Heat Index Calculator

Because sweating only cools you when the sweat evaporates, and evaporation slows as the air fills with moisture — at high humidity, sweat drips instead of evaporating, delivering zero cooling while still draining fluids. The heat index quantifies that lost cooling capacity as an equivalent temperature. It's why 95°F in Phoenix (10% humidity, index ~90) is a different physiological event than 95°F in Houston (60% humidity, index ~108), and why 'dry heat' is a legitimate distinction rather than a boast.

The NWS thresholds: 80–90° caution (fatigue with prolonged exposure), 90–103° extreme caution (heat cramps and exhaustion possible), 103–125° danger (heat exhaustion likely, stroke possible with sustained activity), 125°+ extreme danger (stroke imminent). Calibrate by activity: the categories assume ordinary exposure — hard labor or running shifts everything a category worse, which is why OSHA and athletic programs mandate work-rest cycles keyed to the index, not the thermometer.

Heat exhaustion is the warning: heavy sweating, weakness, dizziness, nausea, headache, cool clammy skin — respond with shade, water, rest, and cooling; recovery is expected. Heat stroke is the emergency: confusion or altered behavior, hot skin that may be dry (sweating has failed), core temperature at or above 103°F, possible collapse — call 911 and cool aggressively while waiting (ice, cold water, fans on wet skin). The transition can take minutes, and confusion is the tell: a person who stops making sense in the heat is a medical emergency, full stop.

The number assumes shade and light wind — the NWS states full sunshine can raise the effective index by up to 15°F, turning a 95° 'extreme caution' reading into a 110° 'danger' reality on a roof, a field, or a stadium bleacher. That's the honest reason outdoor workers and athletes suffer heat illness on days that don't sound extreme. Other multipliers the equation can't see: direct pavement heat, heavy clothing or PPE, dehydration, certain medications, and the first hot days of the season before the body acclimatizes (which takes one to two weeks).