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About The Relative Humidity


Relative humidity is a simple concept as far as weather phenomena go, but it has significant, far-reaching consequences for how we must take care of ourselves on a hot or wet day.


GS I: Geography

Dimensions of the Article:

  1. Understanding Humidity
  2. Importance of Relative Humidity
  3. Ways to Measure Relative Humidity
  4. Importance of Wet-Bulb Temperature in Understanding Relative Humidity

Understanding Humidity

  • Humidity is the amount of moisture in the air.
  • There are three ways to track it:
    • Absolute humidity – the mass of water vapour in a given volume of air and water vapour mixture, expressed as kg/m3.
    • Specific humidity – equal to the mass of the moisture divided by the mass of air. It is expressed as a dimensionless number (but sometimes as grams per kilogram among other similar units).
    • Relative humidity – factors in the amount of vapour that air can hold at different temperatures. It is the vapour density of the air divided by the saturation vapour density at dry-bulb temperature.
  • Relative humidity is important because it takes into account the temperature factor, making it a more accurate measure of the amount of moisture in the air.

Importance of Relative Humidity

  • Sweating helps our body cool down on hot days by releasing sweat via the skin surface, which then evaporates and absorbs heat from the skin, thus cooling it.
  • Relative humidity (RH) of air refers to the amount of moisture present in the air at a particular temperature, and it impacts the ability of sweat to evaporate from the skin.
  • High relative humidity means air is already filled with moisture, and sweat cannot evaporate easily from the skin surface, leading to heat accumulation in the body.
  • High humidity levels can be dangerous, and it is recommended to maintain a comfortable range of 30-60% RH for safe and comfortable conditions.
  • Environments with lower levels than this typically use humidifiers to increase the humidity, while fans can help move the air around in high RH environments to assist with evaporation.
  • In both cases, drinking water is important to replenish the fluids lost due to sweating.

What does relative humidity imply physically?

  • Warmer air has a greater capacity to hold moisture than cooler air at the same absolute humidity.
  • Relative humidity of warmer air will be lower than that of cooler air, even if they have the same absolute humidity.
  • The capacity for moisture changes non-linearly with temperature, meaning that the improvement in the air’s vapour-carrying capacity is much greater when it warms over 20º C from 30º C to 50º C, compared to the improvement as it warms over 20º C from -20º C to 0º C.
  • For instance, in Chennai, while the absolute humidity was 70% at 2:30 pm on April 23, the relative humidity was lower at around 60% due to the ambient temperature of 32°C.

Ways to Measure Relative Humidity:

Conduct a simple experiment with household items:
  • Requires two mercury thermometers, cotton, rubber bands, and cold water
  • Soak cotton in cold water and wrap it around the bulb of one thermometer
  • Hold both thermometers under a fan and record readings after five minutes
  • Subtract the wet-bulb temperature from the dry-bulb temperature
  • Use a relative humidity chart to get the value
Use mathematics:
  • Two methods to estimate relative humidity
    • Method 1: Divide actual vapor pressure by saturated vapor pressure
    • Method 2: Use the Clausius-Clapeyron equation
  • The vapour pressure is the amount of moisture that air contains.
  • The ‘saturated’ value is the maximum possible amount.
  • The given formula requires the dew point (Td), which you can obtain from a weather website. RH = (e/es)*100 e = 6.11 x 10((7.5 x Td)/(237.3 + Td)) For es, use the same formula but replace Td with T, the actual temperature. 4
Use an app or device:
  • Search for “hygrometer” in the app store to locate options
  • Hygrometers and psychrometers are available in physical and online stores
  • Note that the app will only work if your phone has a sensor that can sense humidity.

Importance of Wet-Bulb Temperature in Understanding Relative Humidity

  • The wet-bulb temperature is the lowest temperature a surface, such as your skin, can reach when water evaporates from it. It is equal to the dry-bulb temperature when the relative humidity is 100%.
  • A wet-bulb temperature in an environment of 32-35ºC or higher can be quickly lethal, even if you are not doing any physical activity or are in the shade.
  • Heatwaves are becoming more common, frequent, spread out, and potent over the Indian subcontinent, and the climate crisis is exacerbating the situation.
  • A study published in May 2022 suggests that the next frontier for attribution science is to inform adaptation decision-making in the face of unprecedented future heatwaves.
  • It is essential to keep an eye on the relative humidity, drink lots of water, and cool yourself as ways to adapt to high temperatures and humidity levels.

-Source: Indian Express

February 2024