Wet bulb temperature

As water evaporates, it takes heat from the surrounding air, causing cooling. This cooling continues until the air becomes fully saturated with moisture, meaning it cannot hold any more water vapor. At this point, the temperature stabilizes—this is the wet bulb temperature.

Wet bulb temperature is important because it helps us understand how heat and humidity interact. It plays a critical role in human comfort and safety, weather forecasting, and various industries. Since it determines how effectively sweat can cool the human body, it is a key indicator of heat stress. High wet bulb temperatures can be dangerous or even deadly, particularly when they exceed 35°C (95°F), as the human body can no longer cool itself effectively.

Where is wet bulb temperature used?

Wet bulb temperature has applications across multiple fields, including:

• Weather and climate science: Meteorologists use it to assess humidity levels and how hot it actually feels outside.
• Heat stress and human safety: Wet bulb temperature is a key factor in determining whether conditions are safe for physical activity. If it gets too high, sweating is no longer effective, increasing the risk of heat exhaustion and heatstroke.
• Cooling systems and HVAC: Engineers design air conditioning and evaporative cooling systems based on wet bulb temperature, as it determines how much cooling can be achieved through evaporation.
• Agriculture: Farmers use it to protect crops and livestock from heat stress, as both plants and animals are sensitive to extreme heat and humidity.
• Aviation: Pilots consider wet bulb temperature when evaluating engine performance and the potential for ice formation on aircraft.

How is wet bulb temperature measured or estimated?

There are two main ways to determine wet bulb temperature: direct measurement and estimation based on air conditions.

• Direct measurement: A wet bulb thermometer consists of a standard thermometer with a wet cloth wrapped around its bulb. As water evaporates from the cloth, the thermometer cools down. The drier the air, the more water evaporates and the lower the temperature drops. If the air is already very humid, there is less evaporation and the temperature remains closer to the actual air temperature.
• Estimation: If a direct measurement isn’t available, wet bulb temperature can be estimated using air temperature, humidity, and atmospheric pressure. Since evaporation depends on how much moisture the air can hold, a higher humidity means the wet bulb temperature will be closer to the actual air temperature.

How wet bulb temperature affects how we feel heat and humidity

Wet bulb temperature plays a major role in how humans perceive heat and humidity. It helps explain why a hot, dry day feels different from a hot, humid one—even if the air temperature is the same.

The role of sweat in cooling the body

The human body regulates temperature by sweating. When sweat evaporates, it removes heat from the skin, cooling the body. The effectiveness of this cooling process depends on humidity:

• On a dry day, sweat evaporates quickly, helping the body stay cool.
• On a humid day, the air is already full of moisture, so sweat evaporates more slowly or not at all, making us feel much hotter.

Wet bulb temperature reflects this effect. It tells us the lowest temperature our skin can cool to through sweating. When the wet bulb temperature is close to the actual air temperature, it means little to no evaporative cooling is happening, making the heat feel much worse.

Why wet bulb temperature is a better measure of heat stress

Traditional temperature readings don’t always capture how dangerous heat can be. For example, 35°C (95°F) on a dry day might be uncomfortable but manageable, while 35°C (95°F) with high humidity can be deadly. This is because high humidity prevents the body from cooling effectively, increasing the risk of heat stress, heat exhaustion, and heatstroke.

Scientists and health experts use wet bulb temperature as a better indicator of how heat affects people. A wet bulb temperature above 35°C (95°F) is considered the limit for human survival—beyond this point, even a healthy person resting in the shade with unlimited water cannot cool down, leading to fatal overheating.

How wet bulb temperature applies to weather forecasts and safety

Weather services often use wet bulb temperature to issue heat warnings, especially in regions with extreme heat and humidity. It is also used to guide safety protocols for outdoor work, exercise, and events. For example:

• Workplaces like construction sites and farms use wet bulb temperature to determine safe working conditions.
• Sports organizations monitor it to prevent heat-related illnesses in athletes.
• Military and emergency response teams rely on it for training and disaster planning.

By understanding wet bulb temperature, we can better prepare for extreme heat and stay safe in high-humidity environments.