The Weather Guys

On the cold and windy days of last week, you probably tried to keep yourself warm by wearing appropriate clothing and seeking shelter from the wind.

It feels colder in the wind because the wind sweeps away heated air in contact with your body and replaces it with colder air.

Whereas still air is a poor heat conductor (which is why storm windows have air trapped between glass panes), moving air is not!

The cooling power of the wind is measured by the wind chill factor. The wind chill describes the increased loss of heat by the movement of the air. The wind chill is relevant to humans and other animals needing to maintain a constant temperature that is higher than the surroundings.

The wind chill factor cannot be measured with a thermometer; it must be computed. The wind chill temperature translates your body’s heat losses under the current temperature and wind conditions into the air temperature with a 3-knot wind that would produce equivalent heat losses.

This is not an easy conversion. The original wind chill formula was devised by Antarctic explorer Paul Siple in 1945. More recent research has revealed some flaws in Siple’s work, such as assuming that the wind at face level is equal to the wind at 33 feet above the surface. The National Weather Service updated its wind chill temperature calculation in November 2001.

Dr. Ed Hopkins, Wisconsin’s assistant state climatologist, has computed the wind chill temperatures for all the hourly temperature and wind speed combinations available from Truax/Dane County Regional airport since January 1948. Madison’s lowest wind chill temperature was -54.3 at 4 a.m. and 5 a.m. on January 20, 1985 — the same day of President Ronald Reagan’s second inauguration, which was held indoors because of the cold Washington, D.C., weather and because it was a Sunday. In the cold air outbreak of last week, the coldest wind chill temperature was -43 at 9 a.m. on Jan. 6.

Southern Wisconsin experienced freezing rain and freezing drizzle just before the start of the holidays.

When particles falling from clouds reach the surface as precipitation, they do so primarily as rain, snow, freezing rain or sleet.

Where we live, most precipitation particles are frozen at one time during their formation. What determines the precipitation type at the surface is the temperature between the cloud bottom and the ground.

In winter, precipitation usually begins falling out of a cloud as ice particles. If the temperature underneath a cloud stays below freezing all the way to the ground, the ice crystals never melt and snow falls.

If the temperature is above freezing below the cloud bottom to the ground, the frozen particles melt into liquid droplets that reach the surface and this is called rain.

Ice storms occur when precipitation particles melt and then fall through a layer of subfreezing air near the ground.

Freezing rain forms when a very shallow layer of cold air is at the surface, causing the freshly melted raindrops to freeze on contact with exposed objects on the ground, which has a temperature below freezing.

Sleet results when the layer of subfreezing air at the surface extends upward far enough so that raindrop freezes into a little ball of ice.

When sleet hits the surface, it bounces and does not coat objects with a sheet of ice. Freezing rain covers everything in a sheet of ice, creating shimmering and treacherous road conditions.

Freezing rain is the precipitation type with the highest rate of accidents and death during the weather event. The number of deaths due to accidents is larger for snow events, but that is because snow is more common than freezing rain. Southern Wisconsin averages only about 20 hours of freezing rain a year.