Dear Tom,
You recently said the last time we had a July with no 90s was back in 2000. What happened the rest of that summer?
Samantha Higgins, Chicago
Dear Samantha,
It has been nine years since the city has experienced a July without a 90. In terms of days in the 90s, the summer of 2000 actually started out cooler than the current one which has already recorded three consecutive highs of 94 degrees on June 23-25. In 2000 the city did not record its first official 90 until Aug. 15 when it finally reached 92 degrees. The heat-challenged summer of 2000 went on to produce only three more 90-degree days which occurred consecutively Aug. 31-Sept. 2. The highest temperature that year was only 93 degrees, logged on Sept. 1. The weather remained warm into early October with many days in the 80s, but the mercury never again reached the 90-degree mark.
Dear Tom,
In Erik Larson's book "Devil in the White City" there are frequent weather references, among them one about unusually warm weather on December 15, 1890. Are these references historically correct?
Jerry Halperin
Dear Jerry,
With the help of Chicago climatologist Frank Wachowski we cross-checked several of the weather references in the book with the original Chicago weather records and they are indeed correct. The day in question, December 15, 1890, the high climbed to a balmy 48 degrees melting the last traces of what remained of the city's seven inch snowpack that fell earlier in the month. We previously answered a similar question about Larson's reference to a funnel cloud that blew through the Midway at Columbian Exposition's on July 9, 1893 and found that reference to be meteorologically correct as well.
Dear Mr. Skilling,
I recall a rather cool and cloudy summer, like we are experiencing now, in 2001 or 2002. Can you confirm my recollection?
Mark J. Girolamo, Aurora
Dear Mark,
Memories can be deceiving. It's certainly true that this summer has been cool. As of July 26 (when you wrote to us), it was running 2.7 degrees below normal and it ranked 5th coolest out of 50 years of temperature data at O'Hare Airport. On average, 10 of the annual tally of O'Hare's 17 days at or above 90 degrees should already have occurred, but we had logged only three.
However, the summer of 2002 (through July 26) was hot: 2.6 degrees above normal, 7th hottest, 16 days in the 90s. In 2001, summer ran near normal: 0.4 degrees below normal, 28th warmest, 10 days in the 90s.
Might 2000 have been the cool summer that you recall? With an average temperature of 68.9 degrees and no 90-degree days, it ranked 9th coolest.
Dear Tom,
How many Julys and Augusts have failed to record a 90?
Norman H. Jannusch, Schaumburg
Dear Norman,
Using the city's official station weather records which began in 1871, Chicago has recorded only six Julys that failed to produce a high temperature of at least 90. This July stands to become the seventh and the first since 2000. Augusts without 90s are a little more common here, having occurred 16 times, most recently in 1986. Further checking reveals that there have been only two years, 1875 and 1915, where neither July nor August recorded a 90. In fact, 1875 is the only year that failed to produce any 90s at all. It looked like 1915 would become the second such year, but a late-season temperature surge pushed the mercury to the elusive 90-degree mark on Sept. 14 for the year's single occurrence.
Dear Tom,
In contrast to this summer, I remember some of Chicago's highest dew points when my daughter was born on July 31, 1999. Am I correct?
Ralph Bornhoeft, Green Oaks, Ill.
Dear Ralph,
Your daughter was born just one day after the city sweltered through one of its steamiest days in history. For nearly 16 hours, from the evening of July 29 to midday on the 30th, Chicago dew points remained at Amazon-rain-forest levels of 80 degrees or higher, peaking at an all-time record high of 83. At 9:33 a.m. on July 30 the official O'Hare temperature stood at 90. Combined with that record 83 dew point, the heat index soared to a suffocating 113.
Still humid but slightly drier air moved in during the afternoon of the 30th, allowing the dew point to fall to 69, but with the mercury soaring to 101 the heat index only dropped to 108.
Dear Tom,
Why are all planets spherical instead of some square, triangular, etc. Does it have to do with gravity?
Fred Spitzzeri
Dear Fred,
Indeed, it does. Gravity is a force that pulls everything evenly toward the enter of the planet. Over time, this results in a spherical planetary shape.
Even "solid" materials like rock flow like a liquid, albeit incredibly slowly, when they are pulled by a strong, steady force for a very long time. "Softer" solids such as ice respond more readily, and that is one of the mechanisms of glacial motion. Despite gravity, planets are not perfectly spherical. Every feature of the terrain -- mountains, valleys, buildings, even transient features like ocean waves -- constitute a deviation from a perfectly spherical planet.
