Dear Tom,
As a kid in New Jersey, I cannot recall hearing about tornadoes, but in Chicago we hear about them. Does the Atlantic Ocean reduce the tornado threat?
Andy Charen
Dear Andy,
Your question lacks a definitive answer. The conditions that generate tornado-producing thunderstorms depend upon a proper union of atmospheric and terrain components. Those conditions are satisfied most completely in the so-called "tornado alley" of the Great Plains, and the tornado threat diminishes with increasing distance from that region.
However, local features also affect tornado frequency. Cool air from Lake Michigan usually weakens thunderstorms, but the lake breeze "cold front" sometimes strengthens them, as does the Atlantic Ocean sea breeze front. New Jersey, far from tornado alley, usually lacks other atmospheric conditions conducive to severe weather.
Dear Tom,
How can it rain for four or five days as it has at times this year? At some point doesn't the atmosphere in any local geographical area become depleted of moisture until evaporation can replace it?
Rob Neufeld
Dear Rob,
The total amount of water evaporated in a column of air above a location is known as precipitable water. It is the amount of water that would fall to the surface if all the water vapor in the atmosphere above that spot condensed and fell as rain.
A precipitable water value of 2.00 inches is exceptionally high and rarely occurs in the Midwest. Nevertheless, rain totals far greater than 2.00 inches occur quite often. This implies that moisture, carried by wind, moves to the location at which rain is falling and continuously re-supplies the rain-producing mechanism (such as a thunderstorm). And that is exactly what occurs in heavy rain events.
Dear Tom,
I remember June 13th as a very stormy and rainy day in the Chicago area. It rained again this year. How often has the Chicago area had rain or storms on this date?
Bill Ooms, Alsip
Dear Bill,
A check of Chicago's precipitation records (1871-2009) on June 13 confirms your suspicions. At least some rain has fallen on that date 78 times out of a possible 139 days, and that computes to rain on 56 percent of the days. The normal mid-June precipitation frequency is 35 percent.
June 13 is also infamous in Chicago weather history. In 1976, a devastating F4 tornado killed two and injured 20 as it carved a "J"-shaped path of destruction through Lemont. The twister tossed cars through the air and destroyed an eight-block area that included half of the Lemont High School. The storm dropped 2-inch diameter hail and flooding rains that totaled 7.5 inches in the Oak lawn area.
Dear Tom,
Instead of rising, why does smoke from a chimney descend before a storm?
--Hilary Schnadt
Dear Hilary,
"When the smoke goes low, there will be a blow," and there is some truth to that proverb. In calm, dry weather, hot air from a chimney continues to ascend as long as its temperature is higher than that of the surrounding air. This is more pronounced during cool weather when the hot chimney air is most buoyant. The opposite situation prevails when a rain bearing low-pressure system approaches. Southerly winds ahead of the system bring higher temperatures and humidity. The warmer temperature weakens the current of hot air and, in addition, the smoke particles absorb moisture, become heavier and sink. A drooping smoke plume thus indicates warmer, damper weather and an approaching storm.
Dear Tom,
What causes heat lightning?
Mike Murphy, Chicago
Dear Mike,
Heat lightning is nothing more than ordinary lightning produced by a distant thunderstorm too far away for the thunder to be heard. With summer thunderstorms routinely building to heights in excess of 50,000 feet, lightning from these storms can be visible at distances beyond 125 miles to anyone with a clear view of the horizon. In contrast, the sound of thunder can rarely be heard beyond a distance of about 15 miles. In fact, the parent thunderstorm cloud producing the lightning may actually be below the horizon, but due to the curvature of the Earth's surface, flashes emanating from the upper levels of the cloud are visible. The term heat lightning came into use because people spent time outdoors on hot, humid summer evenings and were able to view the distant lightning.
Dear Tom,
I remember a June 11 back in the late 60s or early 70s with a killing frost that wiped out most of my garden. Details?
