WebToolArcHeat Index Calculator
Calculate the National Weather Service heat index from relative humidity or dew point, inspect the exact Steadman/Rothfusz path and humidity adjustment, reverse-solve risk thresholds, compare nearby conditions, and export repeatable browser-local heat-index evidence.
Heat Index Verification & Scenario Studio
Compute the shaded, light-wind NWS heat index from relative humidity or dew point, see exactly which formula path and humidity adjustment was used, reverse-solve chart thresholds, and compare nearby conditions without uploading weather data.
Formula path & adjustment trace
Reverse-solve NWS chart boundaries
| Target HI | RH needed at current air temp | Air temp needed at current RH |
|---|
Nearby-condition heat-index matrix
A 5×5 neighborhood around the current temperature and humidity helps show how quickly apparent heat changes. Cell colors use the NWS heat-index chart bands.
Batch conditions · up to 200 rows
One row per temperature,relative humidity using the currently selected temperature unit.
| # | Air | RH | Heat index | Band |
|---|
Explicit local scenarios
Save a scenario only when you choose. Entries stay in this browser and are never included in the share URL.
How to use heat index without overstating what the number means
The NWS calculation has two stages
The calculator first evaluates a simpler Steadman-compatible expression and averages it with the air temperature. If that preliminary result is hot enough, it switches to the Rothfusz regression and then applies the NWS low- or high-humidity adjustment when its narrow conditions are met.
Relative humidity and dew point are two ways to describe moisture
You can enter either. Dew point is converted to relative humidity at the selected air temperature, then the same NWS heat-index workflow is used. The derived humidity is shown so the conversion can be audited.
Full sun is context, not an alternate official formula
The NWS heat index assumes shade and light wind. NWS guidance says full sunshine can increase apparent heat by up to roughly 15°F. This page displays that as an upper-context value rather than pretending it is another official heat-index equation.
Heat index and WBGT answer different questions
Heat index uses temperature and humidity to estimate apparent heat. WBGT can account for radiant heat, air movement and other environmental factors and is commonly used in occupational heat-stress programs. Do not substitute one for the other.
Use the matrix to see sensitivity, not to forecast weather
The scenario matrix recalculates nearby temperature/humidity combinations with the same NWS workflow. It is useful for planning and understanding sensitivity, but it does not predict which conditions will actually occur.
Risk bands are context, not personalized medical advice
The familiar Caution, Extreme Caution, Danger and Extreme Danger bands summarize the NWS heat-index chart. Individual risk also depends on exposure, exertion, clothing, acclimatization and health factors that are outside this calculator.
Batch mode is for repeated conditions
Paste up to 200 temperature/RH rows to calculate them consistently and export a CSV. Each row records the NWS method and band, making it easier to audit event, school, field or operations planning data without uploading it.
Use official local products for decisions
Heat advisories and warnings can depend on location, duration, overnight temperatures and local criteria. This tool explains and reproduces the heat-index equation; it is not an official forecast or warning service.