Wet-bulb temperature refers to the combination of heat and humidity; this, if too high, might impair the human body’s capacity to cool itself.

A record-breaking heat wave has been heating parts of India and Pakistan for weeks; exposing more than a billion people to dangerously hot conditions with no relief in sight.

While temperatures in the region have cooled slightly this week; “scorching heat is expected to return in the coming days and spread east, where rising “wet-bulb temperatures”; an esoteric measurement that has been little known outside meteorology circles until now — may endanger human survival, according to experts.

According to Friederike Otto, a climate scientist at Imperial College London, this is the type of problem that is growing more relevant as climate change increases the frequency and severity of extreme heat events.