Another force is also at work. Gravity maintains a planet's near-spherical shape, but rotation introduces centrifugal force that causes it to bulge at its Equator.
Dear Tom,
My thermometer has been registering temperatures in the 60s this July. If global warming is the real truth, why don't we have readings in the 100-degree range by now?
Bruce Ameismeier, Chicago
Dear Bruce,
Given this summer's unusually cool weather (third coolest in 82 years at Midway Airport), your point is understandable. However, Chicago is not the world and it is risky to suppose our cool summer is representative of the global situation, because it is not.
NOAA's Climate Diagnostics Center provides a daily global picture (derived from infrared satellite imagery) of areas of above-normal and below-normal temperatures. The global picture is not what you might expect: Areas currently experiencing above-normal temperatures (such as the Western U.S.) greatly exceed areas running below normal.
Dear Tom,
On a sunny spring afternoon, a strange wind pattern in the newly plowed cornfield behind our house raised dust and corn husks into the air in a circular fashion. Coincidentally, thunderstorm winds caused minor damage at our house that evening. Was the first incident a prediction for the evening storm?
Irene and Ed Sandner
Dear Irene and Ed,
Your characterization of the two phenomena as coincidental is correct -- they were not related. The first event was a dust devil. On sunny, tranquil days, a shallow layer of hot air develops above soil that is being strongly heated by sunlight. That air rises and the resultant in-rush of replacement air will sometimes spiral inward, setting into motion the whirlwind that you witnessed. Dust devils develop at ground-level, whereas thunderstorm winds result from the down rush of rain-chilled air that surges outward beneath and ahead of the storm.
Dear Tom,
The flight times of commercial jetliners flying between Chicago and Los Angeles, cruising at 35 thousand feet, are not affected by jet streams nearly as much as aircraft in the 1950s and 1960s, cruising at 20 thousand feet. Are today's jets flying above the jet stream?
Al Claus, Vernon Hills
Dear Al,
Retired United Airlines pilot Phil Rider tells us today's commercial aircraft are much less affected by unfavorable winds than in the past not because they cruise above jet streams (which occur between 20 and 55 thousand feet) but because of "... better flight planning and the (pilot's) flexibility to choose among more routes and altitudes."
Rider explains that on-board computers can quickly compare flight times over a variety of routes, upper wind forecasts are better and today's flexible air traffic system allows pilots to chose the most advantageous flight path and altitude.
Dear Tom,
During a big thunderstorm in May, I and my family heard a distinct "pop" or
"click" that was instantaneously followed by a tremendous crash of thunder.
Lightning struck a tree only a hundred feet from the house. What was that
clicking noise?
Richard Johnson, Chicago
Dear Richard,
Lightning expert Ron Holle of Vaisala Inc. of Tucson Ariz., tells us any
source of such a sound requires being within a few hundred yards of the
ground strike point.
Holle explains that "The clicks may have been static discharge from upward
streamers from the house, the ground, or other nearby objects; these are
released just when a flash strikes the Earth's surface. Or, there may have
been a buildup of static charge on parts of the house just before the
strike. Sometimes on (AM) radios, you can hear this buildup as a whine or
series of clicks until the flash strikes the ground."
Dear Tom,
What, if any are the effects of tides on the Great Lakes?
Vince Venturella Carmel, Indiana
Dear Vince,
The Great Lakes, like all bodies of water, are affected to some extent by the gravitational pull of the moon and sun and therefore do experience tides. As on ocean, these tides occur twice daily but, unlike their ocean counterparts that are miniscule and are totally masked by short-period water-level fluctuations caused by wind and changes in air pressure. On Lake Michigan, these tiny tides range from about a half to 1.5inches and are unnoticeable.
Dr. Dave Schwab of the Great LakesEnvironmental Research Lab in Ann Arbor does acknowledge the existence of a larger Lake Michigan tidal swing in the Bay of Green Bay where local geography can generate about a four-inch tide.
Dear Tom,
Hurricane forecasting is supposedly better now than ever before, but Hurricane Katrina in 2005 makes me wonder if that is really true. What are your thoughts?
James Van Horn
Dear James,
Hurricane forecasts are more accurate. Dr. Hugh Willoughby, a Distinguished Research Professor in the Department of Earth Sciences at Florida International University, says, "The chances of dying in a hurricane have been reduced by a factor of 100 during the course of the 20th Century."
