# Wet Bulb Calculator

Calculate wet-bulb temperature from air temperature and humidity. Assess heat stress risk and understand human survivability limits in extreme heat.

## What this calculates

Wet-bulb temperature is the lowest temperature air can reach through evaporative cooling alone. It combines heat and humidity into a single number that matters for human survival: at a wet-bulb of 35°C (95°F), the human body can no longer cool itself through sweat, and prolonged exposure is fatal even for healthy people in the shade.

## Inputs

- **Dry-Bulb Temperature** (°F) — min -40, max 140 — Standard air temperature (what your thermometer reads).
- **Relative Humidity** (%) — min 0, max 100 — Relative humidity as a percentage.

## Outputs

- **Wet-Bulb Temperature** (°F) — Wet-bulb temperature in Fahrenheit.
- **Wet-Bulb Temperature** (°C) — Wet-bulb temperature in Celsius.
- **Heat Stress Risk** — formatted as text — Risk level based on wet-bulb temperature.
- **Dew Point** (°F) — Dew point temperature for reference.

## Details

**How wet-bulb temperature works:**

Imagine wrapping a wet cloth around a thermometer and fanning it. The evaporating water cools the thermometer below the air temperature. How much it cools depends on the humidity. In dry air, lots of evaporation happens and the wet-bulb drops well below the dry-bulb. In humid air, evaporation slows down and the two temperatures converge.

This calculator uses the **Stull (2011) approximation**, which is accurate to within 0.3°C for typical conditions.

**Risk levels by wet-bulb temperature:**
- **Below 25°C (77°F):** Low risk, normal activity
- **25-28°C (77-82°F):** Moderate, take regular breaks
- **28-31°C (82-88°F):** High risk, limit exertion
- **31-35°C (88-95°F):** Very high, dangerous for outdoor work
- **Above 35°C (95°F):** Human survivability limit

**The 35°C threshold:**

At a wet-bulb of 35°C, even a perfectly healthy person sitting naked in the shade with unlimited water cannot maintain a stable core temperature. The body produces about 80W of metabolic heat at rest, and when the air's wet-bulb reaches skin temperature (about 35°C), sweat simply cannot evaporate fast enough to dissipate it.

**Real-world context:**

Wet-bulb temperatures above 35°C have historically been extremely rare, but they are becoming more frequent with climate change. Parts of the Persian Gulf, South Asia, and the Mississippi River valley have already recorded brief exceedances.

## Frequently Asked Questions

**Q: What is wet-bulb temperature?**

A: Wet-bulb temperature is the lowest temperature that air can be cooled to by evaporating water into it at constant pressure. It combines the effects of heat and humidity into one number. At 100% humidity, the wet-bulb equals the dry-bulb (air) temperature. In drier air, the wet-bulb is lower.

**Q: Why is 35°C wet-bulb the survivability limit?**

A: At 35°C wet-bulb, the air is too warm and humid for sweat to evaporate from skin. Since the human body constantly generates heat through metabolism, it cannot cool itself and core temperature rises uncontrollably. Prolonged exposure (roughly 6 hours) is fatal regardless of fitness, shade, or water intake.

**Q: How is wet-bulb different from heat index?**

A: Heat index tells you what the temperature "feels like" accounting for humidity. Wet-bulb is a physical measurement of how much evaporative cooling is possible. Wet-bulb is more useful for assessing fundamental limits of human thermoregulation, while heat index is oriented toward comfort and general heat safety guidelines.

**Q: What is a wet-bulb globe temperature (WBGT)?**

A: WBGT is a composite index that adds solar radiation and wind to the wet-bulb measurement. It is the standard used by OSHA and the military for setting work-rest guidelines. WBGT requires a specialized instrument with a black globe thermometer, a natural wet-bulb, and a dry-bulb sensor.

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Source: https://vastcalc.com/calculators/physics/wet-bulb
Category: Physics
Last updated: 2026-04-08