EXTREME WET BULB TEMPERATURE

EXTREME WET BULB TEMPERATURE

Subject: Science & tech

Context: A new study suggests that large swaths of the tropics will experience dangerous living and working conditions if global warming isn’t limited to 1.5 degrees Celsius.

Concept:

• A study published Monday suggests that sharply cutting emissions of greenhouse gases to stay below that limit, which is equivalent to about 2.7 degrees Fahrenheit of warming since 1900, will help the tropics avoid episodes of high heat and high humidity — known as extreme wet-bulb temperature, or TW — that go beyond the limits of human survival.

Extreme Wet Bulb Temperature

• The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked cloth (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 35 °C (95 °F) – theoretically equivalent to a heat index of 70 °C (160 °F), though the heat index doesn’t go that high.