A wet bulb event is a situation where the wet-bulb temperature reaches a critical level, making it difficult for the human body to cool itself through sweating. Wet-bulb temperature is a measure of heat that considers humidity as well as other factors. It 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. Wet-bulb temperature is measured using a thermometer that has its bulb wrapped in cloth, called a sock, that is kept wet with distilled water via wicking action. The wet-bulb depression is the difference between the dry-bulb temperature and the wet-bulb temperature. If there is 100% humidity, dry-bulb and wet-bulb temperatures are identical, making the wet-bulb depression equal to zero in such conditions.

Wet-bulb events are becoming more frequent and intense due to climate change, and they pose a serious threat to human health and safety. Recent research has found that we may already be nearing the threshold values for human survivability of temperature and humidity for short periods in some places of the world, and that this threshold may actually be far lower than previously thought. The human body likes to maintain its temperature at approximately 37 °C (98.6 °F), and a body temperature above 40 °C (104 °F) can become life-threatening. The problem is that the body’s ability to cool itself rapidly falls apart when the humidity rises, and wet-bulb temperature is a measure of heat-stress conditions on humans. The closer the humidity gets to 100%, the less effective cooling methods like fans and water become. Tropical regions with a lot of humidity, especially those along the monsoon belt, are generally at greatest risk of experiencing lethal wet-bulb temperatures.