Indeed, hurricane forecast accuracy is improving at an average rate of one to two percent per year in recent years. Hurricane Katrina in 2005 was very well forecast, even three days in advance. The horrific death toll in Louisiana and Mississippi speaks to lack of adequate hurricane preparedness, not to forecast accuracy.
Dear Tom,
I was reading my mom's old Nancy Drew book that mentions a cloud formation caused by "orographic uplift." What is that?
Chloe Brougham, 5th grader
Dear Chloe,
Orographic lift is a rising air flow caused by mountains. As air moving with the prevailing wind flow encounters a mountain range, it is forced to rise. This causes it to cool and condense, forming clouds and eventually rain if the air contains sufficient moisture. This process, known as orographic lift, is a major precipitation-producing mechanism. In the foothills of the Rockies, east winds push air up the eastern slopes, producing thunderstorms in summer and foggy, drizzly, rainy and snowy weather in winter. Orographic lift accounts for much of the precipitation in some of this planet's rainiest locations including Mt. Waialeale in Kauai, Hawaii; Mawsynram, India; and Lloro, Columbia.
Dear Tom,
You recently mentioned that the summer of 1947 was cool, much like this summer, but didn't it turn hot in August?
--A.J. Simmons
Dear A.J.,
It certainly did. The summer of 1947 was running cool with only six days of 90 degrees or higher on the books through the end of July. Every month since February had logged below-normal temperatures. August appeared to be following suit, opening on a chilly note with a record low of 52 and an afternoon high of only 78 on Aug. 1. However, the weather pattern changed dramatically and blistering heat swept into the city. August 1947 stands as the city's hottest August on record with an average temperature of 80.2 degrees and the third-warmest month on record here. The month had 18 days with highs of 90 degrees or higher, including four days in the 100s.
Dear Tom,
We're in a dry climate here in Arizona, with temperatures in the 100s but low humidity. However, in your Chicago weather reports I never see a reference made to humidity levels. Why?
Elise Pearce
Dear Elise,
Much of Arizona lies in the Sonoran Desert, whose arid climate is so radically different from Chicago's that words like "humid" (and "hot") have very different implications in the two areas. We do use humid in our Chicago forecasts, but only when the dew point temperature nears or exceeds 70 degrees, a level rarely achieved in desert cities like Phoenix or Tucson.
In Chicago, "hot" appears in weather forecasts when readings rise to the lower 90s or higher, but in Phoenix that term is used only in reference to temperatures near or above 110 degrees. Here's a typical Phoenix forecast: "Mostly sunny, with a high near 104" -- no mention of heat.
Dear Tom,
I heard that the first leg of I-355 was finished early during the summer of 1988 because of good weather. How did that summer compare with other summers?
Tom Muelleman West Chicago
Dear Tom,
The summer of 1988 provided a mixed bag for road crews. On the plus side it was a drought summer with very few rainy days to halt construction. During a typical Chicago summer measurable rain falls on about 30 days, an average of 10 each month, but the summer of 1988 had measurable rain on only 17 days--4 in both June and July, and 9 in August--so there was little down time because of rain. On the negative side was the summer's intense heat that created brutal conditions for outdoor work. Officially at O'Hare International Airport there were 47 days with highs of 90 degrees or higher including 7 days with triple-digit heat.
Dear Tom,
The hurricane season in the Atlantic Basin seems to be quiet this year. What is the prediction for the rest of the season?
T. L. Oksana, Dundee, Ill.
Dear T.,
Forecasters at the Climate Prediction Center anticipate a near-normal hurricane season. They ascribe a 70 percent chance to the occurrence of 9 to 14 named storms somewhere in the Atlantic Basin, of which four to seven will be hurricanes (sustained winds of 74 m.p.h. or greater).
However, the outlook for the 2009 hurricane season is a tough call because of the presence this year of "competing climatic factors." On the one hand, a 20-40 year cycle of enhanced hurricane activity began in the Atlantic Basin in 1995. Conversely, an El Nino event is now under way. El Nino suppresses hurricane activity by increasing vertical wind shear (change of wind speed with height) over the tropical Atlantic Ocean.
Dear Tom,
I just read that we should expect an El Niño winter in 2009-10. Does this mean warmer temperatures for the upcoming winter season? Are we looking at more or less snowfall?
