It's called wet-bulb temperature - it's measured by a thermometer whose bulb is covered by fabric and water slowly drips on it. It shows how much can evaporatio

It's called wet-bulb temperature - it's measured by a thermometer whose bulb is covered by fabric and water slowly drips on it. It shows how much can evaporation (sweating) help to cool us down in those particular weather conditions. If the humidity is 100%, the wet-bulb temperature will be equal to the normal temperature - because evaporation stops, therefore no cooling effect. And if the wet-bulb temperature is more than our body temperature, we are overheating. So air temperature over 37 degrees with 100% humidity is indeed potentially lethal.