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What are wet bulb temperatures? UK prepares for hottest June
Wet bulb temperatures offer valuable insights into atmospheric moisture and human comfort levels.
It plays a vital role in climate monitoring and helps explain the impact of humidity on daily life.
One of the most traditional and widely used measures of humidity is the wet bulb temperature.
The Met Office say: “This value provides insight into how saturated the air is and is essential for calculating other humidity-related variables such as relative humidity and dew point temperature.”
What is a wet bulb temperature?
Measured using a thermometer wrapped in a damp muslin wick, wet bulb temperature represents the lowest temperature achievable through evaporative cooling.
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The reading drops as water evaporates from the wick, drawing heat from the thermometer. This cooling continues until the air becomes saturated and can absorb no more moisture.
On foggy mornings or in fully saturated air, wet bulb temperature matches the dry bulb temperature—the actual air temperature.
In all other conditions, the wet bulb reading is lower.
The underlying principle is simple: evaporation requires energy.
Water vapour draws heat from the surrounding air and thermometer, leading to a cooler reading.
The larger the gap between dry bulb and wet bulb temperatures, the drier the air.
When there is no difference, the air is fully saturated and evaporation stops.
Traditionally, wet bulb temperature is measured using a psychrometer—an instrument with paired dry and wet bulb thermometers.
However, most modern weather stations no longer use this classic setup.
“Instead, they rely on two separate thermometers housed in a Stevenson screen: one dry bulb (measuring air temperature) and one wet bulb (wrapped in a moist wick connected to a water reservoir).”
Air moves over both thermometers, allowing evaporation from the wet bulb and resulting in evaporative cooling.
The dry bulb remains unchanged.
The temperature difference helps meteorologists calculate relative humidity and other important humidity metrics.
Modern technology has introduced capacitive sensors to the process.
These sensors measure humidity by detecting voltage changes caused by varying amounts of water vapour in the air.
Readings are then converted into relative humidity or dew point values, providing an automated and more precise method of measurement.
Wet bulb temperature is closely tied to other indicators, such as dew point temperature—the point at which air becomes saturated and moisture begins to condense.
On cool mornings, this is seen as dew forming on grass.
If the dew point or wet bulb temperature equals the air temperature, relative humidity is 100 per cent.
Both values are critical for understanding atmospheric moisture.
They enable meteorologists to assess the likelihood of rain, fog, or heat stress for people and animals.
The Met Office added: “Wet bulb temperature remains a fundamental tool in meteorology for assessing humidity and its effects on weather, climate, and human health.
“Whether measured using traditional psychrometers or advanced sensors, it provides valuable insights into atmospheric conditions.
“By understanding and monitoring wet bulb temperatures, the Met Office continues to deliver accurate forecasts and support public safety across a wide range of sectors.”
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