When it comes to predicting the end of winter, Brookfield Zoo's groundhogs are, in weather terms, partly sunny.
With Groundhog Day on Saturday, the Chicago Sun-Times compared results -- whether the hog saw its shadow and returned to its burrow, which supposedly means six more weeks of winter -- with historical weather data. If the groundhog doesn't see its shadow, winter will end sooner, so the legend goes.
In a complex -- and, oh, OK, totally debatable unscientific formula -- we determined the return of spring by temperatures breaking the 50-degree mark, amounts of sunshine and snowfall levels before and after March 16 -- the six-week mark.
By our reckoning, looking at 10 years of available data, Brookfield's hogs got it right 60 percent of the time.
In 2001, economist Paul Sommers of Vermont's Middlebury College analyzed nearly 50 years of weather and groundhog data about Pennsylvania's Punxsutawney Phil. Based on recorded temps, groundhogs by that name were correct about 70 percent of the time.
In a Mathematical Association of America journal, Sommers wrote that his formula for determining success was TEMP=a + b1 TIME + b2 PHIL.
"PHIL is a dummy variable which is equal to 1 during the six weeks following Feb. 2 and 0 otherwise,'' wrote Sommers.
Of course, everybody knows that. Overall, wrote Sommers: Phil's a "phenomenal phorecaster.''
Patty Anderson, lead keeper at Brookfield Zoo's children's zoo, said the predictive practice may have begun in 18th century Europe, where farmers watched hedgehog activity to schedule spring planting.
This year's chores at the west suburban zoo will be handled by female groundhog Cloudy.
Currently in hibernation, Cloudy takes only one breath a minute, her heart beats a meager four to six times every 60 seconds, and her body temperature has dropped from 90 degrees to 38 degrees, said Anderson.
Groundhogs -- which are actually just big rodents -- go into deep doze when air temperatures drop to 50 degrees or below for an extended period.
Cloudy will emerge (or not) from a Plexiglass burrow at around 10:30 a.m. Saturday in the children's zoo, lured perhaps by a carrot-shaped sweet potato cake.
Saturday's forecast from the National Weather Service: "considerable cloudiness.''
We can only hope.
GROUNDHOG DAY SCORECARD
Legend is that if a groundhog emerges and sees its shadow, there will be six more weeks of winter. If it doesn't see its shadow, spring will arrive sooner. A rundown of recent groundhog history:
2007: Did not see shadow: CORRECT
2006: Did not see shadow: CORRECT
2005: Saw shadow. INCORRECT
2004: Did not see shadow: CORRECT
2003: Did not see shadow: CORRECT
2002: Saw shadow: INCORRECT
2001: Saw shadow: INCORRECT
1998: Did not see shadow: INCORRECT
1997: Saw shadow: CORRECT
1996: Saw shadow: INCORRECT
1995: Did not see shadow: CORRECT
1994: Did not see shadow: INCORRECT