Behind the Forecast: How wind chill is calculated

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

Science Behind the Forecast: How wind chill is calculated

LOUISVILLE, KY (WAVE) - It seems like we’ve skipped over fall and headed straight for winter this year. With much colder temperatures in the forecast for the upcoming week, the wind chill will have a big impact on our plans and, of course, our outfits.

The wind chill explains how cold people and animals feel when outside. It is based on the amount and rate of heat lost from skin exposed to wind and cold.

It is only defined for temperatures at or below 50° and wind speeds above 3 mph. Sunshine can actually increase the wind chill temperature by 10° to 18°F.

As wind speed picks up, it pulls heat from our bodies and drives down skin temperatures; eventually it will also drop internal body temperatures. This is what makes it feel colder.

The wind chill doesn’t have as much of an effect on inanimate objects. It only shortens the amount of time it takes for an object to cool down. An inanimate object, like your car, will not drop below the air temperature.

Actual air temperatures must be below 32° for frostbite to occur on exposed flesh; wind chills below freezing cannot cause frostbite. Hypothermia is a definitive possibility in temperature is above freezing and there’s extended exposure to the cold.

Wind chill is calculated using the formula below:

Windchill (°F) = 35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16)

  • T = Air temperature in Fahrenheit
  • V = Wind Speed in mph

The formula was introduced by the NWS in late 2001 after the NWS and the Meteorological Service of Canada worked to improve the old wind chill formula.

Now while everyone is affected by the wind and cold a little differently, the formula is based on the following:

  • Wind speed is measured at an average height of 5 feet
  • It’s based on a human face model
  • Incorporates modern heat transfer theory
  • It lowers the normal calm wind threshold from 4 to 3 mph
  • Assumes there’s no sunshine (clear night sky)
  • Uses a consistent standard for skin tissue resistance
This chart quickly shows wind chills based on various wind speeds and temperatures.
This chart quickly shows wind chills based on various wind speeds and temperatures. (Source: National Weather Service)

The National Weather Service (NWS) issues wind chill warnings, watches and advisories for specific situations:

  • Wind Chill Warning: NWS issues a wind chill warning when dangerously cold wind chill values are expected or occurring. If you are in an area with a wind chill warning, avoid going outside during the coldest parts of the day. If you do go outside, dress in layers, cover exposed skin, and make sure at least one other person knows your whereabouts. Update them when you arrive safely at your destination.
  • Wind Chill Watch: Be Prepared: NWS issues a wind chill watch when dangerously cold wind chill values are possible. As with a warning, adjust your plans to avoid being outside during the coldest parts of the day. Make sure your car has at least a half a tank of gas, and update your winter survival kit.
  • Wind Chill Advisory: Be Aware: NWS issues a wind chill advisory when seasonably cold wind chill values but not extremely cold values are expected or occurring. Be sure you and your loved ones dress appropriately and cover exposed skin when venturing outdoors.
Wind Chill Advisory criteria for each NWS office in the north-central U.S.
Wind Chill Advisory criteria for each NWS office in the north-central U.S. (Source: National Weather Service)

Each office has their own criteria for issuing Warnings, Watches and Advisories

  • Wind Chill Advisory: Wind chills of -10° F with winds of 10 mph or greater 
  • Wind Chill Warning: Wind chills of -25°F or colder with winds of 10 mph or greater 

WILMINGTON:

  • Wind Chill Advisory: Wind chill of -10º F to -24º F with wind of 6 MPH or greater
  • Wind Chill Watch/Warning: Wind chill of -25º F or colder or less with wind of 6 MPH or greater 
  • Wind Chill Advisory: Wind chill of -15°F to -24°F with winds of 10 mph or greater 
  • Wind Chill Warning: Wind chill of -25°F or colder with winds of 10 mph or greater 
Wind Chill Warning criteria for each NWS office in the north-central U.S.
Wind Chill Warning criteria for each NWS office in the north-central U.S. (Source: National Weather Service)

Copyright 2018 WAVE 3 News. All rights reserved.