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.
Please explain the physical cause of low and high pressure areas that are shown on weather maps.
— Jana Palmer
Air pressure measurements, as determined by barometers, serve as excellent indicators of where air is rising or sinking on a broad scale — across hundreds of miles. We’re all familiar with the horizontal motion of air along the Earth’s surface — we call that motion “wind” — but less noticeable is the vertical motion of air: upward and downward.
At the Earth’s surface, air pressure is higher when the air above it is sinking because the weight of the sinking air is effectively pressing against the ground. That’s the situation in areas of high pressure. Conversely, in areas of low pressure, air is rising from the surface, effectively reducing the weight of the air pressing on the ground.
You have stated that Chicago never had a month-long streak of below-zero temperatures. However, didn’t we have a string of below-freezing days that lasted more than a month?
You’re correct on both counts. In official temperature records dating from the winter of 1870-71, Chicago’s longest consecutive string of days with minimum temperatures of -1 degrees or lower stands at ten. That record occurred during the period Jan. 4-13, 1912.
With regard to daily high temperatures, on average each winter we can expect to shiver through an 11-day string during which daytime temperatures remain continuously below freezing — that is, continuously no warmer than 31 degrees. However, the record period of sub-freezing days stands at 43, from Dec. 28, 1976, through Feb. 8, 1977.
Has there ever been a month Chicago where absolutely no precipitation was recorded? What was the city’s wettest month?
The city has never experienced a rainless month, but it has come close. In September 1979, Chicago measured just 0.01 inches of rain that fell in a single 13-minute, late-afternoon shower on Sept. 1. Other than a few sprinkles on the 24th and 30th, that was the only rain of the month. In sharp contrast was August 1987, the city’s wettest month. It logged 17.10 inches of rain with more than half of it falling in the city’s benchmark heavy-rain event when 9.35 inches of rain inundated areas around O’Hare Airport. That soggy month featured two other heavy precipitation episodes with 2.90 inches of rain falling on the 16th and another 2.46 inches on the 24th.
When was the last time we have seen a white Thanksgiving in the Chicago area?
— Eamon Homedi, Northbrook
White Thanksgivings (an inch of snow on the ground at some time on Thanksgiving Day), while certainly not the norm, have occurred 13 times in the Chicago area since snow records began in 1884. Coincidentally, several of them have occurred in clusters, occurring in 1893 and 1895, in 1947, 1949, 1950 and 1953, and 2002 and 2004, the city’s most recent White Thanksgiving. That year, a storm the day before Thanksgiving dropped 4.3 inches of snow on the city, snarling holiday travel.
The city’s “whitest” Thanksgivings were in 1975 and 1895, when 8 inches of snow covered the ground. The most snow to ever fall on the holiday occurred in 1980, when 3 inches fell.
I was in South Dakota a few years ago and saw a beautiful aurora borealis that stretched across the northern sky. I have always wondered how high up in the air those displays occur.
— Margo Russell
Norwegian physicist and mathematician Dr. Stormer (1874-1957) and his research group simultaneously photographed auroras from many locations in Europe and, using triangulation and geometry, calculated the height in the atmosphere at which auroras occur. Dr. Stormer found that auroras occur in two “zones of activity,” one lower in the atmosphere and the other higher. The lower zone is somewhere between about 50 to 200 miles above the Earth’s surface and the upper zone about 350 to 630 miles up. Stormer also found that most auroras originate in the lower zone.