According to the local Weather Underground website, at that same moment a thermometer at Greenhills School registered 2.3 degrees. Yet the official temperature at the Ann Arbor Airport came in at 9 below.
Turns out that the airport represents a worst-case scenario. "The Ann Arbor Airport sensor is very poorly sited, located at a low spot (swamp) area of the airport," emails U-M weather observer Dennis Kahlbaum. "On clear, calm nights, cold air pools around the sensor, hence the very low temperature readings." Just 100 feet away, he says, temperatures may be five or more degrees Fahrenheit warmer.
"The Ann Arbor area is ripe for microclimates," Kahlbaum explains. "Areas around and north of the Huron River are hilly, while areas south are relatively flat. The hilly areas can experience all sorts of microclimates, ranging from cold-air pooling to inversion intrusion." Prevailing winds also carry heat given off by the city to the north and east. That's why areas to the south and west often get the worst of it-and why a morning walk along the Huron can sometimes chill you to the bone.
[Originally published in March, 2009.]