Heat Index Calculator
Estimate the “feels like” temperature from air temperature and relative humidity using standard NOAA heat index equations.
Practical Weather Engineering Guide
Heat Index Calculator: Understand “Feels Like” Temperature
The heat index combines air temperature and relative humidity to estimate how hot it feels to people in the shade. This guide explains the equations behind the Heat Index Calculator, how to use it correctly, what assumptions matter, and how to interpret results for safety and field decisions.
Quick Start
- 1 Enter the air temperature \(T\). Use shaded, ambient dry-bulb temperature—don’t use a sun-heated surface reading.
- 2 Enter relative humidity \(RH\) as a percentage from 0–100%. If you’re pulling this from a weather app, use the current ambient RH.
- 3 Leave Heat index (known) blank unless you are solving for \(RH\) or \(T\).
- 4 Choose your Algorithm. NOAA / Rothfusz is the standard used by U.S. weather services. The Simple method is a quick approximation.
- 5 Pick Output units (°F or °C). Internally the calculator converts to °F for NOAA equations, then converts back.
- 6 Read the Calculated result and the Heat index category in Quick Stats to interpret risk.
- 7 If the output surprises you, do a quick sanity check: confirm you used shaded temperature and realistic RH (sensor errors in humidity are common).
Tip: Heat index is defined for shade and light wind. Direct sun can make it feel ~10–15°F (≈6–8°C) hotter than the heat index alone.
Common mistake: Using a temperature from a sun-exposed thermometer or from a rooftop/vehicle. Those are surface temperatures, not air temperature.
Choosing Your Method
There are a few ways engineers and forecasters estimate “apparent temperature.” Your calculator offers two, each useful in different situations.
Method A — NOAA / Rothfusz Regression (Standard)
This is the equation behind most U.S. heat index charts. It’s a regression fit to Steadman’s physiological model, with NOAA corrections at very low and very high humidity ranges.
- Matches official weather-service warnings and public charts.
- Best for hot, humid conditions where heat stress is a concern.
- Includes edge-case adjustments for very dry or very humid air.
- Defined for \(T \gtrsim 80^\circ\mathrm{F}\); below that it blends toward a simpler model.
- Assumes light wind and shade; not a full microclimate model.
Method B — Simple / Steadman Approximation
A compact approximation that tracks the same physics but with fewer terms. It’s helpful for quick estimates or mild temperatures.
- Fast, simple, and stable over a wide range of inputs.
- Useful for “ballpark” checks or classroom work.
- Less accurate in extreme heat/humidity.
- Not the basis of official NOAA warnings.
If you’re comparing against public forecasts, safety plans, or OSHA-style heat guidelines, use the NOAA method. If you’re exploring sensitivity, teaching, or doing back-of-the-envelope checks, the Simple method is fine.
What Moves the Number the Most
Heat index rises super-linearly with temperature. A 5°F increase at high humidity can push HI into a much higher risk band.
Higher RH reduces sweat evaporation, so the same air temperature feels hotter. The effect accelerates above ~60% RH.
Below \(80^\circ\mathrm{F}\) the heat index is close to the air temperature. Above it, humidity drives a strong divergence.
The equation assumes shade. In direct sun, radiant loading can add ~10–15°F to perceived heat depending on clothing and surface reflectance.
Wind improves convective and evaporative cooling. Calm air makes conditions feel hotter than HI suggests; strong wind can make it feel cooler.
HI is a general index, not a personalized physiological model. Heavy labor, PPE, low fitness, or poor acclimatization increase risk at the same HI.
Worked Examples
Example 1 — Calculate Heat Index from \(T\) and \(RH\)
- Air temperature: \(T = 95^\circ\mathrm{F}\)
- Relative humidity: \(RH = 60\%\)
- Algorithm: NOAA / Rothfusz
- Goal: Find heat index \(HI\)
If you switch output to °C, the calculator will convert: \[ HI_{^\circ C} = (HI_{^\circ F}-32)\frac{5}{9} \approx 45^\circ\mathrm{C} \]
Example 2 — Solve for Relative Humidity from Known Heat Index
- Air temperature: \(T = 90^\circ\mathrm{F}\)
- Heat index known: \(HI_{known} = 105^\circ\mathrm{F}\)
- Algorithm: NOAA / Rothfusz
- Goal: Find \(RH\)
This “solve for RH” mode is handy when you have a reported heat index but want to back-estimate humidity for a model or field log.
Common Layouts & Variations
Heat index is a standardized shade-and-light-wind index. Real sites add microclimate effects. Use the table below to interpret results in context.
| Situation / Configuration | What Heat Index Assumes | Practical Adjustment / Note | Typical Use Case |
|---|---|---|---|
| Shaded outdoor work, light breeze | Matches definition closely | No adjustment needed | Standard safety planning, forecasts |
| Direct sun on dark surfaces | Shade (no radiant loading) | Add ~10–15°F (6–8°C) to perceived heat | Solar sites, asphalt, rooftops |
| High wind / coastal breeze | Light wind | Perceived heat may be lower than HI | Bridge work, open fields |
| Urban canyon / still air | Some airflow | Perceived heat can exceed HI; reduce work-rest ratio | Downtown construction, enclosed yards |
| Indoor spaces with high RH | Outdoor shade model | HI still useful, but check ventilation and WBGT if possible | Warehouses, plants |
| Cold or mild conditions \(T<80^\circ\mathrm{F}\) | HI ≈ T | Humidity has limited effect; calculator blends toward simple model | Shoulder seasons |
For high-risk occupational heat studies, consider Wet-Bulb Globe Temperature (WBGT). It explicitly includes radiant heat, wind, and solar load, while HI does not.
Specs, Logistics & Sanity Checks
Heat index is often used for safety triggers and operational planning. The key is matching the number to the decision you’re making.
Risk Bands (NOAA)
- < 80°F: Comfortable / low risk
- 80–90°F: Caution — fatigue possible with prolonged exposure
- 90–103°F: Extreme caution — heat cramps/exhaustion possible
- 103–125°F: Danger — heat illness likely without controls
- > 125°F: Extreme danger — heat stroke highly likely
Field Inputs to Verify
- Temperature measured in shade at ~1.5–2 m height.
- Humidity sensor not in a wet-bulb pocket or sun-heated enclosure.
- Calm vs windy site conditions noted in log.
- Time of day and cloud cover recorded.
Sanity Checks
- If \(RH \lt 20\%\), HI should be close to \(T\) even when hot.
- If \(RH \gt 70\%\) and \(T \gt 90^\circ\mathrm{F}\), HI often jumps 10–25°F above \(T\).
- Compare to a local forecast HI; large differences usually indicate a measurement issue.
- Re-run with the Simple method to see if results are consistent in mild conditions.
Remember that HI is a planning metric, not a medical diagnosis. Use conservative margins when scheduling field work, especially where PPE, confined air, or radiant loading increases stress. If your project has strict heat-safety requirements, pair HI with site-specific monitoring and an acclimatization plan.