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Wet Bulb Temperature

2022 MAR 7

Preliminary   > Environment and Ecology   >   Global warming   >   Climate change

Why in news?

  • Recently, sixth assessment report of Intergovernmental Panel on Climate Change (IPCC), emphasised on the trend in the ‘Wet Bulb’ Temperature in South Asia.

About ‘Wet Bulb’ Temperature:

  • Wet bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure.
  • WBT is a limit that considers heat and humidity beyond which humans cannot tolerate high temperatures.
  • The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow.
    • An adiabatic process is one in which no heat is gained or lost by the system.
  • Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin.

Dry Bulb Temperature:

The Dry Bulb temperature, usually referred to as air temperature, is the air property that is most commonly used. When people refer to the temperature of the air they are normally referring to the dry bulb temperature.

  • The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air.
  • The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air.
  • The evaporation is reduced when the air contains more water vapour.
  • The wet bulb temperature is always lower than the dry bulb temperature but will be identical with 100% relative humidity (the air is at the saturation line).
  • A wet-bulb temperature of 31°C is exceedingly harmful to humans, while a temperature of 35°C is un-survivable for more than 6 hours

Add ons:

  • Lucknow and Patna, were among the cities predicted to reach wet-bulb temperatures of 35°C if emissions continued to rise, while Bhubaneswar, Chennai, Mumbai, Indore, and Ahmedabad are ‘at risk’ of reaching wet-bulb temperatures of 32°C-34°C with continued emissions.
  • This will have consequences such as a rise in heat-wave linked deaths or reduced productivity

 

PRACTICE QUESTION:

Consider the following statements:

1. At 100% relative humidity, the wet-bulb temperature is equal to the dry-bulb temperature.

2. Wet-bulb temperature is a limit that considers heat and humidity beyond which humans cannot tolerate high temperatures

Which of the statements given above is/are correct?

(a) 1 only

(b) 2 only

(c) Both 1 and 2(d) Neither 1 nor 2

Answer