A straightening of the jet stream promises warmer air in Chicago and the Northeast by Friday of this week, Tom Skilling, weatherman for WGN-TV in Chicago, tweets. But Valentine’s Day, Sunday, was a snowy day across the Chicago area, as predicted.
— Skilling (@Skilling) February 13, 2016
A weather observer at Midway Airport on the city’s South Side said 2.2 inches of snow that fell there melted down to yield only 0.08 inches of water. That gives a snow:water ratio of about 27:1, much higher than the 12:1 ratio typical for the Chicago area.
With a 12:1 ratio, 1 inch of rain on a warm day drops as much water on the area as 12 inches of snow would bring. With a ratio of 27:1, 1 inch of rain would fluff up to 27 inches of snow.
The ratio of snow:liquid has been as low as 4:1 or 6:1 earlier this season, making for some very wet snow that’s heavy to lift. Light snow, such as the snow that fell today, is usually powdery or fluffy; it’s light to lift.
The day also brought cold temperatures and freezing drizzle in the evening hours, with a temperature of +1°F reported at Waukegan north of the city, according to wunderground.com.
And it was windy. Midway Airport reported a weather reading as follows: KMDW 150553Z 15007KT 5SM -SN BR OVC012 … T10721106. What that means in English is that the temp at 11:53 PM Central time on Sunday was –7.2°C with a wind from the southeast at 7 mph. Light snow and mist was also reported.
With low air temperatures, any wind at all feels cold, but wind speeds above 4 mph have a particularly chilling effect. You can look up the wind chill in a table, which gives you an approximate value, or you can calculate it with the following formula:
where t is the temperature in degrees Fahrenheit and v is the wind speed in miles per hour.
This is the new and improved formula, updated in 2001 by the US National Weather Service and the Canadian Weather Service. It’s based on experiments that tested how fast the faces of volunteers cooled down in a wind tunnel with various combinations of wind and temperature.
(The old formula, used since the 1940s, was based on experiments involving how quickly water would freeze in a plastic cylinder that was exposed to wind in Antarctica.)
A temperature of –7.2°C translates to about 19°F, and the computed wind chill is about 10°F. Note that the effective wind chill is positively affected by sunlight. During the day, not at 11:53 PM, the wind chill is actually about 10–18°F warmer than the formula would compute, depending on the angle and intensity of the sunlight.