Dear Tom,
In July 1987 while driving through the Mojave Desert on our way from Los Angeles to Las Vegas, heavy thunderstorms produced severe flooding and we were ordered off the road. How rare is this?
Kevin Vahey, Evanston
Dear Kevin,
It's not that rare. Rain is limited in the Mojave, averaging less than six inches a year. The desert's western flank receives the majority of its precipitation in the winter, while the eastern desert gets a substantial portion of its rainfall from summer thunderstorms in addition to the amount from winter storms. However, during the summer monsoon season, thunderstorms can form when moist air moves north out of the gulfs of Mexico and California. With ample moisture, strong heating and orographic lift provided by the mountains, the storms can bring blinding downpours.
Dear Tom,
On the evening of Aug. 10 I was checking the weather for many cities coast to coast and they were all 79 degrees. Is that rare and is it more likely to occur in summer?
Norman Alexandroff
Dear Norman,
Having a large expanse of uniform temperatures across the nation is quite unusual and would be far more likely to occur in summer than any other time of the year. Temperature contrasts are almost always greater in the winter with arctic air and snow cover much more frequent across the northern states. Summer's sluggish weather patterns are more conducive to smaller temperature ranges -- especially at night when sunshine is not a factor. Conditions that would bring similar temperatures on a regional basis are far more common and can occur throughout the year given the right conditions.
Dear Tom,
I was married on Dec. 8, 1945 and it was a very warm day. A week later when my friend got married on Dec. 15 it was below zero. Can you provide the details?
Delores Faltynek Antioch
Dear Delores,
We asked climatologist Frank Wachowski to check the weather archives and he found your recollections to be right on---illustrating how quickly Chicago weather can change this time of the year. You wedding day was indeed very mild with a balmy high of 53 degrees. Your friend was not as fortunate as winter hit with a vengeance in the following week. A cold front dropped temperatures into the 20s and a major snowstorm followed on Dec.13 and 14 bringing about 4 inches of snow. Your friend's Dec. 15 wedding took place on a very wintry day with a high of just 11, a low of 2 below zero and 4 inches of snow on the ground.
Dear Tom,
I am putting in a bid for sidewalk snow removal in the Lincoln-Belmont-Ashland shopping area. Can you provide the average frequency of the number of snows per season of various amounts?
Bill Haderlein, Chicagoland Deck Cleaning & Sealing
Dear Bill,
Indeed we can. The following statistics are derived from 80 years (1929-2008) of Midway Airport snowfall data, courtesy of Chicago weather historian Frank Wachowski.
On average, Chicago receives at least one-half inch of snow 17 times per snow season. Snow storms in the range of 1-3 inches occur about 7 times; 4-6 inches, twice; 7-10 inches, once; more than 10 inches, about once every other year.
A note of caution: The number of storms that might occur in any given snow season varies greatly from year to year. The extreme winter of 1978-79 put down 10 snows in excess of 4 inches and a season total of 89.7 inches.
Dear Tom,
What are the extremes for Chicago's average annual temperatures? Any sign the average is rising due to global warming?
Robert Roschke, Chicago
Dear Robert,
In 138 years of official data, Chicago's average annual temperatures have ranged from 45.3 degrees (1875) to 54.5 degrees (1921). The trend of temperatures has been upward, but it's difficult to draw conclusions because the observation site has been relocated several times and each move disrupted the climatological continuity of the temperature record.
However, 80 years of data (1929-2008) from a single location -- Midway Airport -- indicate a warming trend, probably due more to a strengthening urban heat island effect than to global warming. Annual temperatures rose from 50.6 degrees during the first half of the period (1929-1968) to 51.2 degrees during the second half (1969-2008).
Dear Tom,
With winter approaching, how soon can we expect temperatures to drop below
zero?
