Back to the daytime WBGT Atlas
The night half of heat
We track daytime heat closely, but the body does most of its cooling after dark. A night that stays hot denies that recovery, so the strain of one scorching day carries into the next instead of resetting. This is why hot nights, more than hot afternoons, are what makes heat deadly.
Fifty years apart
Drag across the map to wipe between the 1970s and the last decade. Look first at the edges of the tropics, where the band of dangerous nights pushes outward, then at the subtropics filling in behind it. Places spared within a single lifetime now lie inside it.
The body recovers from heat at night, in the hours that are no longer cool enough to let it.
The change itself
This map shows the difference, not the heat. Warmer tones mark where tropical nights have multiplied since the 1970s. The deepest colour gathers across the humid tropics and along the growing margins of the subtropics, where some regions now endure dozens of additional tropical nights each year.
Understanding the night
A tropical night is a night that stays warm: its lowest temperature never drops below 20 °C, so the cool, recovering hours simply do not arrive. The night is when the body offloads the day's heat and repairs itself, and across much of the world that nightly relief is fading. Here is what the map is really measuring.
Air temperature alone does not tell you how heat lands on a body. This map uses the Universal Thermal Climate Index, which folds those four ingredients together and returns one equivalent temperature: the figure that would stress you the same amount in calm, shaded conditions.
All day the body stores heat. After dark it sheds it, radiating warmth and evaporating sweat much as a building vents the day's heat once the sun is down. When the night stays hot and humid that release is blocked: the heart works harder, sleep frays, and the strain builds instead of clearing.
Our daytime map uses Wet-Bulb Globe Temperature, an older index built for workplaces, sport and the military, weighted toward humidity and sun. This night map uses the Universal Thermal Climate Index, a whole-body model of outdoor strain, and the only one of the two that can be read after dark. Same goal, different recipes; their two scales are not interchangeable.
The real danger is compounding. A scorching day followed by a night that never cools gives the body no window to reset, so the load carries from one day into the next. These back-to-back day-and-night events are where heat turns deadly, and they are exactly the pairing becoming more frequent and longer.
How hot can a tropical night get?
A night counts as tropical once its overnight low stays at or above 20 °C. The bands above grade how punishing it then becomes, on the feels-like scale.
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