Calcolatore Wind Chill - Temperatura Percepita
Calcola la temperatura wind chill (temperatura percepita) dalla temperatura dell aria e dalla velocita del vento. Calcolatore gratuito. Senza registrazione.
How Wind Chill Is Calculated
Wind chill temperature estimates how cold the air feels on exposed human skin when wind accelerates heat loss. The current formula used by the US National Weather Service and Environment Canada (adopted in 2001) is:
Wind Chill (°C) = 13.12 + 0.6215 × T − 11.37 × V0.16 + 0.3965 × T × V0.16
Where T is the air temperature in °C and V is the wind speed in km/h measured at 10 meters height (standard weather station height).
Example: At −5°C with 30 km/h wind: Wind Chill = 13.12 + 0.6215 × (−5) − 11.37 × 300.16 + 0.3965 × (−5) × 300.16 = 13.12 − 3.11 − 19.48 − 6.79 = −12.3°C. The air is −5°C but feels like −12.3°C on exposed skin.
The formula is valid when air temperature is at or below 10°C and wind speed is above 4.8 km/h. Above 10°C, the actual temperature is already close to what bare skin perceives, making wind chill less meaningful. At wind speeds below 4.8 km/h, wind has negligible cooling effect beyond natural convection.
Important: wind chill is a subjective measure for human skin, not a physical temperature. A thermometer will still read the actual air temperature regardless of wind. Objects (cars, pipes, water) cannot cool below the actual air temperature from wind alone — wind only accelerates the rate at which they reach ambient temperature.
Wind Chill Reference Table
Use this table to quickly estimate wind chill. Values are in °C.
| Temp \ Wind | 10 km/h | 20 km/h | 30 km/h | 40 km/h | 50 km/h | 60 km/h |
|---|---|---|---|---|---|---|
| 5°C | 3.3 | 1.5 | 0.3 | −0.6 | −1.3 | −1.9 |
| 0°C | −2.2 | −4.4 | −5.9 | −7.0 | −7.9 | −8.6 |
| −5°C | −7.7 | −10.4 | −12.1 | −13.4 | −14.5 | −15.3 |
| −10°C | −13.2 | −16.3 | −18.3 | −19.9 | −21.0 | −22.0 |
| −15°C | −18.7 | −22.3 | −24.5 | −26.3 | −27.6 | −28.7 |
| −20°C | −24.2 | −28.3 | −30.8 | −32.7 | −34.2 | −35.4 |
| −25°C | −29.7 | −34.2 | −37.0 | −39.1 | −40.7 | −42.1 |
| −30°C | −35.2 | −40.2 | −43.2 | −45.6 | −47.3 | −48.8 |
Frostbite risk levels: Above −10°C: low risk. −10°C to −25°C: moderate risk — cover exposed skin. −25°C to −35°C: frostbite possible in 10–30 minutes. −35°C to −45°C: frostbite likely in 5–10 minutes. Below −45°C: frostbite in under 5 minutes — outdoor exposure is dangerous.
Common Use Cases
- Running in cold weather: Runners create their own wind by moving forward. A runner doing 12 km/h into a 15 km/h headwind effectively faces 27 km/h of wind against exposed skin. At −5°C actual temperature with 27 km/h effective wind, the wind chill drops to about −12°C. This determines clothing choices: face protection, gloves, and layering strategy. Many runners use the pace calculator alongside wind chill to plan winter training sessions.
- Winter hiking and mountaineering: At altitude, temperatures drop roughly 6.5°C per 1,000m of elevation gain. Combined with ridge-top winds that can exceed 60 km/h, summit conditions often produce extreme wind chill even when valley temperatures seem moderate. A summit at −10°C with 50 km/h wind creates a wind chill of −21°C — frostbite territory for exposed skin in under 30 minutes.
- Commuting and school closures: Many school districts use wind chill thresholds to determine outdoor recess policies or school closures. Common thresholds: below −20°C wind chill triggers indoor recess; below −35°C may cancel school transportation.