Myra Weisbach
Dear Myra,
Chicago¹s earliest encounter with subzero weather took place nearly 60 years ago on the frigid Thanksgiving morning Nov. 23, 1950 when the mercury plunged to 1 below. The next day was even colder, with a low of minus 2 degrees. Subzero cold that early in the season is unusual, with the bulk of the city¹s negative temperatures occurring from mid December through mid February. The frigid season in Chicago peaks during February¹s opening days, a time of year that coincides with the winter¹s most persistent snow cover. Early March usually marks the end of below zero weather, though the 1 below zero reading on March 22, 1888 is the latest on record. Last winter Chicago logged 10 below zero days--the coldest was a minus 18 on Jan. 16.
Dear Tom,
If I throw a bucket of water from a hot air balloon at say 15,000 feet, will this water appear as rain drops or as big globs when it approaches ground level?
Don Sadecki, Westchester
Dear Don,
You'll get raindrops at the ground. Larger drops fall faster than smaller ones, but drop size has an upper limit. It's a matter of drop stability.
German physicist Dr. Philipp Lenard made the first accurate measurements of raindrop sizes, shapes and fall speeds around 1900. He found that raindrops can attain sizes no greater than one-fifth inch in diameter; they become unstable and shatter at larger diameters, and the largest drops fall at about 20 mph.
As a large raindrop falls faster and faster, the friction of its passage through the air causes it to flatten, then to bulge upward in the middle rather like an inverted paper bag, and finally to shatter into smaller drops that fall more slowly.
Dear Tom,
What is Chicago's longest string of daily lows at 10 degrees below zero or lower? What about zero or lower? And what is our longest string of highs 90 degrees or higher? What about 100 or higher?
Lou Liebovich, Hoffman Estates
Dear Lou,
Computer scans of three data sets -- Chicago's official temperatures (1871-2009) and data from Midway Airport (1928-2009) and O'Hare International Airport (1959-2009) --provide the answers. Chicago's longest string of days with low temperatures of 10 degrees below zero or lower is 7 days (Jan. 14 to 20, 1994); zero or lower: 10 days (Jan. 4 to 13, 1912, and Jan. 2 to 11, 1979); highs at or above 90 degrees: 12 days (July 6 to 17, 1936). That 90-degree string also included the city's longest string at or above 100 degrees: a sweltering 8 days (July 7 to 14, 1936.
Dear Tom,
My 5-year-old asks when the dinosaurs disappeared and if weather was the cause.
Jennifer Krause
Dear Jennifer,
Five major "life extinctions" have occurred on Earth, one of which ended the reign of the dinosaurs about 65 million years ago (though recent evidence suggests some dinosaurs survived and today's birds are descended from them).
A widely accepted theory to explain the dinosaur extinction draws together information from a variety of independent investigations. An asteroid estimated to be 6-10 miles in diameter crashed into Mexico's Yucatan Peninsula. The impact threw a massive amount of debris into the atmosphere, and heat generated as it rained down planetwide caused much of Earth's vegetation to burn. Volcanic eruptions elsewhere contributed additional atmospheric dust, and temporary global cooling hastened the dinosaur extinction.
Dear Tom,
On Nov. 11, 1911 Springfield. Missouri experienced a huge temperature drop attributed to a "Great Blue Norther." Can you elaborate?
Les Herbst Lake Villa
Dear Les,
A "Blue Norther" is a fast-moving cold front accompanied by a strong north or northwest winds, a dark blue-black sky and a large, rapid temperature drop. Such was the case on Nov. 11, 1911 in Springfield, Missouri when the cold front passed. Earlier in the day, with clear skies and brisk south winds, the temperature soared to a record high of 80 degrees. Around 3:30 p.m. dark
ominous clouds rapidly approached followed by an abrupt wind shift to the northwest. Sustained winds of 54 mph and gusts to 74 mph damaged numerous trees and houses. Temperatures plunged, falling to 13 degrees by midnight, setting a record low for the day. Nearly a century later the record low still stands, but the 80 degree high was equaled in 1989.
Dear Tom,
With the holidays approaching I remember a big Thanksgiving Eve snowstorm that probably was in the middle 70s. What year was that?
