Guide

Dry bulb, wet bulb and globe temperature

What is the difference between dry bulb and wet bulb temperature?

Dry bulb is an ordinary shaded thermometer — the air temperature. Wet bulb is the same thermometer with a wet wick, so evaporation cools it, and it always reads lower unless the air is saturated. Globe temperature is a thermometer inside a matt black sphere, which responds to radiant heat and in sun reads far higher than both.

Three instruments, three questions

The three thermometers, and what each one is sensitive to.
Thermometer	Setup	Sensitive to	On a hot sunny day
Dry bulb	Shaded, ventilated, dry	Air temperature only	32°C
Wet bulb	Shaded, ventilated, wet wick	Air temperature and humidity	26°C
Black globe	Centre of a 150 mm matt black sphere, in the open	Radiant heat, plus wind	52°C

Those example figures are what this site's WBGT estimator produces at 32°C, 60% humidity, light wind and a high summer sun. The spread is the point: a single "temperature" cannot describe a situation in which one instrument reads 26 and another reads 52.

How they relate

The wet bulb and dry bulb together determine the humidity — that is what a psychrometer does, and it is why two thermometers were enough to measure humidity for two centuries.

The globe is independent. It responds to radiation, which the other two are shielded from by design. That is exactly why WBGT needs all three: humidity from the first pair, radiant load from the third.

Worked example: the same air, in sun and in shade

Dry bulb: 32°C in both cases — the air is the air
Wet bulb, shaded: 26°C in both cases
Black globe, in sun: 52°C
Black globe, in shade: Close to 32°C

4°C The difference the globe makes to WBGT between sun and shade, from a weighting of only 0.2. Which is why standards use WBGT and not the heat index.

The half of this subject that is not about people

Search for "dry bulb temperature" and much of what comes back is HVAC engineering: psychrometric charts, enthalpy, sensible and latent loads, coil sizing. That field uses the same two thermometers for a completely different purpose — designing equipment rather than protecting people — and its calculators convert between dry bulb, wet bulb and relative humidity as an end in itself.

This site does not cover that, for a simple reason: a calculator whose output is relative humidity is a humidity tool, not a heat-stress tool, and mixing the two would produce pages that answer neither question well.

What is here:

Wet bulb from temperature and humidity, because the output is a wet bulb.
Estimated WBGT, because the output is a heat-stress index.
What wet bulb temperature is and WBGT explained for the physical picture.

Reading a standard that mentions all three

Two traps:

"Wet bulb" in a WBGT context means the natural wet bulb — out in the open, exposed to sun and ambient wind — not the shaded, ventilated psychrometric one. It reads higher. This is the single most common misreading of ISO 7243.
The weights change indoors. Outdoors WBGT is 0.7 natural wet bulb + 0.2 globe + 0.1 dry bulb. Indoors or in full shade it is 0.7 natural wet bulb + 0.3 globe, with no dry bulb term at all.

Common questions

Why is the wet bulb always lower than the dry bulb?

Because evaporation only takes heat away, never adds it. Water leaving the wet wick carries latent heat with it, cooling the bulb below the surrounding air. The gap closes as humidity rises and disappears entirely at 100% relative humidity, where nothing can evaporate.

What is the difference between wet bulb and dew point?

Dew point is the temperature at which the air would saturate if cooled without adding or removing water. Wet bulb is the temperature reached by evaporating water into the air. The wet bulb always sits between the dew point and the dry bulb, and the three converge at saturation.

Why is the globe black and 150 mm across?

Matt black absorbs nearly all incoming radiation, so the sphere's temperature tracks radiant load rather than air temperature. ISO 7243 specifies 150 mm because at that diameter the sphere's thermal response time is comparable to a human body's — tens of minutes — so it does not react to every passing cloud.

Is this the same as psychrometrics in HVAC?

The measurements are the same; the purpose is not. HVAC uses dry bulb and wet bulb to find enthalpy and size cooling coils, working from a psychrometric chart. Heat safety uses them to work out whether people can shed heat. A great deal of the dry-bulb material online is the HVAC version, which will not answer a heat-safety question.

Can I measure wet bulb at home?

Roughly, yes. Wrap the bulb of a thermometer in damp cotton and either whirl it around or aim a fan at it, then read it once it stops falling. It is easy to get a couple of degrees wrong through insufficient airflow or a wick that has dried out, which is why calculating it from temperature and humidity is usually more reliable.

Sources

ISO 7243 — Assessment of heat stress using the WBGT index
Stull, R. (2011). Wet-Bulb Temperature from Relative Humidity and Air Temperature
Liljegren, J. C. et al. (2008). Modeling the Wet Bulb Globe Temperature Using Standard Meteorological Measurements

Published 30 July 2026