Jim Marrone Lake Bluff
Dear Jim,
The very cold morning in the Chicago area that you remember was on June 11, 1972. In the city, the then official Midway Airport site logged a record low of 41 degrees (since tied in 1980), but the real chill was out in the suburbs where readings dropped into the lower and middle 30s. The morning's coldest temperature was logged in northwest suburban Marengo, a frosty 29 degrees. Other suburban lows that morning included Antioch, 33; Aurora 35; Barrington 30; Waukegan 34; Joliet 35; and Park Forest. 36.
The warming effect of the city's heat island and Lake Michigan was evident from the 46 degrees registered at the site in Grant Park near Buckingham Fountain.
Dear Tom,
I remember a June 11 back in the late 60s or early 70s with a killing frost that wiped out most of my garden. Details?
Jim Marrone Lake Bluff
Dear Jim,
The very cold morning in the Chicago area that you remember was on June 11, 1972. In the city, the then official Midway Airport site logged a record low of 41 degrees (since tied in 1980), but the real chill was out in the suburbs where readings dropped into the lower and middle 30s. The morning's coldest temperature was logged in northwest suburban Marengo, a frosty 29 degrees. Other suburban lows that morning included Antioch, 33; Aurora 35; Barrington 30; Waukegan 34; Joliet 35; and Park Forest. 36.
The warming effect of the city's heat island and Lake Michigan was evident from the 46 degrees registered at the site in Grant Park near Buckingham Fountain.
Dear Tom,
Is there such a thing as dry lighting?
Audrey Clark
Dear Audrey,
Dry lightning is a colloquial term for lightning that occurs from a thunderstorm that is not producing precipitation. It's not a meteorological term and its implications are factually incorrect for two reasons.
Lightning, the transient flash of light produced by an electric discharge in the atmosphere (usually in thunderstorms, but also in volcanic ash clouds and, very rarely, in sandstorms) is never wet in the first place.
Secondly, thunderstorms always produce precipitation, but rain falling from a storm sometimes falls through a layer of dry air and evaporates before it reaches the ground. This is known as a dry thunderstorm (also technically a misnomer). Such storms occur in the western U.S. in the summer and are the leading natural cause of forest fires.
Dear Tom,
What percent of U.S. tornadoes touchdown west of the Mississippi River compared to east?
Hank Hartzell Burlington, Wisc.
Dear Hank,
Based on average annual tornado data for 1953-2004 from the National Climatic Data Center and the Storm Prediction Center, areas west of the Mississippi River log 62 percent of this nation's twisters as compared to 38 percent in areas to the east. The vast majority of the twisters west of the Mississippi occur in the southern and central Plains from Nebraska to Texas, an area frequently referred to as "Tornado Alley."
With an average of 139 twisters annually Texas dominates all states in tornado activity, followed by Oklahoma with 57 and Kansas and Florida tied with 55.
Illinois ranks second in tornadoes for states east of the Mississippi averaging 35 each year, trailing only Florida with 55.
Dear Tom,
On our wedding day June 20, 1953 it was 104 and on our 35th anniversary in
1988 it was also 104. How often does Chicago's temperature reach 100 on the
same date.
Frank and Vi Jasonowicz Palatine
Dear Frank and Vi,
Since 1871 Chicago's official thermometer has reached triple digits 61
times. There were only a dozen days with two 100 degree highs, three days
that logged three and just one day, July 10, where the thermometer reached
100 degrees four times. The first time was a high of 102 on July 10, 1901.
The next encounter was during the "Dust Bowl" in 1936 with another 102,
followed by a 100 in 1976 and most recently 101 in 1989. The city has not
logged a 100 degree temperature in nearly four year, the last occurrence was
on July 24, 2005 when the mercury peaked at a sizzling 102.
Dear Tom,
Can you confirm that there really can be a "green flash," and why it is so rare?
Mac Sims
Dear Mac,
The "green flash" is a brief flash of green light emanating from the upper rim of the rising or setting sun. It occurs because sunlight is bent slightly upward as it passes through the atmosphere, and the bending is greatest when the sun is on the horizon.