Ray Consentino, Homer Glen
Dear Ray,
The storm you remember took place on Nov. 26-27, 1975. The snow began Wednesday afternoon and fell with increasing intensity throughout the evening rush hour. The heavy snowfall clogged highways and canceled flights paralyzing holiday travel. Snowfall that Wednesday totaled 7.5 inches, and when the snow finally ended on Thanksgiving morning the city's official site at Midway Airport had logged 8.6 inches of snow. On Thanksgiving morning the snow was eight inches deep, tying 1895 for the city's deepest Thanksgiving snow cover. Just five years ago, another traffic-snarling Thanksgiving Eve snowstorm dropped 4.3 inches on Nov. 24, 2004.
Dear Tom,
My family visited Glacier National Park in July and a park ranger said the glaciers were in retreat. Could you provide more information?
The Stein family, Chicago
Dear Stein family,
U.S. Geological Survey researchers are carefully monitoring the disappearance of glaciers and snowfields in Glacier National Park and they indicate that it is occurring quickly.
One report, prepared in 2008, says, "Glaciers don't respond to anything else except climate change, unlike, for example, forests that can adapt. ... In 1850 there were an estimated 150 alpine glaciers of 25 acres or larger (in the park). Now there are only 25 glaciers left. By 2030, computer models suggest that all the remaining glaciers and snowfields will be gone."
Park snowfall has increased 10 percent in the past 50 years, researchers say, but higher temperatures and melting have overwhelmed the heavier snows.
Dear Tom
Someone told me that Lakes Michigan and Huron are really one lake. Is This true?
Ken Cleys Woodridge
Dear Ken,
Though they have long been, and will always be considered to be two different lakes, geologically and hydrologically, Lake Michigan and Lake Huron are actually one lake joined at the Straits of Mackinac. They lie at the same surface elevation of 577 feet, both lakes' level rise and fall together and flow of water between the lakes can reverse through the Mackinac Straits.
Great Lakes' water levels reported by the Great Lakes Environmental Research Lab and the U.S. Army Corps of Engineers provide only one reading for the Michigan-Huron duo. Because of their large size, early explorers gave the lakes different names. Combined, lakes Michigan and
Huron would cover more than 45,000 square miles, larger than Lake Superior's nearly 32,000 square miles.
Dear Tom,
From Nov. 12-14, 1964, Chicago firemen battled numerous multi-alarm major fires. What were the weather conditions like?
Doran Swan, Chicago
Dear Doran,
The weather was windy, dry and unseasonably warm -- perfect conditions for fires to spread rapidly and burn out of control. Chicago climatologist Frank Wachowski found high temperatures reached the 60s on Nov. 11-12 and peaked at 71 degrees on the 14th. Afternoon humidities fell to near 30 percent and winds were strong and gusty from the south and west with a maximum gust to 59 mph on Nov. 12. Three of the biggest fires were at the old Goldblatt's warehouse at 4000 S. Kedzie, buildings near 25th and Michigan, and in an abandoned railroad warehouse near 18th and Clark. Wachowski noted that weather reports from Midway Airport reported heavy smoke northeast of the field.
Dear Tom,
I was just a kid but I think I remember a big snowstorm that occurred just after Halloween. It must have been in the early 1950s. Am I correct?
Brian Schwartz
Dear Brian,
The year was 1951 and the first week of November featured two major snowstorms. Halloween was a chilly affair with highs only reaching the lower 40s. A surge of cold air followed and held the mercury to just 29 degrees on Nov. 2, setting the stage for a fast-moving Alberta Clipper that dumped 4.4 inches of snow on the city during the afternoon of the 3rd. That was just the beginning as a second, more powerful snowstorm hit on Nov. 6-7, bringing another 9.3 inches of snow. That storm was accompanied by howling northeast wind gusts to 65 mph at the lakefront Meigs Field airport. The winds produced high waves that brought major flooding and property damage along the lakefront.
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.
