With the heatwave that has been raging in India and Pakistan since April, an unusual measurement is often mentioned in the media: wet bulb temperature. This indicator has attracted so much attention because it provides a simple way of assessing when a human being is at risk of dying from heat:
In this tutorial, we'll take a closer look at wet bulb temperature and how to calculate it with Python.
What's the point of wetting a thermometer?
Wet bulb temperature is... the temperature measured by a wet bulb thermometer, usually by covering the thermometer bulb with a cloth soaked in a supply of water.
Unlike other thermal comfort indicators, such as the wet globe thermometer, UTCI or humidex, which are recent inventions, this measurement has been widespread since the 19th century. Why is this? Because comparing the values displayed by two thermometers, one dry and the other wet, enables us to assess the humidity of the air - a very useful piece of information for many industrial processes or in agronomy.
In our case, it's not these applications that interest us. The advantage of a wet bulb thermometer is that it measures air temperature, but is also cooled by the evaporation of the water it comes into contact with. It thus reproduces the two main mechanisms the human body uses to regulate its temperature: contact with ambient air and evaporation through perspiration.
When a wet thermometer displays a temperature equal to or higher than that of our body, it means that these two mechanisms are no longer able to cool us down. Body temperature will rise, causing fatal hyperthermia within a few hours.
In theory, the maximum wet bulb temperature a human can withstand is set at 35°C. Beyond that, heatstroke is inevitable.
However, a recent study suggests that this threshold could be lower, around 31°C. And it's important to remember that heat has serious health impacts before it reaches these extreme values.
Calculate wet bulb temperature
When it cannot be measured directly, the wet bulb temperature can be calculated from the ordinary temperature and humidity.
The most frequently used formula for this is the Stull formula:
Where T is air temperature in degrees Celcius and RH% is relative humidity in percent.
However, this is only an estimate: this formula is valid under most usual conditions, but it has its limits, and other methods exist.
Implementing Stull's formula in Python
Even if Stull's formula is fairly simple (much simpler, for example, than the competing Davies-Jones method), calculating it by hand will soon become tedious.
Here's how to calculate it with Python :
This function can be used, for example, to plot the evolution of wet bulb temperature with ordinary temperature as a function of relative humidity:
Here's the result:
This graph shows some of the properties of the wet bulb temperature:
It is always lower than the ordinary temperature,
At fixed humidity, it increases almost linearly with temperature,
The higher the humidity, the faster it increases.
About: Callendar is a start-up specializing in the development of innovative solutions for climate risk assessment. Aware of the challenge of adapting to climate change, we strive to share our expertise through free tools and tutorials like this one.