Because the shortest wavelengths of sunlight (blues and greens) are bent upward the most, those are the colors that appear first at sunrise and last at sunset. Most of the blue wavelengths are removed by atmospheric scattering, and that leaves green for the flash.
The green flash is seen most often when viewed across the expanse of a large body of water. It requires a nearly perfect combination of clear skies, excellent visibility and atmospheric conditions that enable the air to act as a lens and magnify the flash.
Dear Tom,
What is the difference between a solstice and an equinox?
Todd Flessner
Dear Todd,
During the course of a year, four days are noteworthy as the Earth revolves around the sun.
The summer solstice occurs around June 21, when the sun is directly overhead at its northernmost latitude (the Tropic of Cancer). It is the first day of astronomical summer and the day with the most daylight in the Northern Hemisphere. The winter solstice, occurring about Dec. 21 (when the direct rays of the sun reach their southernmost position at the Tropic of Capricorn), marks the start of astronomical winter and gives the Northern Hemisphere its least amount of daylight.
The equinoxes are the two dates the overhead sun crosses the equator. The vernal equinox, about March 21, marks the onset of spring; the autumnal equinox, about Sept. 21, heralds the start of fall.
Dear Tom,
What is the difference between a Solstice and an Equinox?
Todd Flessner, Woodridge
Dear Todd,
During the course of the year, four days are noteworthy as the Earth revolves around the sun. The summer solstice occurs around June 21, the date on which the sun is directly overhead at its northernmost latitude, the Tropic of Cancer. It is the first day of astronomical summer and the day with the most daylight in the Northern Hemisphere.
The winter solstice, occurring about December 21, occurs when the direct rays of the sun reach their southernmost position and it marks the start of astronomical winter. It provides the Northern Hemisphere its least amount of daylight.
The equinoxes are the two dates the sun crosses the equator and the length of day and night are about equal at all points on Earth. The vernal equinox occurs about March 21, denoting the start of astronomical spring; the autumnal equinox, about September 21, heralds the start of astronomical autumn.
Dear Tom,
You have said Havre, Montana, is the US city with the largest temperature difference between its all-time highest and lowest temperatures. What US city has the smallest difference between its highest and lowest temperatures?
Scott Thomas
Dear Scott,
Let's review the statistics. Chicago's all-time high and low temperatures
are 105 degrees and -27 degrees, respectively, for a spread of 132 degrees.
Comparable figures for Havre, Montana, are 111 and -57, for a spread of 168
degrees, a US record.
Honolulu, Hawaii, stands at the other end of the temperature scale, and
that is the answer to your question. Its all-time highest and lowest
temperatures are 94 and 53 degrees, for a range of 41 degrees. Honolulu's
exceptionally moderate climate results from the tempering effect of the
surrounding Pacific Ocean and a very small seasonal variation in energy
received from the sun.
Dear Tom,
Your use of "normal temperatures" and "average temperatures" is confusing. Wouldn't "seasonal" be better than "normal"?
Jim Hurley, Glenview, Ill.
Dear Jim,
Seasonal temperatures refer to readings that occur commonly during a particular season, such as during the spring. The term does not have a precise definition in the atmospheric sciences. Meteorologists define "average" and "normal" in ways that are somewhat different from standard usage, and that is an unfortunate source of confusion.
In the atmospheric sciences, an average (such as an average temperature) is just what you think it is -- the sum of all the values in a set divided by the number of values. A normal is different. It is the specific average over a standard 30-year period, currently 1971-2000. For example, a normal temperature is the average temperature over the 30-year period from 1971 through 2000.
Dear Tom,
Why are forecasts from "computer models" sometimes in error?
Aaron Turnier
Dear Aaron,
Computer models consist of sets of mathematical equations that describe how the atmosphere operates. The equations consider such quantities as temperature, moisture and wind, and how those quantities will change as time (from a few hours to several days) passes.
