Wind chill formula (NWS)

The National Weather Service uses this formula (valid for T ≤ 50°F and wind ≥ 3 mph):

Wind Chill (°F) = 35.74 + 0.6215T − 35.75(V^0.16) + 0.4275T(V^0.16)
where T = air temp (°F), V = wind speed (mph)

Frostbite risk

Wind chill	Frostbite risk
≥0°F (−18°C)	Low risk with normal outdoor clothing
−20 to −18°F (−29°C)	Frostbite in 30 min on exposed skin
−20 to −40°F (−34°C)	Frostbite in 10 min
<−40°F (<−40°C)	Frostbite in 5 minutes or less

Wind chill history and methodology

The original wind chill index was developed by Paul Siple and Charles Passel in 1945, based on the time required to freeze water in a plastic cylinder. That formula significantly overestimated the cold sensation experienced by humans. In 2001, the NWS and Meteorological Service of Canada replaced it with the current formula, derived from thermal manikin experiments and human volunteer studies that directly measured heat loss from actual skin. The new formula is more conservative and more accurate for real-world conditions.

Wet vs. dry cold

Wind chill assumes dry conditions. Wet clothing conducts heat approximately 25× faster than dry clothing, dramatically increasing heat loss beyond what the wind chill formula predicts. This is why staying dry is critical in cold weather; the “feels like” temperature for a wet person is far colder than the reported wind chill.

Prevention and protection

Layers: moisture-wicking base layer, insulating mid-layer, windproof outer shell.
Cover extremities: hands, feet, and face lose heat fastest; use insulated gloves, wool socks, and a balaclava.
OSHA cold stress guidelines: for outdoor workers, OSHA recommends warm break shelters, buddy systems, and limiting exposure time at wind chills below −15°C (5°F).
Rule of thumb: −15°C in calm air feels approximately the same as 0°C with a 50 km/h (31 mph) wind for frostbite risk.