The cold wave that hit the area from Tuesday, January 13 through Friday, January 16, 2009 occasionally exhibited ideal conditions for cold air drainage due to varying terrain elevation. The typical weather pattern leading to these conditions features a large high pressure system bringing the requisite clear skies, calm wind and dry air.
Clear skies and calm winds allow the dense and heavier cold air to drain into lower areas such as river valleys or broad low prairies. Heat from the day escapes, or radiates into space as long wave radiation. For those who have digital thermometers in their vehicles, you may have noticed how it will fluctuate many degrees while traveling from high to low areas (10 deg differences are not unusual!). See image below:
Sometimes a cloud deck will roll in and temperatures will suddenly halt their free-fall or begin to warm up. This is due to the insulating effect of the clouds. The clouds will reflect the long wave radiation back to earth. The cold air drainage circulation is interrupted and temperatures will stabilize.
The image below shows a topographic map that highlights the varying terrain elevations across southern Wisconsin. The most dramatic differences are out along the Wisconsin and Mississippi Rivers. Also notice the valleys in Rock county, around Janesville. The Kettle Morrain stretching across southeast Wisconsin is a notable geographic feature. The image below is highlighted with areas that are known for being much colder than surrounding areas.
The image below shows the minimum temperatures Friday morning, January 16, 2009. It was the coldest morning during that cold spell. Not all low areas experienced efficient radiative cooling. Other factors will inhibit cooling, such as, urban heat islands, local wind influences and the big one is proximity to Lake Michigan.
Here is a summary of temperatures that morning:
Lone Rock (KLNR) -35 (12th coldest ever. Coldest since -37 on Jan 17 1982) - southwest corner of Sauk County.
Hustisford (Dodge) -32 (Coop)
Argyle (Lafayette) -31 (Coop)
Markisan (Green Lake) -30 (Coop)
Monroe (Green) -28 (Coop)
Black Earth (Dane) -27 (Coop)
Stoughton (Dane) -27 (Coop)
Monroe (KEFT) -27
Mineral Point (KMRJ) -26
Lodi (Columbia) -26 (Coop)
Juneau (KUNU) -26
Afton (Rock) -24 (Coop)
Oconomowoc (Wauk) -24 (Coop)
Watertown (KRYV) -24
Delavan (Walworth) -23 (Coop)
Campbellsport (Fond) -22 (Coop)
Holy Hill (Washington) -22 (Coop)
Fond du Lac (KFLD) -21 (Coldest since records began Jul 2000)
Kenosha (KENW) -21 (Coldest since records began Jul 2000)
Burlington (Racine) -21 (Coop)
Baraboo Dells (KDLL) -21
Janesville (KJVL) -20
Madison (KMSN) -19
Whitnall Park (Milw) -18 (Coop)
Burlington (KBUU) -18
Racine (KRAC) -17 (Coldest since records began Jul 2000)
Waukesha (KUES) -15
Sheboygan (KSBM) -14
Port Washington -13 (Coop)
West Bend (KETB) -13
Milwaukee (KMKE) -12