The models have three main sources of error. The first is our incomplete knowledge of atmospheric processes: The equations themselves aren't perfect. Then there are "initialization errors." At the outset, it is impossible to describe the atmosphere perfectly because of lack of data, especially over the oceans. Finally, there are "resolution" problems. Calculations are performed on volumes of air perhaps 20 miles on a side. Weather features (like thunderstorms) smaller than that are not handled very well.
Dear Tom,
Your excellent Tornado Seminar last April left me wondering, what is the worst tornado ever to occur in Illinois?
Jon Palmer
Dear Jon,
It was the Tri-State Tornado of March 18, 1925. Not only does that occurrence stand as Illinois' worst tornado disaster (in terms of human deaths and injuries), it ranks as the worst ever to strike the United States.
Tornado historian Tom Grazulis has written eloquently about the tornado in his book, Significant Tornadoes, describing it as "... the largest, longest, fastest, most destructive and most deadly tornado in U.S. history. In all respects, this remarkable tornado stands in a class of its own."
The Tri-State Tornado cut a continuous 219-mile path from southeast Missouri across southern Illinois into southwest Indiana. Along the way, 695 people died (613 in Illinois) and at least 2,000 were injured.
Dear Tom,
With summer approaching, I am reminded of the wickedly hot summer of 1988 here in Chicago. I had an apartment with no air conditioning then and I suffered. How hot was it?
Allen Katz
Dear Allen,
Hot, very hot -- and dry. In addition to the heat, the spring and summer of 1988 brought severe drought conditions to the Midwest and much of the nation. That also was the summer during which wildfires raged across 36 percent of Yellowstone National Park.
Here in Chicago, the summer was a scorcher. Midway Airport logged 48 days at or above 90 degrees (an all-time record for the city), and eight days at or above 100. With an average temperature of 77.4 degrees, it was Chicago's second hottest summer. The summer's average high temperature, 87.8 degrees, is also a record. Without air conditioning, there is no doubt that you suffered.
Dear Tom,
Why is the shortest day of the year, Dec. 21, not near the midpoint of the coldest weather of the season; and why is the longest day of the year, June 21, not near the hottest weather of the season?
--Martin Vesole, Highland Park
Dear Martin,
The annual cycle of temperatures lags behind the annual cycle of daylight by about one month---the so-called lag of seasons. Chicago's temperature cycle bottoms in mid-January and peaks in mid-to-late July. Even though heat input from sunlight increases after Dec. 21, the Earth is still losing more heat to space than it receives and temperatures continue to fall. By mid-January, though, increasing heat gain overcomes the loss and temperatures begin to rise. In the summer, heat gain, declining after June 21, exceeds heat loss until mid-July.
Dear Tom,
I work in a high school and found an old picture of the 1930 graduating class on what looked like a warm, windy day. What was it like in Chicago on May 13, 1930?
Nancy Eliason
Dear Nancy,
Your observations were pretty much right on. We had Frank Wachowski check the archives, and he confirmed your inferences. It was a indeed a warm day with the high temperature reaching 75 degrees. After some morning sprinkles, skies cleared to set the stage for a mostly sunny afternoon. Winds were gusty, blowing from the southwest in the morning but from the northwest in the afternoon after the skies cleared. It was a good thing that the picture was taken on May 13. The weather turned chilly after that and did not warm up for more than a week until it reached 85 on May 21.
Dear Tom,
Is there somewhere relatively near Chicago where one could enjoy a spectacular view of the stars of the Milky Way, without the interference of nearby city lights?
Lillian Durante and John Zygowicz, Wauconda, Ill.
Dear Lillian and John,
Astronomer Dan Joyce of the Cernan Earth and Space Center at Triton College bemoans the advent of outdoor illumination and how it has muted what he calls "the grandeur of the night sky." For sky viewing, Joyce recommends Apple River Canyon State Park near Galena, the Green River Conservation Area west of DeKalb and Wildcat Mountain State Park west of Lake Delton, Wisc.
