Humidity in air is a measure of the absolute or relative amount of moisture or gaseous water (water vapor) present at a given temperature and air pressure. At the air temperature where dew forms, gaseous water is in equilibrium with liquid water, the dewpoint temperature, or simply, the dewpoint.
There are many ways to measure this complex phenomena. It’s complex because there are so many variables involved, temperature being one of the most important, but not the only one.
The information pages at About Temperature & Moisture Sensors provides a wealth of information plus leads to charts and software used to simplify the complexity that can arise in many situations.
Humidity measurement is is a unique use of temperature sensors and can lead one to an interesting side trip to the realm of humidity and pressure sensors.