A few parts of the metropolitan area have received a dusting or an inch of snow in December. But at O'Hare there has been only sleet, ice pellets or a few minutes of snow that did not leave a measurable amount. A trace of snow is anything under one-tenth of an inch, Castro said. A trace has been record nine times this month, with the largest amount of one-tenth of an inch occurring Dec. 12 at the other weather station at Midway Airport, Castro said.