UPSC Prelims 2025 Q64: Wet-Bulb Temperature 35 °C — Explained
The World Bank warned that India could become one of the first places where wet-bulb temperatures routinely exceed 35 °C. Which of the following statements best reflect(s) the implication of the above-said report?
I. Peninsular India will most likely suffer from flooding, tropical cyclones and droughts.
II. The survival of animals including humans will be affected as shedding of their body heat through perspiration becomes difficult.
Select the correct answer using the code given below.
- I only
- II only
- Both I and II
- Neither I nor II
Answer: (c) Both I and II
Statement-wise explanation
- I: correct — UPSC's final key accepts this statement, so it reads the report's heat warning as one part of a wider climate-risk picture for India. That wider picture is warming that also brings heavier extreme rainfall, stronger cyclones and longer dry spells. Peninsular India is exposed on three fronts. It has a long coastline facing the Bay of Bengal and the Arabian Sea, which brings cyclones and storm-surge floods. Its interior Deccan lies in a rain shadow and is drought-prone. The World Bank's November 2022 cooling-sector report, the source of the wet-bulb warning, is mainly about heat. Its projection of wet-bulb temperatures above 35 °C is for a few places in northern India, so this statement is a broader inference rather than a direct quote.
- II: correct — Wet-bulb temperature combines heat and humidity. At a wet-bulb temperature of 35 °C, sweat can no longer evaporate fast enough to carry heat away from the body. Body temperature then keeps rising even in shade with water, and survival is at risk within hours. The World Bank report calls 35 °C the survivability threshold and notes that India's wet-bulb temperatures today rarely, if ever, go beyond 32 °C. Other mammals that cool by sweating or panting, such as cattle, face the same limit.
How to eliminate
Statement II follows directly from what wet-bulb temperature means, and you can confirm it from basic physiology: humid heat stops sweat from evaporating, so the body cannot shed heat. Almost every candidate should mark II as correct, which rules out the options for 'I only' and 'Neither I nor II'. The real choice is whether to add Statement I. It talks about floods, cyclones and droughts, none of which follow from wet-bulb physics, so many candidates dropped it. UPSC's key still accepted it because the question asks what 'best reflects the implication' of a climate warning for India, and warming increases all three hazards in peninsular India. Basic knowledge cannot fully settle this: Statement I is the weaker, debatable one. The pattern to remember is that when UPSC's question stem asks about 'implications' of a climate report, it tends to accept broad, plausible climate impacts and does not limit itself to the narrow mechanism. Fact to remember: wet-bulb temperature of 35 °C is the human survivability limit, and India today rarely goes above 32 °C (World Bank, November 2022).
Know more
Wet-bulb temperature is the lowest temperature a wet surface can reach through evaporation. A thermometer with its bulb wrapped in a wet cloth measures it. In dry air, water evaporates fast and the reading falls well below air (dry-bulb) temperature. In saturated air (100% relative humidity), wet-bulb and dry-bulb readings are equal.
Why 35 °C matters. Human skin stays at about 35 °C. When the wet-bulb temperature reaches that level, sweat stops cooling the body, and core temperature climbs toward heat stroke. Lab studies since 2022 suggest the real limit for healthy young adults may be lower, around 31 °C, and lower still for the elderly.
| Measure | What it captures | Use |
|---|---|---|
| Dry-bulb temperature | Air temperature only | IMD heat-wave criteria |
| Wet-bulb temperature | Heat + humidity (evaporative cooling limit) | Survivability threshold |
| Heat index | Heat + humidity as 'feels like' | Public advisories |
| Wet-bulb globe temperature | Adds sun and wind | Workplace and sports safety |
IMD heat wave (plains): declared when the maximum temperature is at least 40 °C and 4.5–6.4 °C above normal, or when it reaches 45 °C or more. 'Severe' means more than 6.4 °C above normal or 47 °C or more. Coastal stations use 37 °C as the base and hill stations 30 °C.
The World Bank report (November 2022), Climate Investment Opportunities in India's Cooling Sector:
- India could be among the first places where heat waves cross the survivability limit.
- 160–200 million Indians could face lethal heat waves each year by 2030.
- About 34 million jobs could be lost to heat-stress productivity decline.
- Cooling demand may rise eightfold by 2037.
- A greener cooling pathway could open a USD 1.6 trillion investment opportunity.
Policy link: The India Cooling Action Plan (MoEFCC, 2019) aims to cut cooling demand by 20–25% by 2037–38. Heat Action Plans are prepared in heat-prone States with NDMA and IMD support.
In the news
The World Bank released Climate Investment Opportunities in India's Cooling Sector in November 2022, after India's record-hot March and the severe heat waves of spring 2022. The report's line that India could be among the first places to cross the 35 °C wet-bulb survivability limit was widely reported. Heat waves in 2023–24 kept the topic in the news.
NCERT Concepts to Revise
- Wet-bulb temperature: The whole question rests on what a 35 °C wet-bulb temperature means for the body's ability to cool itself.
- Heat wave: Separates IMD's air-temperature-based heat wave from the humid-heat danger that wet-bulb temperature measures.