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Wet-bulb temperature

Wet-bulb temperature is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wet-bulb temperature rather than just read about it. In short: The wet-bulb temperature is the lowest temperature that can be reached by the evaporation of water under current ambient conditions. It combines humidity and temperature to better reflect heat stress on humans, as the body's ability to cool itself through sweating is reduced at higher humidity.

Wet-bulb temperature — main illustration
Wet-bulb temperature — illustration

Key takeaways

  • Wet-bulb temperature belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Wet-bulb temperature to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Wet-bulb temperature from memory before moving on to harder problems.

Reference excerpt

The wet-bulb temperature is the lowest temperature that can be reached by the evaporation of water under current ambient conditions. It combines humidity and temperature to better reflect heat stress on humans, as the body's ability to cool itself through sweating is reduced at higher humidity. At 100% relative humidity, wet-bulb temperature is equal to air temperature, or dry-bulb temperature. At lower humidity, the wet-bulb temperature is lower than dry-bulb temperature due to evaporative cooling. Wet-bulb temperature is formally defined as the temperature of a parcel of air cooled adiabatically to saturation (100% relative humidity) by the evaporation of water into it, with all latent heat supplied by the parcel. Wet-bulb temperature can be measured using a wet-bulb thermometer, such as in a psychrometer.

Principles If a thermometer's sensor (e.g. the bulb of a mercury thermometer) is wrapped in a water-moistened cloth, its temperature will depend on the humidity in addition to air temperature. The drier and less humid the air is, the faster the water will evaporate. The faster water evaporates, the lower the thermometer's temperature will be relative to air temperature. Water can evaporate only if the air around it can absorb more water. This is measured by comparing how much water is in the air to the maximum that could be in the air—the relative humidity. 0% means the air is completely devoid of water, and 100% means the air contains all the water it can hold in the present circumstances and it cannot absorb any more water (from any source). This is part of the cause of apparent temperature or 'feels like' temperature in humans. The drier the air, the more moisture it can take up beyond what is already in it, and the easier it is for extra water to evaporate. The result is that sweat evaporates more quickly in drier air, cooling down the skin faster. If the relative humidity is 100%, no water can evaporate, and cooling by sweating or evaporation is not possible.

When relative humidity is 100%, a wet-bulb thermometer can also no longer be cooled by evaporation, so it will read the same as an unwrapped thermometer. The wet-bulb temperature is thus effectively the lowest temperature that may be achieved by evaporative cooling of a water-wetted, ventilated surface. By contrast, the dew point is the temperature to which the ambient air must be cooled to reach 100% relative humidity assuming there is no further evaporation into the air; it is the temperature where condensation (dew) and clouds would form. For a parcel of air that is less than saturated (i.e., air with less than 100 percent relative humidity), the wet-bulb temperature is lower than the dry-bulb temperature, but higher than the dew point temperature. The lower the relative humidity (the drier the air), the greater the gaps between each pair of these three temperatures. Conversely, when the relative humidity rises to 100%, the three figures coincide. For air at a known pressure and dry-bulb temperature, the thermodynamic wet-bulb temperature corresponds to unique values of the relative humidity and the dew point temperature. It therefore may be used for the practical determination of these values. The relationships between these values are illustrated in a psychrometric chart. Lower wet-bulb temperatures that correspond with drier air in summer can translate to energy savings in air-conditioned buildings due to:

Reduced dehumidification load for ventilation air Increased efficiency of cooling towers Increased efficiency of evaporative coolers

Thermodynamic wet-bulb temperature The thermodynamic wet-bulb temperature is the temperature a volume of air would have if cooled adiabatically to saturation by evaporation of water into it, all latent heat being supplied by the volume of air. The temperature of an air sample that has passed over a large surface of liquid water in an insulated channel is the thermodynamic wet-bulb temperature—the air has become saturated by passing through a constant-pressure, ideal, adiabatic saturation chamber. Meteorologists and others may use the term "isobaric wet-bulb temperature" to refer to the "thermodynamic wet-bulb temperature". It is also called the "adiabatic saturation temperature", though meteorologists also use "adiabatic saturation temperature" to mean "temperature at the saturation level", i.e. the temperature the parcel would achieve if it expanded adiabatically until saturated. The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapor. The value indicated by a simple wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature. For an accurate wet-bulb thermometer, "the wet-bulb temperature and the adiabatic saturation temperature are approximately equal for air-water vapor mixtures at atmospheric temperature and pressure. This is not necessarily true at temperatures and pressures that deviate significantly from ordinary atmospheric conditions, or for other gas–vapor mixtures."

Temperature reading of wet-bulb thermometer

… excerpt ends here. Continue reading the full article.

Illustrations

Wet-bulb temperature: A sling psychrometer. The small fabric sock covering the bulb of the upper thermometer is wet with distilled water and whirled around for a minute or more before taking the "wet-bulb" thermometer's reading. The lower thermometer provides the "dry-bulb" temperature.
A sling psychrometer. The small fabric sock covering the bulb of the upper thermometer is wet with distilled water and whirled around for a minute or more before taking the "wet-bulb" thermometer's reading. The lower thermometer provides the "dry-bulb" temperature.
Wet-bulb temperature illustration
Wet-bulb temperature: A wet-and-dry hygrometer featuring a wet-bulb thermometer
A wet-and-dry hygrometer featuring a wet-bulb thermometer

Worked examples

Example 1 — a first encounter with Wet-bulb temperature

Start with the simplest possible case. Write down what Wet-bulb temperature claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wet-bulb temperature before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wet-bulb temperature ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wet-bulb temperature

In research
Wet-bulb temperature appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Wet-bulb temperature in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Wet-bulb temperature is common in secondary-school and first-year university syllabi. It links to neighbouring topics Atmospheric thermodynamics, Meteorological data and networks, Temperature, so understanding it makes those chapters shorter.
In everyday life
Look for Wet-bulb temperature outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Wet-bulb temperature in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Wet-bulb temperature means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Wet-bulb temperature out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wet-bulb temperature in simple terms?

The wet-bulb temperature is the lowest temperature that can be reached by the evaporation of water under current ambient conditions. It combines humidity and temperature to better reflect heat stress on humans, as the body's ability to cool itself through sweating is reduced at higher humidity.

Why does Wet-bulb temperature matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wet-bulb temperature?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wet-bulb temperature.

Tags

  • Atmospheric thermodynamics
  • Meteorological data and networks
  • Temperature

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