Joyce also suggests that you attend a general meeting (they're free) of one of the local astronomical societies and ask for advice. Besides Chicago Astronomical Society, there is Calumet A.S., Northwest Suburban Astronomers, Naperville Astronomical Association, and Skokie Valley A.S. to name a few.
Dear Tom,
What is the coolest high temperature that has ever been recorded here in meteorological summer?
Nick Recchia, River Grove
Dear Nick,
One of the things than never ceases to surprise a student of Midwestern weather is its stubborn tendency to defy generalizations. The three-month period of meteorological summer began on June 1 and, by popular reckoning, summer is supposed to be the season of heat. It is, of course. In an average year Chicago logs 24 days (Midway Airport data) at or above 90 degrees, and 20 of them occur from June 1 through August 31.
But summer days also include a surprising number of cool days. On average, about one summer day in eight fails to reach 70 degrees; much less frequently, one in 78 remains below 60 degrees. Six summer days have even registered wintry chill -- highs only in the 40s. The coldest of them was June 5, 1895, with a high of 45.
Dear Tom,
Is Phoenix's highest temperature still 122 degrees? If it is, did you know that so that date of the record 6/26/90 adds up to the temperature?
Mabel Shinouskis, Schaumburg
Dear Mabel,
We checked and the 122 degree high set on June 26, 1990 still stands as the all-time hottest reading in Phoenix so your numerology is still valid. That record came close to being challenged a few years ago when the mercury peaked at 121 degrees on July 28, 1995. Temperature records at Phoenix date back to 1896 and since then have varied from that 122 degree high to an all-time low of 16 degrees on January 7, 1913. So far this year (through June 9) the Arizona city has recorded 24 days of triple digit temperatures well on their way to annual average of 89. In 1989 Phoenix logged 143 days in the 100s, the most on record.
Dear Tom,
What city in the world has the worst pollution? Not just air pollution, but all-around unhealthy conditions resulting from a degraded environment.
Richard Bemins
Dear Richard,
The Blacksmith Institute, a New York-based environmental group, recently released the results of an exhaustive survey of more than 300 of the world's most polluted places. The Blacksmith investigators, an international team of environmental and health experts that included faculty members from Johns Hopkins and Mt. Sinai Medical Center, carefully narrowed the list to ten cities. Here they are, grouped by primary pollutant: Lead contamination: Haina, Dominican Republic; La Oroya, Peru; Rudnaya Pristan, Russia; Kabwe, Zambia. Radioactive waste: Chernobyl, Russia; Mailuu-Suu, Kyrgyzstan. Coal dust: Linfen, China. Exotic chemicals: Ranipet, India; Dzerzinsk, Russia; Norilsk, Russia.
Dear Tom,
I kept a diary during grade school and made an entry for July 4, 1956: "very cold, it snowed in Chicago a little." I don't think that officially occurred but could a flake or two have been reported?
Margaret Patterson
Dear Margaret,
It was a very cool 4th of July in 1956, but I can assure you that it did not snow. Chicago area temperatures did not climb out of the 60s that day, one that was marked by overcast skies, light rain and brisk east winds that gusted to 20 m.p.h. We had Chicago climatologist Frank Wachowski check to see if any hail occurred, but there were no thunderstorms in the area. In nearly 140 years of Chicago weather records the period from June 3 through Sept. 24 has remained completely snow free. July 1956 did start out on a very hot note with a record high of 103 degrees on the 1st before the cool weather moved in for July 4.
Dear Tom,
We are ending the first week in June and the city has not officially recorded an 85 degree high yet. When was the last time weather that warm was so delayed?
Barb Maple
Dear Barb,
In a typical year Chicago first officially reaches the 85 degree benchmark around May 15, but so far this season the warmest O'Hare Airport has been able to muster has been a high of 84 on three occasions: April 24 and again on May 20 and 21. It's been a dozen years since we've gone so late into the warm season without reaching 85 -- back to June 19, 1997. Before that it took until June 6 in 1995 and June 10 in 1983. Despite the late onset of warm weather in 1983, that summer turned out to be a real scorcher with 42 days of 90 degrees or higher -- including 2 days where the thermometer peaked at 100. Checking back through the archives, the latest date when the city reached first 85 was July 3 back in 1935.
