Wet bulb temperature is the lowest temperature air can cool to just by evaporating water into it, under the current humidity and pressure.

What is wet bulb temperature?

In simple terms, imagine wrapping a regular thermometer in a wet cloth and letting air blow over it: as the water evaporates, it cools the thermometer until evaporation can’t cool it any further.

The final, stabilized reading is the wet bulb temperature, which is always less than or equal to the normal air (dry bulb) temperature.

Key points:

  • It is the lowest temperature reachable by evaporation alone in current conditions.
  • When air is very dry, evaporation is strong, so wet bulb temperature is much lower than air temperature.
  • When air is saturated (100% relative humidity), wet bulb equals the actual air temperature because no more evaporation can occur.

A classic mental picture:

You’re in a hot desert at 40°C. If you wrap a thermometer bulb in a wet cloth, it might cool to about 25–28°C. That lower value is the wet bulb temperature because evaporation is very efficient in dry air.

How is it measured in practice?

Wet bulb temperature is measured with a wet‑bulb thermometer , often paired with a normal (dry‑bulb) thermometer in a device called a sling psychrometer.

Typical setup:

  • One thermometer is dry: this shows the normal air (dry‑bulb) temperature.
  • The other has its bulb wrapped in a wet cloth or “sock”, kept moist with distilled water.
  • Air is passed over both (for example by slinging the device through the air or using a fan) until the wet thermometer reading stops falling.

Once both readings are stable, the difference between dry and wet bulb temperatures can be used, with a psychrometric chart or formulas, to calculate relative humidity and other air‑moisture properties.

Why does wet bulb temperature matter?

Even though it sounds like a niche meteorology term, it’s now a trending topic because of heatwaves, climate change, and human health.

1. Heat stress and human survivability

Wet bulb temperature is a direct measure of how well sweat can cool the human body.

  • When wet bulb temperatures approach the mid‑30s °C (around 35°C), the body can no longer effectively cool itself by sweating, even in the shade with plenty of water.
  • Research has highlighted that very high wet bulb events are becoming more likely in a warming climate, raising concern for outdoor workers, athletes, and vulnerable populations.

This is why you may see headlines or forum posts discussing “dangerous wet‑bulb temperatures” during extreme heat episodes.

2. Weather, forecasting, and humidity

Meteorologists use wet bulb temperature to:

  • Infer relative humidity and dew point when you have wet and dry bulb readings.
  • Help forecast fog, cloud formation, and precipitation because it ties closely to how close air is to saturation.

3. Engineering and cooling systems

In engineering, wet bulb temperature is a key design variable:

  • Cooling towers and evaporative coolers are fundamentally limited by the local wet bulb temperature; they cannot cool water or air below that limit.
  • Designers use it to estimate how effective evaporative cooling will be in a particular climate.

Relation to other temperature terms

To keep the vocabulary straight, here’s how the main terms relate:

  • Dry bulb temperature :
    Normal air temperature, measured by a regular thermometer shielded from direct sun.
  • Wet bulb temperature :
    The cooled temperature from evaporation at the current humidity.
  • Dew point temperature :
    The temperature to which air must be cooled for water vapor to start condensing (100% humidity) without changing pressure or moisture content.

For unsaturated air (less than 100% humidity), the relationship is:

  • Dew point ≤ wet bulb ≤ dry bulb, with all three equal only when the air is fully saturated.

Forum and “trending topic” angle

On forums and social media, “wet bulb temperature” often comes up in these contexts:

  • Discussions about whether certain regions might become unlivably hot and humid under climate change, especially along coasts and in the tropics.
  • Questions in “explain like I’m five” threads asking why a “wet bulb” gives a different temperature and what it means for safety during heatwaves.
  • Practical questions from runners, outdoor workers, and coaches about how to adjust training or work schedules when wet bulb (or related indices like WBGT) crosses certain thresholds.

One common takeaway in these discussions is:

It’s not just how hot it is, it’s how hot and humid it is — and wet bulb temperature compresses those two into one number that tells you how hard it is for your body (or a cooling system) to get rid of heat.

Quick bullet recap

  • Wet bulb temperature = lowest temperature reachable by evaporation of water in current air conditions.
  • Measured with a wet‑bulb thermometer (often part of a sling psychrometer) alongside a dry‑bulb thermometer.
  • Always ≤ dry bulb temperature, equal only at 100% relative humidity.
  • Crucial for understanding heat stress on humans, especially in a warming climate.
  • Important in meteorology and the design of cooling towers and other evaporative cooling systems.

TL;DR: Wet bulb temperature is the “evaporation‑limited” temperature of the air, telling you how far evaporation can cool things down and how dangerous heat and humidity really are for people and cooling systems.

Information gathered from public forums or data available on the internet and portrayed here.