Why does it feel hotter than it is?
Because humid air stops your sweat from evaporating, and evaporation is how your body cools itself. The heat index folds the temperature and the humidity into one number for how hot it feels: 90°F at 60% humidity feels like about 100°F.
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Your body runs on evaporation
When you sweat, the water on your skin evaporates and takes heat with it. In dry air that happens quickly, so 95°F in the desert can feel manageable in the shade. In humid air the air is already holding a lot of water, so sweat evaporates slowly, beads up and drips off without cooling you. Your body keeps heating up.
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What the heat index means
The US National Weather Service's heat index estimates how hot it feels in the shade with a light breeze, for an average adult. Above about 103°F heat exhaustion and heat stroke become much more likely with long exposure, and the NWS starts issuing heat advisories at thresholds that vary by region. Standing in direct sun can add many degrees more.
Watch the dew point
The dew point is the cleanest signal: once it climbs past the mid-60s °F, humid heat becomes hard on the body, and above 70°F it's oppressive. Air Story shows your dew point on its first screen and traces where all that water in the air came from.
Questions people ask
- What is the 'feels like' temperature?
- Most apps use the heat index when it's hot and humid and the wind chill when it's cold and windy, so 'feels like' can be higher or lower than the air temperature.
- Can dry heat feel cooler than the temperature?
- Yes. In very dry air the heat index can come out slightly below the temperature, because sweat evaporates so efficiently. But dry heat still dehydrates you quickly.