What is the definition of "relative humidity" in weather forecast? How is it calculated?

Relative humidity is defined as the ratio of the mole fraction of water vapor to the mole fraction of saturated water vapor.

Meteorological measurement data include air pressure p, temperature t and dew point temperature td.

The water vapor pressure can be calculated by td, and the saturated water vapor pressure at the corresponding temperature and pressure can be calculated by P and T, and the ratio between them is the relative humidity.