Sunday’s high temp of 42 degrees was slightly above normal for December 1st. An influx of warmer air will begin to flow across the area Monday night. Overrunning, a condition that exists when warm, moist air rides up, and over cool air near the ground, is forecast to develop. This will increase rain chances late Monday and Monday night. Snow or sleet may mix with the rain at times. Clouds, and increasing SE winds will keep temps above freezing, and no snow or ice accumulation is expected. Mild air is then forecast to overspread the region Tuesday, with temps rising to mid-November levels. These readings are nowhere near the warmth during the opening days of December, 1970 when high temps hovered around 70 degrees. Polar air is slated to arrive Wednesday night, sending temps tumbling to the coldest levels so far this season.
There are days when jet contrails cover much of the sky. Do contrails make any change to our weather, specifically to temperature?
Bud Bond, Gilberts, Ill.
Dr. David Travis, Professor of Geography and Geology at the University of Wisconsin, reports that contrails do, indeed, affect temperatures. His research indicates that contrails reduce daytime temperatures by blocking sunlight and they raise nighttime temperatures by radiating heat back down to the surface. The net result is a two-degree reduction in the range between afternoon high and nighttime low temperatures.
The grounding of aircraft following the Sept, 11, 2001, terrorist attacks enabled meteorologists to compare temperatures under contrail-free skies with readings in similar weather conditions when contrails were present.
Temperatures on Saturday reached 50 degrees or higher across most of the area. This was the warmest day in Chicago since November 21st, when the official high was 51 degrees. Modest cooling is expected Sunday and Monday as winds turn north to northeast off chilly Great lakes waters. Computer guidance has been very consistent in suggesting major weather changes during the upcoming work week. Intensifying low pressure is forecast to pass west and north of the region on Wednesday, drawing mild, rainy weather into the Midwest, on howling south winds. Polar air massing across western portions of Canada and the U.S. is then forecast to begin spreading east. By Friday, a large amplitude dip in the jet stream is expected to develop, allowing polar air to reach the Midwest. Temperatures next weekend are forecast to remain well below freezing.