Context:
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.
Relevance:
GS I: Geography
Dimensions of the Article:
- Understanding Humidity
- Importance of Relative Humidity
- Ways to Measure Relative Humidity
- 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
