Wet Bulb Temperature and Psychrometers Explained

The wet bulb temperature is one of those measurements that most people have heard of but fewer have actually taken. It appears on weather reports as part of the heat index calculation, on HVAC specifications, in sports science as a measure of heat stress risk, and on laboratory instrument panels. Understanding what it means — and knowing how to measure it — gives you access to information that a standard thermometer and hygrometer alone cannot provide.

A psychrometer is the instrument used to measure both the wet bulb and dry bulb temperature simultaneously. It is also one of the most accurate and reliable methods for measuring relative humidity, which is why it is used as a reference instrument for calibrating other humidity sensors. This guide explains how psychrometers work, what wet bulb temperature tells you, and who actually needs to measure it.

What is wet bulb temperature?

The wet bulb temperature is the lowest temperature that can be achieved by evaporative cooling. It is measured by a thermometer whose bulb is covered by a wet wick — when air passes over the wet wick, water evaporates from it, cooling the thermometer. The rate of evaporation depends on how much water the air can still accept (how far below saturation the air currently is), so the wet bulb temperature drops more in dry air than in humid air.

The dry bulb temperature is simply the standard air temperature measured by a thermometer with no wick — the temperature you would read from any ordinary thermometer. The difference between the wet bulb and dry bulb temperatures is called the wet bulb depression, and this value is used in charts and formulas to calculate relative humidity.

At 100% relative humidity, wet bulb and dry bulb temperatures are identical — the air is already saturated and no evaporation can take place from the wick. As humidity drops, the wet bulb temperature falls further below the dry bulb temperature, and the depression increases.

Why wet bulb temperature matters

Heat stress and outdoor safety

The wet bulb temperature is one of the most important metrics for assessing heat stress risk in outdoor environments. A wet bulb temperature of 95°F (35°C) — which can occur at dry bulb temperatures of 100°F (38°C) or above with moderate humidity — is considered the physiological limit for human survival, because at this point the body can no longer cool itself through sweating. Sports scientists, military trainers, and industrial health and safety managers use wet bulb temperature monitoring to set limits on outdoor activity.

HVAC and building services

Wet bulb temperature is a fundamental calculation in air conditioning and refrigeration system design. Cooling coils remove moisture from the air by cooling below the dew point, and the wet bulb temperature is used in specifying coil performance, calculating loads, and verifying system operation. HVAC engineers take wet bulb measurements regularly during commissioning and maintenance of air handling systems.

Reference humidity measurement

A well-maintained psychrometer is considered one of the most reliable methods for measuring relative humidity precisely. Unlike electronic sensors which drift over time and require calibration against a reference, a psychrometer's accuracy depends primarily on the accuracy of its thermometers and the quality of the wick and airflow — physical parameters that do not drift in the same way as an electronic sensor. For this reason, psychrometers are used as reference instruments for calibrating other humidity sensors.

Agriculture and greenhouse management

Wet bulb temperature is used in evapotranspiration calculations — estimates of how much water plants lose to the atmosphere through evaporation from soil and transpiration from leaves. These calculations feed irrigation scheduling systems and are fundamental to water management in agriculture.

Types of psychrometers and wet bulb meters

How to use a sling psychrometer

A sling psychrometer consists of two thermometers mounted on a frame that can be whirled through the air. One thermometer has a dry bulb; the other has its bulb covered by a wet muslin wick. To take a reading, the wick is moistened with clean water and the instrument is whirled in the air at arm's length for approximately one minute. The wet bulb thermometer cools as water evaporates from the wick; once both readings have stabilised, the temperatures are read and the difference is used with a psychrometric chart or formula to calculate relative humidity.

The accuracy of a sling psychrometer depends heavily on technique: insufficient whirling produces too little airflow and an inaccurate wet bulb reading; using water other than clean distilled or de-ionised water can affect the wick's evaporative properties. For a trained user with a well-maintained instrument, a sling psychrometer gives results that match or exceed electronic hygrometers in accuracy.

