Heat-Adjusted Pace Calculator

Heat Index here uses the National Weather Service's Rothfusz regression -- the same equation behind the public heat-index figure you'd see in a forecast -- computed once Temperature reaches 80°F, with the standard NWS low-humidity and high-humidity correction terms applied on top. Below 80°F the regression isn't defined, so Heat Index simply equals Temperature. Dew Point uses a quick approximation (Temperature minus one-fifth of the humidity deficit), accurate mainly when Relative Humidity is above roughly 50%; treat it as directional at very low humidity. Pace Slowdown comes from an illustrative heat-stress curve, not a precisely validated physiological model: it's flat at zero up to 55°F, then rises in progressively steeper bands as the higher of Heat Index and Temperature climbs, reaching a 12%-per-10°F rate above 100°F.

Heat Risk Level applies its own, more conservative breakpoints than the general-public NWS advisory scale (Low below 75, climbing to "Extreme -- Do Not Run" at 104°F+), since sustained running effort adds substantial metabolic heat on top of ambient conditions -- a caution level a resting person wouldn't need at the same temperature can still matter mid-run. Planned Distance only scales the absolute time lost, not the pace slowdown percentage itself, and Normal Pace's minutes/seconds split doesn't affect Heat Index, Dew Point, or Pace Slowdown at all -- those depend only on Temperature and Relative Humidity. This tool doesn't account for acclimatization, sun exposure, wind, individual sweat rate, or hydration status, all of which materially change real heat tolerance.