Humidity plays a vital role in weather forecasting, climate monitoring, and understanding human comfort levels.
One of the most traditional and widely used measures of humidity is the wet bulb temperature. 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.
Understanding Wet Bulb Temperature
Wet bulb temperature is the lowest temperature that can be achieved through evaporative cooling. It is measured using a thermometer with its bulb wrapped in a moist muslin wick. As air flows over the wet surface, water begins to evaporate, drawing heat from the thermometer and causing the temperature reading to drop.
This cooling effect continues as long as the air can absorb more moisture. If the air is fully saturated - for example on a foggy morning, when relative humidity reaches 100% - evaporation stops. At this point, the wet bulb temperature equals the dry bulb temperature (the actual air temperature). In all other conditions, the wet bulb temperature remains lower than the dry bulb temperature.
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This process reflects the physical principle that evaporation requires energy. As water evaporates from the wick, it absorbs heat from the surrounding air and the thermometer itself, resulting in a lower temperature reading. The greater the difference between the dry bulb and wet bulb temperatures, the drier the air. When both temperatures are equal, the air is fully saturated, and no further evaporation can occur.
How wet bulb temperature is measured
While the psychrometer - an instrument consisting of paired dry and wet bulb thermometers - has traditionally been used to measure wet bulb temperature, most modern weather stations (including those operated by the Met Office) do not use psychrometers in the classic sense. 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).
As air flows over the thermometers, water evaporates from the wet bulb, cooling it through evaporative heat loss. The dry bulb remains unaffected. The difference between the two readings is then used to calculate relative humidity and other humidity indices.
Modern technology has introduced capacitive sensors that measure humidity by detecting changes in voltage caused by varying levels of water vapour in the air. These sensors convert voltage readings into relative humidity or dew point temperature values, offering a more automated and precise method of humidity measurement.
Wet bulb temperature and other humidity measures
Wet bulb temperature is closely related to other humidity indicators such as dew point temperature. The dew point is the temperature at which air becomes saturated and water vapour begins to condense. This is commonly observed as dew forming on grass during cool mornings. If the dew point or wet bulb temperature equals the air temperature, the air is saturated, and relative humidity is 100%.
Both wet bulb and dew point temperatures are essential for understanding atmospheric moisture content. They help meteorologists assess the likelihood of precipitation, fog formation, and the potential for heat stress in humans and animals.
Applications in meteorology and beyond
Wet bulb temperature is used extensively in weather forecasting and climate studies. It helps determine the comfort level of the atmosphere, especially during hot and humid conditions. High wet bulb temperatures can indicate dangerous heat stress levels, as the body’s ability to cool itself through sweating becomes compromised when evaporation is limited.
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In aviation, agriculture, and construction, wet bulb temperature is used to assess working conditions and safety. It is also critical in determining the potential for thunderstorms and severe weather, as high humidity levels can fuel convective activity.
Humidity measurements above the surface are obtained using instruments suspended from helium balloons, known as radiosondes. These devices ascend through the atmosphere, collecting data on temperature, humidity, and pressure. Satellites also contribute by inferring water vapour content and relative humidity from radiative signals. More recently, GPS technology has been employed to estimate atmospheric moisture by analysing signal delays caused by water vapour.
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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