and temperature simultaneously by measurement of dry-bulb temperature and wet-bulb temperature.
In extreme high temperature, low temperature, low humidity, and low pressure, an error becomes large by the object for the measurement in general temperature and moisture environment.

Consisting of two thermometers, one side is a portion which makes a ball part always become wet with water and which is called a wet bulb.
A wet bulb shows a temperature usually lower than another thermometer (dry bulb), in order that water may take evaporation heat by evaporation in a ball part.
However, since a wet bulb is covered in the layer of thin ice when temperature is the freezing point, a temperature higher than a dry bulb may be shown.

In the case of precision measurement, relative humidity is calculated by the formula of Adolf Sprung (1848 to 1909) from dry-bulb temperature or wet-bulb temperature, the difference in temperature between a dry bulb and a wet bulb, and atmospheric pressure.
The correction value of the formula is prepared for every ventilation system.

e  :the partial pressure of water vapor in the air,
esw:saturated water vapor pressure at the wet-bulb temperature
A = 0.000662K-1 : psychrometer coefficient
p  :pressure
Td :the dry-bulb temperature
Tw :the wet-bulb temperature

The correction value of the formula is prepared for every ventilation system.