How can one determine the relative humidity on a particular day at a given place?

Relative humidity (RH) is defined as the percentage of moisture present in the atmosphere compared to its full capacity at a specific temperature [3]. To determine RH, two variables are essential: the actual amount of water vapor present (absolute humidity) and the maximum amount of water vapor the air can hold at that temperature (saturation point) [2]. While absolute humidity provides the mass of water vapor per unit volume, it does not account for temperature [3]. However, the air's capacity to hold moisture depends entirely on its temperature; warmer air can hold more water vapor than cooler air [3]. Therefore, to calculate the ratio that defines relative humidity, one must know both the current absolute humidity and the temperature, as the latter determines the saturation vapor pressure used as the denominator in the RH formula [1].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Relative Humidity > p. 326
2. [3] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 10: Water in the Atmosphere > CHAPTER > p. 86
3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Change In Relative humidity > p. 327