- Workplace safety: Occupational health guidelines require employers to provide warming breaks for outdoor workers when wind chill drops below certain thresholds. OSHA recommends work-warming schedules based on wind chill: at −25°C, outdoor workers should warm up for 10 minutes every 75 minutes.
- Pet and livestock safety: While the formula is calibrated for human skin, wind chill indicates increased risk for animals too. Dogs with short coats need protection below −10°C wind chill. Livestock water systems freeze faster in high wind, and animals burn more calories to maintain body temperature.
Step-by-Step Examples
Example 1: Morning Run in Winter
You plan a 10K run at 5:30/km pace (roughly 11 km/h). The forecast shows −3°C with 20 km/h wind from the north. Your planned route goes north for 5K then returns south.
- Outbound (into wind): effective wind = 20 + 11 = 31 km/h
- Wind chill outbound = 13.12 + 0.6215×(−3) − 11.37×310.16 + 0.3965×(−3)×310.16 ≈ −11.4°C
- Return (wind at back): effective wind = 20 − 11 = 9 km/h
- Wind chill return = 13.12 + 0.6215×(−3) − 11.37×90.16 + 0.3965×(−3)×90.16 ≈ −5.8°C
- Recommendation: Dress for the outbound wind chill (−11°C). Wear a buff/balaclava, wind-resistant outer layer, and gloves. You'll warm up on the return leg.
Example 2: Ski Resort Conditions
The ski resort base reports −8°C with 15 km/h wind. The summit (600m higher) is about −12°C with exposed ridge winds of 45 km/h.
- Base wind chill = 13.12 + 0.6215×(−8) − 11.37×150.16 + 0.3965×(−8)×150.16 ≈ −13.9°C
- Summit wind chill = 13.12 + 0.6215×(−12) − 11.37×450.16 + 0.3965×(−12)×450.16 ≈ −24.4°C
- At −24.4°C wind chill, frostbite can occur on exposed skin in 10–30 minutes. Full face coverage and insulated gloves are essential at the summit.
Example 3: Fahrenheit Conversion
An American runner checks the weather: 20°F (−6.7°C) with 25 mph (40 km/h) wind.
- Convert to metric: T = −6.7°C, V = 40 km/h
- Wind chill = 13.12 + 0.6215×(−6.7) − 11.37×400.16 + 0.3965×(−6.7)×400.16 ≈ −15.7°C (3.7°F)
- Despite the actual temperature being above −7°C, the feels-like temperature is below −15°C — a significant difference that demands proper cold-weather gear.
Tips and Common Mistakes
- Don't confuse wind chill with actual temperature: Pipes won't freeze at a temperature above 0°C regardless of wind chill. Wind chill only describes how quickly heat-generating organisms (humans, animals) lose heat. If it's −2°C with extreme wind, water still freezes at 0°C — wind chill doesn't change that threshold.
- Account for your own movement: When running, cycling, or skiing, add your speed to the headwind (or subtract for tailwind). A cyclist doing 30 km/h on a calm −5°C day faces wind chill equivalent to a 30 km/h wind — roughly −12°C on exposed skin.
- Wind measurements vary by height: Weather stations measure wind at 10 meters. At ground level, wind is typically 50–70% of the reported speed due to surface friction. On exposed ridges or open water, wind can exceed reported values.
- Dress in layers, not bulk: Multiple thin layers trap air (insulation) and let you adjust as effort level changes. A base layer (moisture-wicking), mid layer (insulating), and shell (wind/water-proof) system is far more effective than one thick coat.
- Cover the extremities first: Fingers, toes, ears, and nose are most vulnerable to frostbite because they have high surface-area-to-volume ratios and are the first areas where the body restricts blood flow in cold conditions.
- Wet skin amplifies wind chill: Evaporative cooling from sweat or rain dramatically increases heat loss. In cold, windy, wet conditions, hypothermia risk is much higher than the wind chill alone suggests. This is why wind-proof and water-proof shells are critical.
