What is considered a dangerous heat index?

Heating & HVACNOAA Heat Index Equation

Heat Index Calculator (NOAA Feels-Like Formula)

Calculate the official NOAA National Weather Service Heat Index (apparent temperature) and heat illness safety alert thresholds.

Heating & HVAC Field StandardsNOAA Heat Index Equation

Engineering formulations calibrated to IPC, UPC & NEC specificationsSub-Zero Cold Climate Tolerances

Quick Summary & Key Answer

The Heat Index measures how hot it feels when relative humidity is factored into air temperature. High humidity prevents sweat from evaporating, trapping body heat. An air temperature of 90°F with 65% relative humidity produces an apparent heat index of 103°F (Danger Zone).

Heat Index formula applies when temperatures are at or above 80°F and humidity is 40% or higher.
Heat index values above 103°F trigger National Weather Service Heat Advisories.
Working in direct sunlight adds up to 15°F to the heat index value.

Interactive Calculator

Adjust inputs for real-time results

Ambient Air Temperature°F

°F

Relative Humidity (RH)%

Calculated OutputExtreme Caution: Heat cramps and exhaustion likely

Apparent 'Feels-Like' Heat Index

103°F°F

Dry Bulb Temperature: 90°F
Relative Humidity: 65%
Heat Illness Danger Level: Moderate

Note: Full direct sun exposure can increase heat index values by up to 15°F.

Heat Exhaustion vs Heat Stroke

Heat stroke occurs when core body temperature reaches 104°F; seek immediate emergency medical care.

Frequently Asked Questions

Common questions regarding this calculation and code compliance

A heat index of 103°F to 124°F is categorized as 'Danger', where heat exhaustion and cramps are likely and heat stroke is possible.