The Definition of Wind Chill

Definition of Wind Chill

Wind chill is the cooling effect of wind combined with low air temperature on exposed skin. Moving air strips away the thin layer of warm air next to the skin, so the body loses heat faster than it would in still air at the same temperature. The wind chill temperature expresses this as the still-air temperature that would cause the same rate of heat loss from bare skin.

In the United States and Canada, the National Weather Service and Environment and Climate Change Canada use a wind chill index introduced in 2001. It is based on heat loss from the human face, using wind speed at about 5 feet above the ground. Canada reports the index without a degree symbol, while the U.S. reports it in degrees Fahrenheit.

For outdoor workers, wind chill is one of the main factors in planning work in cold weather. It indicates how quickly frostbite can develop on exposed skin and helps set warm-up breaks and clothing requirements.

How Wind Chill Is Calculated

The NWS formula is: Wind Chill (°F) = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275TV^0.16, where T is air temperature in degrees Fahrenheit and V is wind speed in miles per hour. It is valid for temperatures at or below 50°F and wind speeds above 3 mph.

For example, an air temperature of 0°F with a 15 mph wind gives a wind chill of about −19°F. The National Weather Service notes that frostbite can occur in about 30 minutes at a wind chill of −18°F, and the time drops sharply at lower wind chills.

What wind chill does not do

Wind chill only applies to living bodies that produce heat. Wind cannot cool an object below the actual air temperature. A vehicle radiator will not freeze if the air temperature is above 32°F, even if the wind chill is below freezing. Wind chill also does not account for sun, which can make conditions feel warmer, or wet clothing, which speeds heat loss.

Wind Chill and Cold Stress at Work

Cold stress can lead to hypothermia, frostbite, trench foot and chilblains, and it reduces dexterity, which raises the risk of other injuries. Federal OSHA has no specific cold stress standard, but employers must protect workers from recognized hazards under the General Duty Clause. ACGIH publishes a work and warm-up schedule that sets maximum work periods and breaks based on air temperature and wind speed.

Controls for cold, windy conditions

• Check the forecast and plan heavy or long outdoor tasks for the warmest part of the day.
• Use windbreaks, enclosures or heated shelters near the work area.
• Schedule warm-up breaks in heated areas, more often as wind chill drops.
• Provide layered clothing with a wind-resistant outer layer, insulated gloves, hats, face protection and waterproof boots.
• Keep extra dry clothing on hand, since wet clothing greatly increases heat loss.
• Use a buddy system so workers can watch each other for early signs of frostbite and hypothermia.
• Train workers on cold stress symptoms and first aid.

Example of Wind Chill

A utility crew was called out to restore power lines after a winter storm. The air temperature was 5°F, but winds of 25 mph made the wind chill about −17°F. The crew supervisor used the company's cold weather plan, which tied work and warm-up periods to wind chill.

Crew members wore insulated, flame-resistant layers with face coverings, and the supervisor rotated linemen in the bucket every 40 minutes so they could warm up in the heated truck. A crew member was assigned to check faces and hands at each break for early signs of frostbite. The repairs were finished without any cold injuries.

Frequently Asked Questions: Wind Chill

Can wind chill freeze water or pipes?

No. Wind chill only affects living bodies. Water and pipes will not freeze unless the actual air temperature is at or below freezing.

What wind chill is dangerous for outdoor work?

Risk rises steadily as wind chill drops. At around −18°F, frostbite can occur in about 30 minutes on exposed skin. Many employers set stricter controls or limit nonessential work at lower wind chills.

Does wind chill affect people wearing protective clothing?

Good clothing greatly reduces the effect of wind on covered skin. Wind chill mainly indicates the risk to exposed skin, but wind can still penetrate clothing that is not wind-resistant.