Introduction
This calculator estimates apparent temperature—often called “feels like”—using the two most common U.S. weather indices: windchill for cold, windy conditions and heat index for hot, humid conditions. The goal is practical: help you translate a thermometer reading into a more realistic sense of comfort and risk when you’re outside.
Windchill focuses on how moving air increases heat loss from exposed skin. Heat index focuses on how humidity slows sweat evaporation, making it harder for your body to cool itself. Both are simplified models used by weather services for public guidance.
How to use this calculator
- Enter air temperature in °F.
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Choose a calculation type:
- Auto-detect uses windchill at ≤ 50°F and heat index at ≥ 80°F.
- Windchill asks for wind speed (mph).
- Heat Index asks for relative humidity (%).
- Optionally select activity level and clothing level. These do not change the official formulas, but they do affect the safety notes shown in the results.
- Press Calculate Apparent Temperature to see the feels-like value, risk category, and recommendations.
Tip: If your temperature is between 50°F and 80°F, neither index is typically used. In that moderate range, sun, shade, clothing, and exertion often matter more than windchill/heat index.
Formula (and assumptions)
Windchill (U.S. National Weather Service)
Used when air temperature is at or below 50°F and wind speed is above 3 mph. Wind speed is assumed to be measured at about 5 feet above ground.
- Twc: windchill (°F)
- Ta: air temperature (°F)
- V: wind speed (mph)
Heat index (Rothfusz regression)
Used when air temperature is typically 80°F or higher. The calculator uses the standard multi-term regression and includes common adjustments for very low or very high humidity.
In plain terms: heat index rises quickly when both temperature and humidity are high, because sweat evaporates more slowly.
Worked examples
Windchill example: Air temperature 20°F with wind 15 mph produces a windchill near the low teens (°F). That’s why a “20-degree day” can feel much colder when it’s breezy.
Heat index example: Air temperature 95°F with 80% humidity yields a heat index around 124°F (see the chart below). Even short outdoor work can become dangerous without shade, water, and rest.
Safety notes and limitations
These indices are guidance tools, not medical advice. They do not directly account for sun exposure, wet clothing, altitude, or individual health conditions. Direct sun can add roughly 10–15°F to perceived heat stress, and wet/windy conditions can accelerate cooling beyond what windchill alone suggests.
If you feel symptoms of heat illness (confusion, fainting, nausea) or cold injury (numbness, uncontrollable shivering), seek help and get to a safer environment immediately.
Risk categories (quick reference)
The calculator’s result includes a risk label based on commonly used public guidance thresholds. For cold, the key concern is how quickly exposed skin can freeze (frostbite). For heat, the key concern is heat exhaustion and heat stroke risk, especially during exertion.
| Windchill (°F) | Risk Level | Frostbite Time | Precautions |
|---|---|---|---|
| Above 0°F | Low | Low risk | Dress warmly; cover exposed skin |
| 0 to -20°F | Moderate | 30 minutes | Minimize skin exposure; limit outdoor time |
| -20 to -40°F | High | 10–30 minutes | Cover all exposed skin; avoid prolonged exposure |
| -40 to -60°F | Very High | 5–10 minutes | Avoid outdoor activity; frostbite imminent |
| Below -60°F | Extreme | <5 minutes | Emergency shelter required; life-threatening |
| Heat Index (°F) | Risk Level | Primary Concerns | Precautions |
|---|---|---|---|
| 80–90°F | Caution | Fatigue possible | Stay hydrated; take breaks if exerting |
| 90–103°F | Extreme Caution | Heat cramps, exhaustion possible | Limit strenuous activity; frequent hydration |
| 103–125°F | Danger | Heat exhaustion likely; stroke possible | Minimize outdoor exposure; rest in shade/AC |
| 125°F+ | Extreme Danger | Heat stroke highly likely | Avoid outdoor activity; seek air conditioning |
FAQ
Why does auto-detect hide inputs sometimes?
Auto-detect only shows wind speed when the temperature is ≤ 50°F and only shows humidity when the temperature is ≥ 80°F. In the middle range, neither index is typically used, so both extra inputs are hidden.
Does windchill or heat index include sunlight?
No. Both indices assume shade and standard measurement conditions. Bright sun can make hot conditions feel significantly hotter, and wet clothing can make cold conditions more dangerous.