Dear Tom,
I have seen frost on garage rooftops at sunrise when temperature reports are 38 or 39 degrees. What gives?
Robert Wolfson
Dear Robert,
Frost forms when the shallow layer of air at the ground (or on any other surface such as a rooftop) cools below its saturation temperature (and below freezing) by contact with a cold surface that itself has cooled to a subfreezing temperature. Water vapor in the air condenses directly, in ice-crystal form, onto the cold surface: frost forms.
Under clear, calm nighttime conditions, surfaces like blades of grass (but it works for roofs, too) radiate a great deal of heat and their temperature drops sharply. The cold-air layer can be very shallow, often only a fraction of an inch. Air temperatures in National Weather Service reports are taken by temperature sensors a few feet above the ground and sheltered from radiational cooling.
Dear Tom,
What is the meaning of "northwest winds"? Wind coming from the northwest or blowing toward the northwest?
Ralph Bellendir
Dear Ralph,
By international convention, wind direction always refers to the direction from which the air is moving. The proper interpretation of, say, a northwest wind of 10 mph is that the wind is blowing from the northwest to the southeast at a speed of 10 mph.
While we're on the subject, here's an unusual bit of weather trivia: You'll never hear a reported wind speed of 11 mph. Never. Here's why: Wind speeds are always measured in knots (nautical miles per hour). That's the way it's done, worldwide, and then in the United States that value is converted to statute miles per hour. One knot is 1.1508 statute mph. A value of 9 knots converts to 10 mph and 10 knots becomes 12 mph after rounding to a whole number.
Dear Tom,
We are approaching the 34th anniversary of the sinking of the Edmund Fitzgerald. How rare is it to have a storm of that magnitude on the Great Lakes?
Mike Long
Dear Mike,
The legendary storm of Nov. 9-10, 1975 had a central pressure of 28.95 inches when it crossed Lake Superior, equivalent to a Category 1 hurricane. At its peak, it had sustained winds of nearly 80 mph with gusts above 90 and produced giant waves 25-30 feet high.
Though stronger storms have battered the Great Lakes, this storm was certainly on the high end of severity scale. The term "Gales of November", popularized by mariners long before it was made famous by Gordon Lightfoot, refers to the peak of the Great Lakes' storm season, when late-fall storms fueled by clashing warm and cold air masses gain extra energy from the residual summer heat stored in the waters of the Great Lakes.
Dear Tom,
I was able to see the stars clearly when I moved to Des Plaines 60 years ago, but now I miss seeing them at night. I understand gradually losing the ozone layer and the use of fossil fuels is to blame. Will I ever see the stars again?
Louise Koehler, Des Plaines
Dear Louise,
Triton College astronomer Dan Joyce shares your frustration and laments that "the grandeur of the nighttime sky" (as he refers to it) that was once a common spectacle has faded.
However, the decay of the atmosphere's ozone layer is not to blame because ozone only blocks the ultraviolet component of sunlight. Nor are fossil fuels at fault: the switch from coal to natural gas for heating has greatly reduced particulate pollution and our air is now clearer than ever.
The problem is light pollution. Metropolitan outdoor nighttime illumination is now so bright that it overwhelms our ability to discern the stars.
Dear Tom,
Why is it usually more cloudy in the autumn than in the summer?
Roger Hohman
Dear Roger,
The difference in the amount of heat provided by sunlight is the fundamental cause of seasonal variations in cloudiness.
In the warm season, the nearly overhead sun strongly heats the ground and overlying air. Currents of ascending warm air give rise to cumulus clouds that grow vertically into local cloud patches rather than spreading horizontally into widespread overcasts.
The opposite process prevails in the autumn and winter. Sunshine strikes the ground at a small angle and delivers little heat. The atmosphere grows colder, and the coldest layer is at ground level. Colder air under warmer is stable, and condensation and cloudiness tend to be horizontally stratified, resulting in widespread and long-lasting overcasts.
Dear Tom,
It seems that 2009 has had an abundance of precipitation. Is this going to be a record wet year?
Chris Mall
Dear Chris,
Chicago is definitely having a wet 2009, but so far, this year is taking a back seat to 2008 which reigns as Chicago's all-time wettest year with a 50.86-inch precipitation total. Buoyed by September deluges that brought 13.63 inches of rain, 2008 marked the first time that the city's annual precipitation broke the 50-inch barrier, topping the old record of 49.35 inches established in 1983. At October's close precipitation at O'Hare Airport had officially reached 38.61 inches -- nearly 8 inches above normal -- but last year at the end of October the precipitation tally here was 43.28 inches -- 4.67 inches greater. Barring an incredibly wet close to 2009, last year's record should not be eclipsed.
Dear Tom,
With November beginning, I'm wondering what the month's weather extremes are in Chicago.
--Tarleton Edwards Dear Tarleton,
Dear Tarleton,
November weather in Chicago transitions from autumn to winter, sometimes gradually but often harshly and abruptly. Normal high temperatures decline more rapidly than in any other month --on average 1 degree every other day (from 55 degrees to 40). At its extremes, November weather displays a split personality. The month's highest temperature, 81 degrees (recorded on Nov. 1, 1950), and its lowest temperature, 2 degrees below zero (Nov. 24, 1950), occurred only 23 days apart. November usually brings the season's first accumulating snow, and the average monthly snowfall is 2.2 inches --- but 14.8 inches came down in November 1940.
Dear Tom,
The first day of winter is Dec. 21, the shortest day of the year. Days start lengthening after that and the Earth also gets closer to the sun. So why does Chicago continue to get colder almost to February?
Bob Thurston
Dear Bob,
The sun is 2.8 million miles closer to Earth in early January than it is at its most distant point in early July. However, the effect of that "closeness" is overwhelmed by the effect of the tilt of our planet's axis relative to the sun. That tilt situates the Northern Hemisphere away from the sun's most direct rays, thereby reducing the amount of solar energy available for heating.
Other cooling factors such as round-the-clock darkness and widespread snow cover in the polar north prove far more important in determining temperature than closeness to the sun, and cold air continues to expand its dominance well into January.
Dear Tom,
I moved to Chicago several months ago and I've heard that winters here are brutal. Does the cool weather now mean this winter will be even more brutal than usual?
Tami
Dear Tami,
Take heart. A review of Chicago's autumn and winter temperature statistics in years past shows that a chilly October (such as we're experiencing this year) is usually followed by an abrupt temperature turnaround. Winter is cold, of course -- Chicago's winters are always cold -- but a winter with above-normal temperatures often follows an October with subnormal temperatures.
May we take exception to your characterization of Chicago winters as "brutal"? We believe that's too harsh. If you moved here from a milder climate, as your question implies, Chicago's winter weather will shock you initially, but you'll quickly become acclimated.
Dear Tom,
We're planning an ocean-side vacation at Waikiki Beach in Honolulu, Hawaii, and have become concerned about the tsunami threat. Any thoughts?
Kim Larson
Dear Kim,
The catastrophic tsunami of Dec. 26, 2004, in the basin of the Indian Ocean raised worldwide awareness of the deadly potential of such occurrences. Your concern is justified because, despite the Indian Ocean event, the Pacific Ocean, in which Hawaii is located, is encircled by the notorious "Ring of Fire" and is the world's most tsunami-prone ocean.
That said, be advised that Hawaii has had long experience with tsunamis and, with the National Weather Service's "TsunamiReady Program," has an excellent warning system in place. The Weather Service's Pacific Tsunami Warning Center, located in Ewa Beach, Hawaii, with a staff of 15, maintains 24-hour vigilance.
