Dear Tom,
What do the lines on weather maps mean?
--Jenna Jamieson (5th grade), Bloomingdale, Ill.
Dear Jenna,
Meteorologists use weather maps to display information such as temperatures, air pressure and winds over large areas. The maps give forecasters the ability to see the big weather picture and, at a glance, to understand "what's going on" with the weather. That information is the starting point in the preparation of weather forecasts. Many different lines are drawn on weather maps, but the most common ones are called "fronts." Fronts show the boundaries between different kinds of air masses (such as hot and cold, or humid and dry). Lines called isobars frequently appear on weather maps as well. Isobars connect locations that have the same air pressure, and they show where the air pressure is low (usually with stormy weather) or high (usually with fair weather).
Dear Tom,
You've noted more than once that large raindrops fall faster than small ones. What circumstances cause this apparent contradiction to what Galileo learned, namely objects of different weights fall at the same speed?
-- Dennis McGann, New Lenox
Dear Dennis,
At the Leaning Tower of Pisa, Galileo Galilei (1564-1642) demonstrated that bodies of different weights fall with the same velocities, but due to air resistance, objects such as feathers, with little weight for their surface area, will fall slower than dense objects, such as bricks. In a vacuum, all objects fall at the same rate: Feathers, bricks, hailstones, even misty raindrops.
The great Italian astronomer and mathematician, invented the telescope, but his greatest contribution to the advancement of knowledge was that his work helped establish mechanics as a science.
Dear Tom,
My father used to say "winter's back is broken" once we get into February. What do the statistics actually say?
-David Means
Dear David,
Chicago's weather records make an unequivocal statement that, on average, winter is not as harsh in February as it is in January, and data support your father's claim that "winter's back is broken" by Feb. 1.
February temperatures average 5.0 degrees higher than in January (27.0 degrees versus 22.0) and its snowfall is 2.1 inches less (8.5 inches versus 10.6), but the operative word is average. We live in a climate more prone to large swings above and below the averages than adherence to those averages.
In fact, despite the averages, February has been colder than January in 48 years out of 139 (35 percent), and snowier than January in 58 years out of 125 (46 percent).
Dear Tom,
I remember a 70-degree January day as if it were yesterday. Was it in 1987 or 1989?
--Cathy Disch
Dear Cathy,
It was Jan. 31, 1989, and though it was a sunny, windy and unseasonably warm day in Chicago, it did not reach 70 degrees. The city's official high that day was a record 65 degrees, which fell 2 degrees short of January's record high of 67 degrees, reached on Jan. 25, 1950. For the record, January is the only month in which Chicago has never recorded a 70-degree temperature.
The warmth of that January day in 1989 was short-lived and literally was the "warm before the storm." Temperatures crashed as February opened and a snowstorm followed, dropping nearly 7 inches of snow on the city by Feb. 5. On Feb. 6 the mercury plunged to 4 below zero over the fresh snow cover.
Dear Tom,
January and February are the two coldest months in Chicago. Which months are in third place, fourth, fifth, etc.?
-Bill Philips
Dear Bill,
Here are the months and their average temperatures, ranked from coldest to warmest: January, 24.3 degrees; February, 27.9; December, 28.9; March, 37.7; November, 40.8; April, 49.6; October, 54.5; May, 60.3; September, 66.1; June, 70.5; August, 73.7; and July, 75.2.
We used Midway Airport data from 1929 to 2009 to generate those averages. Although monthly average temperatures are slightly different at other locations in metropolitan Chicago, the ranked order remains the same.
The ranking by season is winter (December through February), 27.0 degrees; spring (March through May), 49.2; autumn (September through November), 53.8; summer (June through August), 73.2. The first half of the year (January through June) averages 45.1degrees and the second half (July through December), 56.6.
Dear Tom,
I remember a huge spring snowstorm in the middle 1970s when it took me 8-1/2 hours to make a usually one hour trip home from work. Details please?
-John Dailey, Fort Smith, Arkansas
Dear John,
You and thousands of other Chicagoans were stuck in one of the worst spring snowstorms in the city's history. Following a night of rain and sleet, the precipitation changed to heavy wet snow during the day on April 2, 1975. The storm dropped 9.8 inches of snow and was accompanied by northeast winds gusting as high as 40 mph. The snow quickly clogged expressways, creating gridlock and forcing motorists to abandon their cars. The area was plagued by power outages and the storm forced O'Hare International Airport to close. Eight fatalities were attributed to the storm, most of them from heart attacks brought on by shoveling.
Dear Tom,
In September or October 1959 we heard the classic a freight-train wind sound for nearly a minute in the Edison Park-Norwood Park area on Chicago's Northwest Side. Trees were down all over the place. Was it a tornado?
Bob Johnson, Buffalo Grove
Dear Bob,
Around 7p.m. Saturday, Sept. 26, 1959, a tornado carved a 12-mile path of destruction from near O'Hare International Airport through Niles and Park Ridge. Most of the damage was F1 but there were pockets of F2 damage. The twister was spawned by severe thunderstorms that packed wind gusts to 75 mph. A garage was demolished near Touhy and Caldwell avenues and a roof was torn off a building at Milwaukee and Touhy avenues. Numerous boats were swamped on Lake Michigan and power failures were widespread. The Evanston and Wilmette areas were especially hard hit, with fallen trees blocking many streets. Storm damage was estimated at more than $250 thousand dollars.
Dear Tom,
We were in Los Angeles in mid-January during their very rainy weather and there were some thunderstorms as well. The residents said thunderstorms never occurred there, which I question. Is it true that Los Angeles almost never has thunderstorms? Is there any place in the world where thunderstorms never occur?
-Ruth Malik
Dear Ruth,
By Chicago's standards, thunderstorms in Los Angeles are relatively infrequent, but they do occur there and the claim of no thunderstorms is incorrect. Chicagoans can expect 38 days per year with thunderstorms; in Los Angeles, it's just five days.
Thunderstorms occur in practically every place on Earth at one time or another, but they are rare in some locations such as the interior portions of the Sahara Desert and the central Arctic Ocean. Only one place never has thunderstorms: interior Antarctica.
Dear Tom,
Is it my imagination or has this been the grayest, cloudiest winter ever?
-- Erin O'Connor
Dear Erin,
Your impressions are right on. It has been a very cloudy winter to date. However, as dreary as it's been, it has been worse.
According to Chicago climatologist Frank Wachowski, since Dec. 1, this winter has logged 27 totally sunless days. Since the inception of sunshine records in 1894, only three winters have had more cloudy days through the end of January: 1997-98 with 33, 1968-69 with 30 and 1940-41 with 28.
Wachowski noted that this winter has featured extended sunless periods; an eight-day string from Dec. 17-24 and more recently a seven-day stretch from Jan. 20-26.
Dear Tom,
Last December we had two full moons, the second one being on New Year's Eve. When was the last time February had two full moons? Wouldn't it have to be in a leap year?
--Pamela Eiten
Dear Pamela,
The answer is never. By nature of the 29.5306-day lunar cycle, February, with 28 or 29 days, can never host two full moons. In fact, it is the only month of the year that can be without a full moon. When this occurs, both January and March will sport double-full moons. That last two times this happened were in 1961 and 1999, and future occurrences will be in 2018, 2037, 2067 and 2094. Astronomer Dan Joyce of Triton College's Cernan Space Center tells us that in general, two full moons in one month occur about every 32 months; the next time being in August 2012.
