What is relative humidity?

Short Answer:

Relative humidity is the measure of moisture present in the air, expressed as a percentage of the maximum amount of moisture that air can hold at a given temperature. In simple words, it tells us how full the air is with water vapor compared to its maximum capacity at that temperature.

For example, if the relative humidity is 70%, it means the air contains 70% of the total water vapor it can hold before condensation starts. It is an important concept in weather forecasting, air conditioning, agriculture, and human comfort levels.

Detailed Explanation:

Relative humidity

Relative humidity (RH) is a very important parameter in the study of psychrometry, which deals with the properties of moist air. It indicates the level of saturation of air with water vapor and is crucial in maintaining comfort, health, and proper system design in many engineering applications like HVAC systems, drying processes, and climate control.

Relative humidity changes with temperature. Warmer air can hold more moisture, while cooler air holds less. So, RH is not just about how much water vapor is present, but how close the air is to being fully saturated at that temperature.

Definition and Formula

Relative humidity is defined as:

Relative Humidity (RH)=(Actual vapor pressureSaturation vapor pressure)×100\text{Relative Humidity (RH)} = \left( \frac{\text{Actual vapor pressure}}{\text{Saturation vapor pressure}} \right) \times 100Relative Humidity (RH)=(Saturation vapor pressureActual vapor pressure​)×100

Or,

RH=(Mass of actual water vapor in airMaximum possible water vapor air can hold)×100\text{RH} = \left( \frac{\text{Mass of actual water vapor in air}}{\text{Maximum possible water vapor air can hold}} \right) \times 100RH=(Maximum possible water vapor air can holdMass of actual water vapor in air​)×100

  • Expressed in percentage (%)
  • 0% RH means completely dry air
  • 100% RH means air is fully saturated (condensation starts)

Importance of Relative Humidity

  1. Human Comfort
    • Ideal RH for comfort is between 40% to 60%.
    • Too high RH causes sweating, tiredness, and mold.
    • Too low RH causes dry skin, throat irritation, and static electricity.
  2. Air Conditioning Systems
    • RH helps in controlling temperature and moisture.
    • Engineers design HVAC systems based on desired RH levels.
  3. Agriculture
    • RH affects plant growth, seed storage, and pest control.
    • Helps farmers know when to irrigate or spray fertilizers.
  4. Food and Pharmaceutical Industries
    • Products like medicines and packaged food need controlled RH to stay fresh.
  5. Drying and Cooling Applications
    • RH plays a key role in drying grains, clothes, or chemicals.
    • High RH slows down drying, while low RH speeds it up.

RH and Temperature Relationship

  • RH decreases when temperature increases, even if the amount of moisture stays the same.
  • For example, air with 60% RH at 25°C may have only 30% RH at 35°C, due to increased capacity to hold water vapor.

This is why summer air may feel dry, even if moisture is present, and winter air may feel damp, even when it’s cold.

Measurement of Relative Humidity

  1. Hygrometer – A simple device used in homes
  2. Psychrometer – Uses dry bulb and wet bulb thermometers
  3. Electronic Sensors – Used in air conditioners, weather stations

These tools help measure and monitor RH for various applications.

Real-Life Example

If you hang clothes to dry on a humid day, they take longer to dry because the air is already holding a lot of moisture, and cannot absorb more. This is due to high relative humidity, which slows down the evaporation process.

Conclusion

Relative humidity is a measure of how much water vapor is present in air compared to the maximum it can hold at that temperature. It affects everything from human comfort and health to industrial processes, agriculture, and HVAC systems. By understanding and controlling RH, engineers can design better environments and more efficient systems for daily life and industry.