Dear Tom,
Chicago's all-time high and low temperatures are 105 degrees and -27 degrees, respectively, for a difference of 132 degrees. What U.S. city has the largest temperature difference between its all-time high and low readings?
Scott McLean, Naperville
Dear Scott,
The most likely candidates are those cities whose locations allow them to experience scorchingly hot summer afternoons on occasion and bitterly cold winter nights. That eliminates the South (not cold enough), the maritime climate of the West Coast (neither hot nor cold) and the East (rather hot but not cold enough).
What's left is the Great Plains, a region that heats strongly in the summer but stands in the path of bitterly cold arctic invasions in the winter. The "winner": Havre, Mont. Its extreme temperatures (111 degrees in 1961, -57 degrees in 1916) span an incredible range of 168 degrees, a U.S. record.
Dear Tom,
Looking back at the records, how many sunny days a year does Chicago enjoy?
Jo'Ann McDonnell
Dear Jo'Ann,
Frank Wachowski, Chicago's sunshine guru tells us that the city typically receives about 54 percent of its possible annual sunshine and enjoys at least some sunshine on about 320 days a year. However there are an average of only 46 spectacular days that record 100 percent of the possible sunshine---logging every minute of sun from sunrise to sunset. While the long daylight of summer provides the city with the most minutes of sun, it is the tranquil autumn period of September and October that normally brings the city its greatest number of totally sunny days. In a 100-year study of Chicago sunshine ending in 1993, Wachowski found that October led all the months with 561 days with 100 percent sunshine, while February trailed the pack with only 279 days.
Dear Tom,
Lightning is in the news again with the Air France jet tragedy over the Atlantic Ocean off Brazil. How often are jets struck by lightning?
Terry Proce
Dear Terry,
Lightning strikes to aircraft are quite common. Federal Aviation Administration statistics indicate that commercial aircraft are struck about once during every 5 to 10 thousand hours of air time. University of Florida lightning expert Dr. Martin Uman says, "Because modern commercial aircraft are essentially all metal, lightning currents seldom penetrate them. ... Generally, lightning leaves pit marks or burn marks on the aircraft's metallic skin or burn or puncture holes through it."
Most strikes cause no significant damage because the electrical current flows harmlessly across the metal skin of the aircraft. In addition, special shielding protects all of the craft's sensitive electronic equipment.
Dear Tom,
If the reliability of weather forecasts diminishes rapidly beyond seven days, how can forecasts of global warming 100 years hence have any validity?
-- Robert J. Roeder, Des Plaines
Dear Robert,
Weather and climate are quite different. A weather forecast predicts the details of day-to-day weather, a process in which small errors grow as the forecast advances in time. These errors eventually overwhelm the accuracy, especially in predicting small-scale features of evolving weather patterns. Climate, however, is average weather over a period of decades or more. A number of the mechanisms that produce large scale alterations in the climate are quite well understood making predictions of climate change possible. Studies have shown that climate models using weather observations from decades ago can accurately predict changes up to the present.
Dear Tom,
My dad says the summer of 1988 had more than 50 days above 90 degrees? Is that true?
Marc Johnson Western Springs
Dear Marc,
You dad is almost correct. The summer of 1988 takes the honors for the most 90 degrees days--- 47 officially recorded at O'Hare International Airport ----not quite the 50 days your dad indicated. Included in that plethora of 90s were seven 100 degree days. The hot weather began with three straight 90s from May 30-June 1. June went on to record 13 days of 90 degrees or higher, July tallied 16 days reaching that mark, August had 14 and September logged a lone 90 (Sept. 17). Though the day's were beastly hot, 1988 was a drought summer with fairly low humidity allowing temperatures to drop at night, with many overnight lows falling into the upper 50s and lower 60s.
