{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"heat-index"},"result":{"entity_type":"calculator","id":"heat-index","calculator_id":"heat-index","canonical_url":"https://tttkmbb.com/weather/heat-index","name":"Heat Index Calculator","title":"Heat Index Calculator – Apparent Temperature from Air Temperature and Humidity (NOAA/NWS Rothfusz Equation)","category":"weather","category_name":"Weather & Environment","tool_name":"calculate_heat_index","featured_mcp_tool":false,"description":"Computes the NWS heat index ('feels like' temperature in the shade) from air temperature and relative humidity with the Rothfusz regression, including the NWS low- and high-humidity adjustments, and classifies the NWS risk category.","use_when":"You need how hot humid weather feels, the NWS heat-index category (caution to extreme danger) or a heat-safety threshold for temperatures of 80 °F (27 °C) and above.","do_not_use_when":"Temperatures are below about 80 °F (27 °C), where the heat index is not meaningful (for cold weather use wind-chill), or you need the dew point itself (use dew-point).","inputs":[{"name":"temperature","label":"Air temperature","type":"number","required":true,"min":-50,"max":150,"description":"Air temperature in the shade, in temperature_unit. The regression is valid for 80–112 °F (27–44 °C).","example":90},{"name":"temperature_unit","label":"Temperature unit","type":"enum","required":false,"default":"f","values":[{"value":"c","label":"Celsius (°C)","aliases":["celsius","°c","degc"]},{"value":"f","label":"Fahrenheit (°F)","aliases":["fahrenheit","°f","degf"]}],"description":"Unit of the temperature input(s).","example":"f"},{"name":"relative_humidity_percent","label":"Relative humidity","type":"number","unit":"%","required":true,"min":0,"max":100,"description":"Relative humidity in percent (70 means 70 %).","example":70}],"outputs":[{"name":"heat_index_f","label":"Heat index","type":"number","unit":"°F","decimals":1,"description":"Apparent temperature in Fahrenheit."},{"name":"heat_index_c","label":"Heat index","type":"number","unit":"°C","decimals":1,"description":"Apparent temperature in Celsius."},{"name":"temperature_f","label":"Air temperature","type":"number","unit":"°F","decimals":1,"description":"Input temperature in Fahrenheit (the regression works in °F)."},{"name":"category","label":"NWS category","type":"string","decimals":4,"description":"Caution (80–89 °F), Extreme caution (90–102 °F), Danger (103–124 °F) or Extreme danger (125 °F and above)."},{"name":"effect","label":"Possible effects","type":"string","decimals":4,"description":"NWS description of heat disorders possible with prolonged exposure or strenuous activity."},{"name":"method_used","label":"Formula applied","type":"string","decimals":4,"description":"Rothfusz regression (heat index ≥ 80 °F) or the simple Steadman-average formula below 80 °F."}],"input_schema":{"type":"object","properties":{"temperature":{"description":"Air temperature in the shade, in temperature_unit. The regression is valid for 80–112 °F (27–44 °C).","type":"number","minimum":-50,"maximum":150,"examples":[90]},"temperature_unit":{"description":"Unit of the temperature input(s).","type":"string","enum":["c","f"],"default":"f","examples":["f"]},"relative_humidity_percent":{"description":"Relative humidity in percent (70 means 70 %). Unit: %.","type":"number","minimum":0,"maximum":100,"examples":[70],"x-unit":"%"}},"additionalProperties":false,"required":["temperature","relative_humidity_percent"]},"output_schema":{"type":"object","properties":{"heat_index_f":{"description":"Apparent temperature in Fahrenheit. Unit: °F.","type":"number","x-unit":"°F"},"heat_index_c":{"description":"Apparent temperature in Celsius. Unit: °C.","type":"number","x-unit":"°C"},"temperature_f":{"description":"Input temperature in Fahrenheit (the regression works in °F). Unit: °F.","type":"number","x-unit":"°F"},"category":{"description":"Caution (80–89 °F), Extreme caution (90–102 °F), Danger (103–124 °F) or Extreme danger (125 °F and above).","type":"string"},"effect":{"description":"NWS description of heat disorders possible with prolonged exposure or strenuous activity.","type":"string"},"method_used":{"description":"Rothfusz regression (heat index ≥ 80 °F) or the simple Steadman-average formula below 80 °F.","type":"string"}}},"formula":"T in °F, RH in %. Simple: HI₀ = 0.5·[T + 61 + 1.2·(T − 68) + 0.094·RH]. If HI₀ ≥ 80: HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH²; if RH < 13 and 80 ≤ T ≤ 112: HI −= (13 − RH)/4 · √((17 − |T − 95|)/17); if RH > 85 and 80 ≤ T ≤ 87: HI += (RH − 85)/10 · (87 − T)/5","method":"The Rothfusz regression reproduces Steadman's apparent-temperature model within ±1.3 °F for shady, light-wind conditions; full sunshine can add up to 15 °F. NWS categories: 80–90 °F caution, 90–103 extreme caution, 103–124 danger, 125+ extreme danger.","sources":[{"name":"NWS Weather Prediction Center – The Heat Index Equation (Rothfusz 1990 regression)","url":"https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml","type":"government","retrieved_at":"2026-09-24"},{"name":"NWS – Heat Index chart and categories","url":"https://www.weather.gov/safety/heat-index","type":"government","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"90 °F, 70 % RH","inputs":{"temperature":90,"temperature_unit":"f","relative_humidity_percent":70},"expected":{"heat_index_f":105.9,"heat_index_c":41.1,"temperature_f":90,"category":"Danger","method_used":"Rothfusz regression"},"url":"https://tttkmbb.com/api/v1/calculate/heat-index?temperature=90&temperature_unit=f&relative_humidity_percent=70"},{"name":"35 °C, 60 % RH","inputs":{"temperature":35,"temperature_unit":"c","relative_humidity_percent":60},"expected":{"heat_index_f":113.1,"heat_index_c":45.1,"category":"Danger"},"url":"https://tttkmbb.com/api/v1/calculate/heat-index?temperature=35&temperature_unit=c&relative_humidity_percent=60"}],"faq":[{"q":"Why does the chart say 105 °F but the calculator 105.9 °F?","a":"The NWS chart rounds Steadman's original table; the regression used here (and by the NWS online calculator) reproduces it within about ±1.3 °F."},{"q":"Does it account for sun or wind?","a":"No. The heat index assumes shade and a light breeze; the NWS notes that direct sunshine can raise the apparent temperature by up to 15 °F (8 °C)."},{"q":"What happens below 80 °F?","a":"The simple Steadman-average formula is reported and the category is 'No heat-index warning', because the full regression is only fitted for heat index values of 80 °F and above."}],"tags":["heat index","feels like temperature","apparent temperature","humidity heat","heat safety","rothfusz"],"related":[{"calculator_id":"dew-point","reason":"Dew point is the other common measure of how muggy the air feels."},{"calculator_id":"wind-chill","reason":"The cold-weather counterpart of the heat index."},{"calculator_id":"temperature","reason":"Plain °C ↔ °F conversion."}],"links":{"html":"https://tttkmbb.com/weather/heat-index","markdown":"https://tttkmbb.com/weather/heat-index.md","json":"https://tttkmbb.com/weather/heat-index.json","api":"https://tttkmbb.com/api/v1/calculate/heat-index","schema":"https://tttkmbb.com/api/v1/calculators/heat-index","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T01:48:54Z"}