Digital wet bulb meters

Modern digital wet bulb meters and handheld weather instruments — including the Kestrel range stocked by Weather Scientific — calculate the wet bulb temperature automatically from electronic temperature and humidity sensor readings using the psychrometric formulas. This approach is less accurate than a well-used sling psychrometer for precision work, but far more convenient for field use, sports monitoring, or HVAC commissioning where a quick reliable reading is needed.

The Kestrel 5000 and above models include wet bulb temperature as a calculated output alongside measured temperature, humidity, and other weather parameters. For anyone who needs wet bulb data in the field without the technique requirements of a sling psychrometer, a Kestrel meter is the practical modern equivalent.

Pricing guide

Frequently asked questions

What is a wet bulb meter?

A wet bulb meter is an instrument that measures or calculates the wet bulb temperature — the temperature a thermometer reaches when its bulb is covered with a wet wick and air flows over it. The wet bulb temperature is used to calculate relative humidity using psychrometric formulas and is an important metric for assessing heat stress risk, HVAC system performance, and evapotranspiration in agriculture.

How do I calculate relative humidity from wet and dry bulb temperatures?

The calculation uses a psychrometric chart or formula. The simplest approach is to look up the dry bulb temperature and wet bulb depression (dry bulb minus wet bulb) in a psychrometric table, which gives the relative humidity directly. Online psychrometric calculators make this even simpler — enter the two temperatures and the calculator returns the relative humidity, dew point, and other derived values. Alternatively, digital wet bulb meters and Kestrel weather instruments perform this calculation automatically.

What is the difference between wet bulb temperature and dew point?

Both wet bulb temperature and dew point are measures related to atmospheric moisture, but they are not the same thing. The dew point is the temperature at which the air becomes saturated — when the air cools to this temperature, condensation forms on surfaces. The wet bulb temperature is always between the dry bulb temperature and the dew point. In saturated air (100% RH), all three are equal. In dry air, the dew point can be far below the wet bulb temperature. For heat stress assessment, wet bulb temperature is the more directly relevant measure; for condensation risk assessment, dew point is more useful.

What is a dangerous wet bulb temperature for humans?

Research suggests that a wet bulb temperature of approximately 95°F (35°C) represents a physiological limit for human survival at rest, because at this point sweating can no longer cool the body. Sustained activity in conditions above 82–86°F (28–30°C) wet bulb temperature carries significant heat stress risk. Sports medicine bodies and military health standards typically set activity limits at wet bulb temperatures well below the survival threshold — many recommend reducing or stopping outdoor activity above 82°F (28°C) wet bulb temperature during sustained exertion.

What is a psychrometer used for?

A psychrometer is used to measure relative humidity accurately by comparing the readings of a wet bulb and dry bulb thermometer. It is used as a reference instrument for calibrating other humidity sensors, for HVAC commissioning and maintenance, for meteorological observation, and for heat stress assessment in occupational health and sports science. A well-maintained psychrometer remains one of the most accurate methods for measuring atmospheric humidity despite being a centuries-old technology.

Shop wet bulb meters and psychrometers at Weather Scientific

For digital wet bulb meters, Kestrel handheld weather instruments, and professional psychrometers, browse our Kestrel weather meters collection and our environmental meters range. For a complete introduction to humidity measurement, see our complete hygrometer buyer's guide.

Bob Batemen

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Bob Batemen is a dedicated contributor to WeatherScientific.com, bringing a wealth of expertise in weather management and environmental science. Bob combines a deep understanding of environmental systems with practical experience in weather forecasting, climate patterns, and the implementation of sustainable weather-related solutions. Over the years, Bob has developed a keen interest in how climate change impacts global weather patterns, disaster risk management, and the mitigation of extreme weather events.

Bob's professional experience spans both private and public sectors, where they have contributed to the development of weather-sensitive infrastructure, environmental policy, and climate adaptation plans.

As a contributor to WeatherScientific.com, Bob shares insightful articles, guides, and analyses on emerging weather trends, cutting-edge weather technologies, and their environmental implications. Their passion for blending science with practical applications continues to shape their work, providing readers with valuable, informed perspectives on the ever-evolving world of weather and environmental management.