Explained: What Is Wet Bulb Temperature And How Hot Is Too Hot

India is currently experiencing a severe heatwave, with temperatures soaring in parts of Uttar Pradesh, Punjab, Rajasthan, Chandigarh and Delhi among others. In some regions, rising mercury, coupled with humidity, makes it even more difficult for people to step out for any work. 

But how is humidity recorded? The Dry bulb, Wet bulb and Dew point temperatures help understand or calculate the state of humid air. With just two of these values, one can deduce critical information such as water vapour content, as well as sensible and latent energy (enthalpy).  

What is the Wet bulb temperature?

Wet bulb temperature is a meteorological term used to describe the lowest temperature that can be reached by evaporating water into the air at constant pressure. It is measured by covering a thermometer bulb with a wet cloth and letting the water evaporate. As the water evaporates, it cools the thermometer, showing the wet bulb temperature. 

This temperature helps measure humidity and understand how much water can evaporate into the air, affecting things like comfort, farming and weather patterns.

The difference between the Dry bulb and Wet bulb temperatures depends on the humidity of the air. Higher humidity means less evaporation and a smaller difference. When the air is fully saturated (100% humidity), the Wet bulb and Dry bulb temperatures are the same. 

What is a safe heat/humidity limit?

A wet-bulb temperature of 35 degrees Celsius is suggested as the maximum safe limit, according to a 2010 study. Beyond this point, the human body can't cool itself by sweating, which is necessary to maintain a stable core temperature. 

However, this was proven inaccurate recently when researchers at Penn State University put young, healthy men and women in a controlled heat environment. Each participant swallowed a pill that monitored their core temperature. They then performed minimal activities such as showering, cooking and eating as the researchers gradually increased the temperature or humidity. 

The researchers discovered the "critical environmental limit" – the point where a person's core temperature starts to rise uncontrollably. Below this limit, the body can keep a stable core temperature for a long time. Above this limit, the core temperature keeps rising, increasing the risk of heat-related illnesses. 

When the body overheats, the heart works harder to pump blood to the skin to release heat. Sweating also depletes body fluids. Prolonged exposure to high heat and humidity can lead to heat stroke, a serious condition needing immediate medical attention.

The studies discovered that the safe limit is lower than 35 degrees Celsius (as was revealed by the earlier study). The ideal wet bulb temperature is approximately 31 degrees Celsius at humidity levels above 50%. This means, 31 degrees Celsius at 100% humidity or 38 degrees Celsius at 60% humidity, showing the significant risk posed by high heat and humidity even for healthy people.