Heat Index · Feel-like temp
How hot does it actually feel out there?
Punch in the air temperature and either the humidity or dew point. We'll blend them into the perceived temperature your body will register — using the NWS heat index model.
Model is most accurate when it's warm and sticky — air temperature above 80°F and humidity over 40%.
Feels like
Heat cramps and exhaustion are possible. Hydrate often.
What the heat index really is
Sometimes called humiture, the heat index is the temperature your body thinks it's experiencing — a mash-up of the actual air temperature and how saturated the air is with moisture. Muggy 83°F air at 70% humidity can feel closer to 88°F, because your sweat can't evaporate fast enough to cool you down.
Heat perception is personal. Hydration, body composition, medications, pregnancy, and even direct sun exposure all shift the number for any given person. Treat the reading as a guide, not gospel.
Where the math comes from
We use the same regression the U.S. National Weather Service publishes — George Winterling's 1978 equation, tuned for temperatures at or above 80°F and humidity of 40% or more. Below those thresholds we fall back to a simpler blended average so the readout still behaves sensibly.
Fed a dew point instead of humidity? We convert it with the Magnus approximation, then run the same model. Standing in full sun can add up to 14°F on top of whatever we report.
Know your zone
A quick reference for what different heat index readings do to the human body. When in doubt, slow down and get water.
Fatigue is possible with prolonged effort. Keep breaks in the plan.
Cramps and heat exhaustion become likely with continued activity.
Heat exhaustion is expected; heat stroke is probable if you push on.
Heat stroke is imminent. Stop, cool down, seek shelter.
Kids are especially vulnerable — smaller bodies, more surface area, less awareness to stop and drink. Thirst is a late signal. Hydrate before, during, and after time outside.