There’s no room for doubts when the weather forecast calls for “100 percent chance of precipitation.” Such was the case on the last day of summer, Sept. 21. An enormous cold front stretched a line of clouds and rain showers from its southernmost point, the State of Michoacan in central Mexico, to Killiniq Island. That’s the northernmost tip of Quebec and Labrador, 4,000 miles from Mexico City.
Other than its length, the front showed the typical pattern of the large traveling weather systems that sweep across this continent. It represented the edge of an air mass arriving from Canada. Like a giant windshield wiper, it swept the warm air out of the eastern U.S., kicking up a bow wave of rainstorms as it did so, and replacing it with drier, much colder air.
This is what scientists call a continental polar air mass. Because the sun still has a long way to go until it reaches its lowest noontime position on Dec. 21, there’s a lot of energy available to warm the ground and, in turn, heat the atmosphere. So at this time of the year, a cold front brings mostly relief from summer heat, and balmy days with clear blue skies.
Even northwestern Canada isn’t experiencing harsh conditions yet.
In Whitehorse, Yukon Territory, it was 48 degrees Fahrenheit while the front was moving through Georgia. Three hundred miles to the north, the town of Dawson reported 31 degrees.
Every so often, an air mass forming over the vast region comprised of Alaska, Yukon, the Northwest Territories, Alberta, Saskatchewan and Manitoba will settle down on the cold ground and spread out in the process.
That’s what brings it to our much warmer state, first with a chance of rain while it is kicking the previous resident, a body of humid warmer air, out to the Atlantic Ocean. Then, as this air mass slowly adapts to local conditions, sunny fall days are the result. It is the Canadian air which makes October our driest month of the year, a favorite for ball games, barbecues, outdoor weddings and other fair-weather events. Weather maps show a cold front as a line with teeth that point in the direction of its travel.
Teeth will indeed be felt three months from now, when the continental polar air coming to Georgia begins to bring much harsher conditions than we are seeing now.
Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.