Wet-Bulb Temperature

The lowest temperature air can reach via evaporative cooling. A wet-bulb above 35°C is fatal — and South Asian coasts are approaching it.

Measurement

What is Wet-Bulb Temperature?

The wet-bulb temperature (WBT) is the lowest temperature you can reach by evaporative cooling — wrapping a thermometer in a wet cloth and exposing it to moving air. The water on the cloth evaporates, drawing heat from the thermometer and lowering its reading. The temperature at which it stabilizes is the wet-bulb temperature.

This is more than a curious physics measurement. The wet-bulb temperature represents the physical limit of how cool the human body can keep itself through sweating — because sweat acts as that wet cloth around your skin.

If the air’s wet-bulb temperature equals or exceeds your skin temperature (about 35°C), sweating cannot cool you further. Your body’s metabolic heat has nowhere to go. Within hours, even healthy adults face heat stroke and death.

For South Asia, the wet-bulb threshold is no longer hypothetical. Several locations have already recorded wet-bulb readings near 33°C — within 2°C of the survivability limit. Climate models project wet-bulb extremes approaching 35°C over most of South Asia, and exceeding it in a few locations, by the late 21st century under high emissions.

How wet-bulb is measured

There are two related but distinct concepts:

Wet-bulb temperature (WBT) — Physical measurement using a wet-cloth-wrapped thermometer. Modern automatic stations calculate it from dry-bulb temperature, humidity and pressure using the Stull or Davies-Jones equations.
Wet-bulb globe temperature (WBGT) — A combined index used by sports medicine and military safety. Combines wet-bulb (humidity), globe (radiation/sun), and dry-bulb (temperature) into one number for outdoor safety thresholds.

For climate science and human survivability research, the simple WBT is used. For occupational safety (industrial, military, sports), WBGT is the standard.

Why 35°C wet-bulb is fatal

Human core body temperature is normally 37°C. Skin temperature is about 35°C (a couple of degrees cooler). The body must continuously shed metabolic heat (about 100 watts at rest, 500-1000 watts during exertion) through three mechanisms:

Radiation — only works if surroundings are cooler than skin.
Convection — heat carried away by moving air. Doesn’t help if air is at body temperature.
Evaporation — sweat absorbs ~580 calories per gram as it evaporates. This is the most important cooling mechanism in hot conditions.

When wet-bulb temperature reaches 35°C:

Air’s evaporative cooling capacity is exhausted.
Sweat can still leave skin but doesn’t carry heat away.
Skin temperature begins to rise.
Core temperature follows within hours.
Heat stroke, organ failure and death follow within a few hours.

This isn’t a survival issue for the elderly or sick — it’s the theoretical survival limit even for healthy adults at rest; laboratory tests on young, healthy adults found the practical limit to be lower. During exertion, even lower wet-bulb temperatures become lethal.

Wet-bulb measurements in South Asia

Several locations have recorded wet-bulb readings approaching the survivability limit:

Jacobabad, Pakistan (Sindh, interior plain):

Wet-bulb above 35°C briefly recorded.
Combined with 50°C+ air temperatures during multi-day heatwaves.

Ras al Khaimah, UAE (Persian Gulf coast):

Similar wet-bulb 35°C events recorded.
Coastal cities of the Persian Gulf are global hotspots for extreme wet-bulb.

Indian coastal cities:

Vizag, Chennai, Mumbai: High wet-bulb events during May-June.
Coastal Tamil Nadu and Andhra Pradesh: Pre-monsoon heat + ocean humidity → one of the hotspots for wet-bulb readings above 31°C.

Bangladesh:

Sundarbans and southern coastal districts: High wet-bulb readings during heatwaves combined with high humidity.

Pakistan’s hot triangle:

Jacobabad, Sukkur, Larkana, Sibi: Some of the highest wet-bulb readings in South Asia, most often around monsoon onset.

Climate change and wet-bulb futures

Climate models project significant increases in wet-bulb extremes:

Under RCP 8.5 (high emissions) by 2070-2090:

Persian Gulf coast could experience wet-bulb 35°C events multiple times per year.
Indo-Gangetic plain (Lucknow, Patna, Dhaka) could see wet-bulb 32-34°C regularly.
Coastal South Asia could see wet-bulb 33-34°C events lasting days.

Under RCP 4.5 (moderate emissions) by the late 21st century:

Lower wet-bulb peaks than RCP 8.5, but still dangerous for outdoor workers.

Under RCP 2.6 (Paris-aligned):

Wet-bulb extremes still increase.

The implications for outdoor workers — construction, agriculture, street vendors — in South Asia are profound. Hundreds of millions of livelihoods depend on outdoor labour. Without rapid emissions cuts, large parts of the region may become functionally uninhabitable during summer months.

Wet-bulb vs heat index — which to use?

For practical day-to-day awareness, heat index / feels-like temperature is more useful. It gives a number similar to the air temperature scale (e.g., 45°C heat index means “feels like 45°C”), so it’s easy to communicate.

For scientific and policy work, wet-bulb temperature is the standard. It’s physical, not empirical, and the 35°C threshold is rigorously defined.

Approximate correspondence:

Heat index 41°C ≈ wet-bulb 29°C — “Danger”
Heat index 54°C ≈ wet-bulb 31-32°C — “Extreme Danger”
Wet-bulb 35°C — off the heat-index chart; survivability limit

Protecting against extreme wet-bulb

When wet-bulb exceeds 30°C, even healthy adults should:

Avoid outdoor work between 11 AM and 4 PM.
Seek air-conditioned shelter — fan-only cooling fails at high wet-bulb.
Hydrate aggressively — sweat loss continues even without cooling effect.
Avoid hot beverages and alcohol — both worsen heat stress.
Recognize early symptoms — confusion, dizziness, cessation of sweating — and act immediately.

Cities can mitigate through:

Cool roofs and shade structures
Mandatory rest breaks for outdoor workers during heat events.
Cooling centres open to anyone without home AC.
Modified working hours (dawn / dusk shifts).
Long-term mitigation through tree planting and urban heat island reduction.

Frequently asked questions

What is the wet-bulb 35°C threshold? The maximum wet-bulb temperature at which a healthy adult human can theoretically survive in shade with adequate water. Above this, the body cannot dissipate metabolic heat — leading to heat stroke and death within hours regardless of fitness or hydration.

Has 35°C wet-bulb been recorded anywhere? Briefly, yes — Jacobabad (Pakistan) and parts of the Persian Gulf have recorded readings above 35°C. Most populated areas haven’t yet experienced sustained 35°C wet-bulb, but climate projections suggest this will change.

How is wet-bulb different from heat index? Wet-bulb is a physical measurement. Heat index is a regression formula calibrated to human comfort. Wet-bulb has a clear 35°C survivability threshold; heat index doesn’t have a single threshold (different US NWS categories).

Why is humid heat more dangerous than dry heat? Because sweating only cools you when sweat evaporates. Humid air absorbs little additional moisture, so sweat evaporates slowly and cools little. Dry 50°C air is dangerous due to dehydration but allows sweating to cool the body; humid 35°C air can be more lethal.

Where can I see live wet-bulb for my city? Mausam Online displays feels-like temperature (closely related) on every city page. For wet-bulb specifically, scientific datasets like ERA5 reanalysis are needed. See Chennai, Vizag, Karachi, Dhaka, Mumbai.

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