Wet-bulb temperature

Topic: Atmosphere: Composition, Structure and Elements of Weather · NCERT: Beyond NCERT

Meaning

Wet-bulb temperature is the lowest temperature that air, or a wet object, can cool down to when water evaporates into it. [1] It measures heat and humidity together, so it shows how well the human body can cool itself by sweating. A wet-bulb temperature of about 35°C held for about 6 hours was proposed as the upper limit of human survival in the open. [1]

Explanation

How it is measured

• The wet-bulb thermometer: an ordinary thermometer whose bulb is wrapped in a wet cloth.
• Water evaporates from the cloth.
• Evaporation takes heat away, so the bulb cools.

• The reading where it stops falling is the wet-bulb temperature.

• The dry-bulb thermometer: an ordinary thermometer kept dry next to it. It shows the normal air temperature.

• Together the two make a hygrometer (wet- and dry-bulb thermometer). This is the instrument used to measure humidity (water vapour in the air), one of the elements of weather.
• The gap between the two readings is called the wet-bulb depression. A bigger gap means drier air.

Why humidity changes the reading

• Dry air → much evaporation → large cooling.

• The wet-bulb reading is much lower than the air temperature.

• Humid air → little evaporation → little cooling.

• The wet-bulb reading is close to the air temperature.

• Saturated air (100% relative humidity) → no evaporation.

• The wet-bulb and dry-bulb readings are the same.

• So the wet-bulb temperature can never be higher than the dry-bulb (air) temperature.

Link to the human body: the survival limit

• The body cools itself the same way the wet bulb does: sweat evaporates from the skin and carries heat away.
• About 35°C wet-bulb for about 6 hours was proposed as the upper limit of human survival in the open (Sherwood and Huber, 2010). [1]
• The air is so hot and humid that sweat cannot evaporate.
• The body cannot lose heat, so its core temperature keeps rising.

• The result is fatal hyperthermia (dangerous overheating of the body).

• Lower real-world limit: later lab tests on people (Vecellio et al., 2022) put the critical limit at about 31°C wet-bulb, especially in humid heat.

• Key point: humid heat at a moderate air temperature can be more dangerous than dry heat at a higher air temperature. A thermometer reading alone hides this danger.

Where it occurs

• Already recorded above 35°C: since 2005, wet-bulb temperatures above 35°C have been recorded nine times. They were in subtropical places such as Pakistan and the Persian Gulf. [1]
• Rising trend: readings of about 32–35°C (90–95°F) more than tripled over the 40 years studied. [1]
• Projected hotspots by 2050: South Asia, the Persian Gulf and the Red Sea coasts. [1]
• Projected hotspots by 2070: eastern China, parts of South-east Asia and Brazil. [1]
• At-risk zones in India:
• Humid coasts: Odisha, Andhra Pradesh, Tamil Nadu, and the Gujarat and Konkan coasts. Sea air keeps humidity high here.

• Indo-Gangetic plain: pre-monsoon heat meets high humidity here.

• Use in India: the India Meteorological Department (IMD) reads wet- and dry-bulb thermometers at its weather stations to measure humidity. In 2023 IMD began issuing an experimental Heat Index for the whole country, which adds humidity to heat warnings. [2]

Don't confuse with

• Dry-bulb temperature: the normal air temperature shown by an ordinary thermometer. It ignores humidity. Wet-bulb temperature includes humidity and is always equal to or lower than the dry-bulb temperature.
• Heat Index (apparent or "feels-like" temperature): how hot the body feels once humidity is added to air temperature. [2] It is usually higher than the air temperature. Wet-bulb temperature is a physical reading that is usually lower than the air temperature. IMD's experimental Heat Index (2023) uses a heat-index equation like the one used by the US National Weather Service (NOAA). [2]
• Dew point: the temperature to which air must be cooled, with no water added, before its water vapour starts to condense. Wet-bulb temperature is cooling done by adding water through evaporation. Normally, dew point ≤ wet-bulb ≤ dry-bulb.
• Relative humidity: a percentage that shows how close the air is to being full of water vapour. It says nothing about heat by itself. Wet-bulb temperature combines heat and humidity in one number.

Prelims Hooks

• Wet-bulb temperature measures heat and humidity together. In humid air the wet-bulb reading is close to the air temperature. In dry air it is much lower. [1]
• Instrument match: humidity is measured with a hygrometer (wet- and dry-bulb thermometer). It is not measured with a barometer or an anemometer.
• Survival limit: about 35°C wet-bulb for 6 hours in theory (Sherwood and Huber, 2010). About 31°C in practice (Vecellio et al., 2022). [1]
• Already crossed: wet-bulb temperatures above 35°C have been recorded nine times since 2005, in Pakistan and the Persian Gulf. [1]
• Projection order: 2050 – South Asia, Persian Gulf, Red Sea coasts → 2070 – eastern China, parts of South-east Asia, Brazil. [1]
• Trap: IMD's experimental Heat Index (2023) is a "feels-like" temperature, not a wet-bulb temperature. Its colour codes are Green below 35°C, Yellow 36–45, Orange 46–55, Red above 55. [2]

Mains Points

• Humid heat is a growing disaster risk (GS-III):
• On India's humid coasts and the Indo-Gangetic plain, high humidity stops sweat from evaporating. This causes heat stress even when air temperatures are moderate.
• South Asia is expected to cross survivable wet-bulb levels by 2050. [1]

• The most exposed people are outdoor workers, the elderly and people living in crowded cities.

• Why dry-bulb warnings are not enough (GS-I/III):

• Heat warnings based only on air temperature can miss deadly humid heat.

• IMD's experimental Heat Index (2023) is a first step because it adds humidity to warnings. [2]

• Policy response: city heat action plans, cooling shelters, changed work hours for outdoor workers, and early-warning systems linked to wet-bulb thresholds such as the 31°C practical limit and the 35°C survival limit.

Related concepts

• Weather
• Climate
• Weather and climate
• Elements of weather

Read more

• Weather, climate and the elements of weather