Feels Like Temperature Calculator

Feels Like Temperature Calculator — Wind Chill, Heat Index, Dew Point

Convert air temperature, humidity, and wind speed into the temperature your body actually perceives. The calculator runs three official meteorological formulas at once — NWS wind chill for cold-and-windy weather, NWS heat index for hot-and-humid weather, and the Australian BoM apparent temperature formula that works in any conditions — and picks the most applicable as the primary result. Also shows dew point (when condensation forms), absolute humidity (water vapor mass per m³), and risk categories with color-coded severity.

What's the difference between wind chill, heat index, and apparent temperature?

Each formula targets a different mechanism of how weather affects the body:

**Wind chill** measures cold weather perception. Below ~10°C, moving air strips heat off exposed skin much faster than still air. The current formula (NWS 2001) was calibrated using volunteers in cold chambers wearing face sensors, replacing the much harsher Siple-Passel formula from 1945. It only applies when the air temperature is at or below 10°C (50°F) and wind speed is at least 4.8 km/h (3 mph).

**Heat index** measures hot weather perception. Above ~27°C with high humidity, the body's sweat doesn't evaporate as efficiently, so cooling slows down. The NWS Rothfusz regression (1990) is a multi-variable polynomial that approximates Steadman's biophysical model. It applies at ≥ 27°C (80°F) when humidity exceeds about 40%.

**Apparent temperature (BoM)** is a single Australian formula that combines temperature, humidity, and wind in one equation: AT = T + 0.33×e − 0.7×ws − 4.0, where e is water vapor pressure. It works at *any* temperature, hot or cold, which is why it's the calculator's fallback when neither wind chill nor heat index applies.

The Feels Like value displayed prominently is whichever of these three is most extreme — heat index when hot, wind chill when cold-and-windy, apparent temperature in between.

Why does wind chill stop working above 10°C?

The NWS 2001 formula is a regression — a polynomial fit to experimental data. The experiments were done with subjects in cold conditions; the formula's coefficients only describe physiology in that range.

Above 10°C, blowing wind still has a cooling effect (especially on sweaty skin), but the mechanism shifts: evaporative cooling dominates over conductive heat loss. The formula doesn't account for humidity (which now matters), and extrapolating outside its calibrated range gives nonsense results — at 25°C with 30 km/h wind, the formula would predict you feel warmer than the air, which is obviously wrong.

The BoM apparent temperature handles this regime instead. It includes a wind term that's still negative (wind cools you) but combined with a humidity term that captures the evaporative effect properly. That's why the calculator switches from wind chill to apparent temperature as you cross 10°C.

For the same reason, heat index stops working below 80°F (27°C) — below that threshold the regression's polynomial is dominated by the wrong terms and the formula returns the air temperature plus noise.

How accurate are these formulas?

Each formula was validated against the model it approximates, with a stated accuracy of about ±1.3°C for the NWS heat index and ±1°C for wind chill within their respective applicable ranges. Real human perception varies more than that — clothing, activity level, age, hydration, and acclimatization all affect how 'feels like' actually feels.

A few specific limitations:

- **Wind chill** is for *exposed skin only*. If you're bundled up, your face still feels the wind chill but your body doesn't. The formula doesn't model your gear.
- **Heat index** is for *shaded conditions*. Direct sun adds 8–15°F (4–8°C) to the apparent temperature; this isn't included.
- **Dew point** assumes the Magnus formula approximation, which is accurate within ±0.3°C for temperatures −45°C to 60°C.
- **Absolute humidity** uses the Clausius-Clapeyron approximation, accurate within 1% in the meteorologically relevant range.

For heat-related warnings: NOAA issues an Excessive Heat Warning when heat index is forecast to reach 105°F (41°C) for at least 2 days, with overnight lows above 75°F (24°C). For cold: a Wind Chill Warning is issued at −25°F (−32°C) wind chill or colder, indicating frostbite within 10 minutes for exposed skin.

What's dew point and why does it matter more than humidity?

Dew point is the temperature at which water vapor in the air condenses into dew. It's a more honest measure of how 'humid' it feels than relative humidity (RH) because it's an *absolute* quantity — a dew point of 21°C means the same amount of moisture is in the air, whether the temperature is 25°C or 35°C.

Relative humidity is misleading because cold air can hold less water. At 5°C, 90% RH is only 0.0048 kg of water per kg of dry air; at 30°C, 60% RH is 0.0163 kg/kg — more than three times wetter, despite lower percentage. That's why winter air feels dry even at 90% RH and summer air feels muggy at 60% RH.

Dew point comfort thresholds, widely used by meteorologists:

- **≤ 10°C (50°F)** — dry. Skin loses moisture easily, lips and eyes may feel parched.
- **10–16°C** — comfortable for most people.
- **16–18°C** — noticeably humid.
- **18–21°C** — uncomfortable, sticky.
- **21–24°C** — very uncomfortable, oppressive.
- **≥ 24°C** — miserable. Tropical conditions; major exertion is risky.