Wind Chill vs Heat Index: Cold and Hot "Feels Like"
Wind chill and heat index are complementary measures that describe how weather feels compared to the actual temperature. Both adjust the raw temperature based on factors that affect human thermoregulation.
| Factor | Wind Chill | Heat Index |
|---|---|---|
| Applies when | Temperature ≤ 10°C (50°F) | Temperature ≥ 27°C (80°F) |
| Key variable | Wind speed | Relative humidity |
| Effect on body | Accelerates heat loss from skin | Impairs sweat evaporation, trapping heat |
| Primary risk | Frostbite, hypothermia | Heat exhaustion, heat stroke |
| Objects affected? | No — only warm-blooded organisms | No — only warm-blooded organisms |
| Formula origin | NWS/Environment Canada (2001) | Rothfusz (NWS, 1990) |
Most weather apps show a single "Feels Like" temperature that uses wind chill when cold and heat index when hot. For runners, understanding both is essential for safe training year-round. Use the temperature adjustment calculator to see how temperature affects your expected running performance, and the hydration calculator for fluid needs in extreme conditions.
In the moderate range (10–27°C), the actual temperature closely matches the feels-like temperature, making those conditions ideal for outdoor exercise without special precautions.
Frequently Asked Questions
Does wind chill affect how cold my car engine gets?
No. Wind chill only applies to objects that generate heat (like human skin). A parked car cools to the actual air temperature, not below it. However, wind increases the rate at which the car reaches ambient temperature — so your engine cools faster on a windy night, but reaches the same final temperature.
Is wind chill the same as "feels like" temperature?
Wind chill is one component. In cold weather (below 10°C), "feels like" equals wind chill. In warm, humid weather, "feels like" uses the Heat Index formula, which accounts for humidity's effect on your body's ability to cool through sweat. Most weather apps automatically switch between the two based on current conditions.
Can I still run safely in extreme wind chill?
Yes, with proper precautions. Down to about −20°C wind chill, running is safe with appropriate layering, face coverage, and extremity protection. Below −25°C, exposed skin can develop frostbite in under 15 minutes, so full skin coverage is essential. Below −35°C, most running experts recommend indoor alternatives. Your lungs are not at risk of "freezing" — air warms to near body temperature by the time it reaches the lower airways.
At what wind chill should I cancel outdoor activities?
General guidelines: −20°C to −25°C: limit time outdoors, ensure all skin is covered. −25°C to −35°C: shorten exposure, take warming breaks every 20–30 minutes. Below −35°C: avoid extended outdoor exposure. Below −45°C: outdoor activity is dangerous for any duration. These thresholds apply to healthy adults; children, elderly, and those with cardiovascular conditions should use more conservative limits.
How does altitude affect wind chill?
Altitude itself doesn't change the wind chill formula, but altitude amplifies its inputs. Temperature drops approximately 6.5°C per 1,000m of elevation gain, and wind speeds increase at higher elevations due to reduced surface friction. A mountain summit at 3,000m might be 20°C colder than the valley with winds 2–3× stronger, creating dramatically different wind chill conditions.
Why was the wind chill formula changed in 2001?
The old formula (Siple and Passel, 1945) was based on experiments freezing water in plastic cylinders on an Antarctic expedition. It significantly overestimated wind chill for human skin. The 2001 formula was developed using human trials with volunteer subjects and modern heat transfer modeling, producing more accurate "feels like" values at face height (1.5m) rather than weather station height (10m).
Does humidity affect wind chill?
The current wind chill formula does not account for humidity. In practice, humid cold air can feel colder than dry cold air at the same temperature because moisture conducts heat away from skin faster than dry air. This is why a damp 0°C day often feels colder than a dry −5°C day. Some researchers advocate for a revised formula that includes humidity, but no standard has been adopted yet.
How should I dress for running in different wind chill ranges?
General guidelines: 0°C to −10°C: long sleeves, tights, light gloves, ear cover. −10°C to −20°C: add wind-proof shell, thicker gloves, buff/neck gaiter, double-layer tights. −20°C to −30°C: full face coverage (balaclava), insulated gloves, multiple layers, consider a pace adjustment of 10–20% slower. Below −30°C: full expedition-weight coverage or run indoors.