India's heatwave turn deadly: Why Wet-bulb temperature is so important? What is it?

New Delhi, Apr 30: In a summer of record-breaking heat in many places, severe humidity has made the outdoors feel particularly unbearable.

While there are numerous ways to measure climate conditions, some experts say it's time to start talking about wet-bulb temperature.

What is wet bulb temperature?

The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked cloth (a wet-bulb thermometer) over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature); at lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of evaporative cooling.

The wet-bulb temperature is defined as the temperature of a parcel of air cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat supplied by the parcel.

A wet-bulb thermometer indicates a temperature close to the true (thermodynamic) wet-bulb temperature. The wet-bulb temperature is the lowest temperature that can be reached under current ambient conditions by the evaporation of water only.

Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32 °C (90 °F), equivalent to a heat index of 55 °C (130 °F). The theoretical limit to human survival for more than a few hours in the shade, even with unlimited water, is a wet-bulb temperature of 35 °C (95 °F) - theoretically equivalent to a heat index of 70 °C (160 °F), though the heat index does not go that high.[3]