Wind chill advisory issued for parts of Rochester region. What is wind chill?

A bitter cold weekend with subzero wind chills will cool parts of the Rochester region this weekend, according to the National Weather Service in Buffalo.

A wind chill advisory was issued for Saturday, between midnight and 11 a.m. for Ontario, Livingston, Wyoming, Allegany and Cattaraugus counties. Forecasters predict "very cold wind chills" as low as 15 to 20 below zero. Monroe County, which was not part of the advisory, will still feel cold, but wind chills are not expected to dip below -10 and it does not meet the standards for an advisory, said Weather Service Meteorologist Jim Mitchell. (The air temperature will reach the mid-teens in Rochester on Saturday.)

"It's going to be pretty chilly overnight and very cold on Saturday, as the wind chill will not rise above zero all day," he said.

Sunday in comparison will feel balmy, he said. Winds are expected to shift and after another cold night (high is predicted to be zero) the high will rise to the low 20s, Mitchell said.

What is wind chill?

As temperatures plummet and the winds howl this winter, you'll hear about the dangers of the "wind chill." So what is it?

The wind chill temperature is how cold people (and animals) feel while outside. Wind chill is based on the rate of heat loss from exposed skin caused by the combination of wind and cold, according to the National Weather Service.

As the wind increases, it draws heat from the body, driving down skin temperature and eventually the body's internal temperature.

“It’s how it feels when you’re out in cold weather with wind blowing,” Bob Oravec, a lead forecaster for the Weather Service at the Weather Prediction Center, told Popular Science.

What do wind chill numbers mean?

Therefore, the wind makes it feel much colder than it really is, so it's been described as a "feels-like" number. If the temperature is 0 degrees and the wind is blowing at 15 mph, the wind chill is 19 degrees below zero.

Low wind chill numbers shouldn't keep you from going out; they should encourage you to dress properly, however. For example, when the wind chill is around 40 degrees below zero, exposed skin can freeze in as little as 10 minutes.

How long can you be in wind chill?

Remember that the wind chill temperature is not some different kind of temperature: Wind chill alone can never make an object colder than the air temperature. Even if the wind chill is 10 degrees, you won't get frostbite or freeze your car's radiator if the air temperature is 32 degrees or above.

The Weather Service began to include wind chill in their forecasts in the early 1970s, several decades after Antarctic explorers first published research about it in the late 1930s.

The U.S. and Canadian weather services revised the wind chill index in 2001, based on greater scientific knowledge and on experiments that tested how fast the faces of volunteers cooled in a wind tunnel with various combinations of wind and temperature.

How is wind chill created?

Here is a chart from the Weather Service for figuring out the wind chill, which is based on a formula that combines the current temperature with the current wind speed:

This article originally appeared on Rochester Democrat and Chronicle: Rochester NY Weather: Wind chill advisory issued. What is wind chill?