A thermometer answers a narrow question: how hot is the air? Your body cares about a broader one: how fast am I losing or gaining heat? The gap between those two questions is why "feels like" numbers exist — and why they're the ones that should drive your outdoor plans.
Heat index: humidity is the villain
Humans cool themselves by evaporating sweat. When the air is already saturated with moisture, evaporation slows dramatically, and body heat has nowhere to go. That's the entire logic of the heat index: 90°F in dry Phoenix air is genuinely manageable, while 90°F at high humidity in Houston or New Orleans can feel like 105°F and carry real risk of heat exhaustion. The dangerous cases aren't always the record-breaking temperatures — they're moderate temperatures paired with oppressive humidity, which people routinely underestimate because the number on the thermometer looks ordinary.
Wind chill: the same story in reverse
Your skin warms a thin boundary layer of air around you. Wind strips that layer away and forces your body to heat fresh cold air continuously, which is why 30°F in still air is a pleasant walk while 30°F with a 25 mph wind bites through everything and can produce frostbite risk on exposed skin. Wind chill applies only to exposed skin, which is exactly why layering, a hat, and covering your face change your experience of a cold day far more than a few degrees of actual temperature.
Planning by "feels like," not by temperature
Practical thresholds most planners use: above a heat index of about 90°F, shorten outdoor sessions and force water breaks; above 103°F, move strenuous activity to early morning or reschedule; below a wind chill of about 15°F, limit exposure and cover extremities; and in either extreme, remember that children, older adults, and pets hit their limits well before healthy adults notice discomfort. Weather or Not flags dangerous heat and cold in its verdict for exactly this reason — a day that's technically dry can still be a day you shouldn't spend outside.
The overlooked factors
Neither index accounts for direct sun, which can add roughly 15°F to what your body experiences compared to shade, nor for pavement and artificial turf, which radiate stored heat long after the air cools. If your event is on asphalt in July, treat the heat index as the optimistic number and plan shade accordingly.