US heat advisories often use dew point directly as their threshold rather than RH or heat index, because dew point predicts heat-stroke risk more reliably.

How does relative humidity, dew point, and absolute humidity relate?

Three different ways to describe the same air mass:

- **Relative humidity (RH, %)** — water vapor in the air *relative to* the maximum the air can hold at that temperature. Easy to measure (hygrometer), but its meaning shifts with temperature.
- **Dew point (°C)** — the temperature you'd have to cool the air to for it to be saturated (RH = 100%). Independent of current temperature; better for comfort comparison.
- **Absolute humidity (g/m³)** — the actual mass of water vapor per cubic meter of air. Independent of pressure and temperature; useful for industrial drying, HVAC sizing, museum conservation.

They're related by the Clausius-Clapeyron equation. If you know any two, you can compute the third. This calculator shows all three so you can see them side by side.

Example: at 30°C and 60% RH, the dew point is 21.4°C and the absolute humidity is 18.4 g/m³. Cool the same air to 22°C without removing moisture and the RH jumps to ~95% — the air still has 18.4 g/m³ of water, but now near its saturation limit. This is why glasses fog when entering a warm humid room from cold outside: not added moisture, just suddenly much higher *relative* humidity.

Why isn't shade or sun in the formula?

Heat index assumes shaded conditions and a light breeze. The NWS publishes a separate adjustment table: in direct sunlight, add about 5°F (3°C) to the heat index for full sun, less for partial cloud. This is why staying in shade matters so much on a hot day — the actual thermal stress on the body can be 8–15°F (4–8°C) higher in the sun.

The calculator doesn't ask about cloud cover because (1) it's hard for the user to estimate precisely, and (2) the adjustment is essentially additive — you can mentally add 3°C if you're standing in direct sunlight.

Other things the formulas don't include but matter:

- **Activity level** — running raises core temperature; the same heat index that's 'caution' for someone walking is 'danger' for someone exercising.
- **Clothing** — light cotton vs. dark synthetic changes radiant heat absorption dramatically.
- **Hydration** — dehydration impairs sweat production and raises thermal stress.
- **Acclimatization** — people who live in hot climates tolerate higher heat indices.
- **Age** — older adults and young children are more sensitive at every temperature.

Use the calculator as a baseline, not a personal guarantee. If conditions are flagged as 'Danger' or worse, take the warning seriously regardless of how you personally feel.

How fast does frostbite happen at low wind chill?

Frostbite risk is the reason wind chill matters. The NWS publishes a guideline based on the 2001 formula:

- **Wind chill warmer than −18°C (0°F)** — minimal frostbite risk for healthy adults.
- **−18°C to −28°C (0 to −18°F)** — frostbite possible within 30 minutes on exposed skin.
- **−28°C to −40°C (−18 to −40°F)** — frostbite within 10 minutes.
- **Colder than −40°C (−40°F)** — frostbite within 5 minutes; bare skin freezes nearly instantly.

The formula was specifically designed so that frostbite risk maps to wind chill, not to air temperature. A windless −20°C night is unpleasant but rarely frostbite-inducing; a windy −5°C afternoon can be more dangerous if you're outside for an hour with exposed cheeks.

Clothing helps enormously: face, hands, and ears are usually the failure points. Modern fabrics (merino wool, fleece, technical synthetics) plus a windproof outer layer can let you tolerate wind chills down to −30°C indefinitely if the rest of your body is properly insulated.

The calculator shows the frostbite-risk category for any wind chill it computes, so you can see at a glance whether the current conditions warrant covering up.

Is this calculator private?

Yes. Every calculation runs in your browser via plain JavaScript:

- The five formulas (NWS wind chill, NWS heat index, BoM apparent temperature, Magnus dew point, Clausius-Clapeyron absolute humidity) are mathematical expressions in the code — no API is contacted.
- The page loads the site's standard assets (Bootstrap CSS, icons) but no third-party weather or location service is called.
- No telemetry of which temperatures or conditions you entered.
- The five scenario buttons populate the input fields with built-in numeric presets; nothing is downloaded.

Verify by opening DevTools → Network and watching the panel while you type — no requests should fire when you change a value or click a scenario. The tool works offline once the page has loaded, and any weather conditions you input stay on your device.

Key Features

• Three official formulas in one tool: NWS wind chill, NWS heat index, BoM apparent temperature
• Magnus formula dew point and Clausius-Clapeyron absolute humidity
• Auto-picks the most applicable formula as the primary result
• °C and °F temperature units, with km/h, mph, m/s wind speed units
• Color-coded risk categories (caution, danger, frostbite risk in 30/10/5 min)
• Five quick-scenario presets for common weather situations
• Shows when each formula is out of its applicable range and why
• Comfort note for dew point (dry / comfortable / sticky / oppressive)
• Real-time recalculation as you type
• Works for cold (−40°C) to extreme heat (+50°C)
• Mobile-friendly layout with side-by-side metrics
• Pure JavaScript — no external libraries
• Works offline after first load
• No external API for weather data
• 100% client-side — your inputs stay in your browser