Dan Becker, Gurnee
Dear Dan,
Indeed, an El Niño event (abnormal warming of water in the equatorial belt of the Pacific Ocean westward from South America) began in June, is likely to strengthen in the autumn and persist through winter. That's the word from the National Weather Service's Climate Prediction Center.
El Niños have little impact on U.S. temperature and rainfall patterns during the summer and early autumn, though they can help to suppress Atlantic Ocean hurricane activity.
Late autumn and winter is another story. At Chicago, "El Niño winters" are usually milder and less snowy than normal, sometimes dramatically so if the El Niño event is especially strong. We'll keep you posted.
Dear Tom,
I received an e-mail saying Mars will be very close to the Earth on Aug. 27 and will appear to the naked eye to be as large as the full moon. Is this true?
Carol Notley
Dear Carol,
It's an e-mail hoax, and it keeps popping up. Containing phrases like, "The Red Planet is about to be spectacular" and, "On August 27th Mars will look as large as the full moon" and, "No one alive today will ever see this again," that e-mail has garnered international attention -- but it's just not true.
It all started when, on Aug. 27, 2003, Mars orbited within 35 million miles of Earth, its closest approach in just under 60,000 years. At that time, Mars did appear as large as the full moon -- but only when viewed through binoculars or a telescope at 75-power magnification. Mars made another close approach to Earth in December 2007: 55 million miles. These approaches had no effect on Earth.
Dear Tom,
For gardening purposes, there are light requirements for plants for which I cannot find answers. When is the hottest time of the day, and when does the hottest time of the day give way to less intense sun?
--Linda Davis
Dear Linda,
Daytime air temperatures usually lag a few hours behind the intensity of sunlight. On any given day, assuming no air pollution and no cloudiness, the most intense sun is always when the sun is highest in the sky. This occurs in Chicago at about noon (1 p.m. daylight-saving time). In the summer, the hottest time of the day in Chicago is usually within an hour of 4 p.m., but, depending on the weather, the time of the highest temperature can vary greatly. On rare occasions, it can even occur at night. By convention, the maximum temperature is the highest reading attained during the 24-hour calendar day.
Dear Tom,
On April 25, Chicago's temperature dropped 28 degrees in 14 minutes. What is the greatest change on record?
Ryan Thomas, Chicago
Dear Ryan,
Lake breezes aside, sudden temperature declines around here are not confined to any particular time of the year, but they do show a marked preference for the spring, and Lake Michigan is the culprit.
Very cool air approaching Chicago from the north travels the full 310-mile length of Lake Michigan, whose water in the spring is still cold. Such air arrives at Chicago with unabated chill. If the city happens to be enjoying a warm spring day, the arrival of cold air from the north sends temperatures plunging.
This happened on May 9, 1963 -- the date of Chicago's most stunning short-term temperature drop. Beginning at 1:47 p.m., the temperature at Grant Park plunged 22 degrees (from 84 to 62) in 150 seconds.
Dear Tom,
A Chicago suburb recently published a newsletter advising that a viaduct or highway underpass offers safety from an approaching tornado. Isn't this ill advised
Carole Jacobsen, Carpentersville, Ill.
Dear Carole,
Your question comes up frequently, and the answer deserves repeating: It's a very dangerous thing to do. Contrary to popular belief, expressway underpasses do not offer safe shelter from the high winds of tornadoes or severe thunderstorms.
Because of channeling and funneling, winds actually blow stronger when they sweep through expressway underpasses and beneath bridges, thereby increasing the risk of injury from airborne debris.
Misconceptions about the safety of underpasses probably originated with a widely circulated video of motorists rushing to an underpass in order to avoid a tornado that was crossing Interstate 35 near Wichita, Kansas, on April 26, 1991.
Dear Tom,
A friend and I have long debated the meaning of a 30 percent chance of rain. Can you help?
Randolph Penna
Dear Randolph,
Regardless of its accuracy, a weather forecast fails if the user does not understand the forecaster's words. The proper interpretation of a 30 percent chance of rain (assuming the forecast verifies perfectly) is that you will have rain on your head three out of ten times that you hear such a forecast.
The forecaster may believe rain will cover 100 percent of the area if the rain arrives, but his confidence that it will arrive is only 30 percent. Alternatively, the forecaster might have great confidence that rain will occur, but he believes it will be scattered showers affecting only 30 percent of the area.
Regardless of the forecaster's rationale, the meaning for you is always the same: The chance of rain on your head is 30 percent.
Dear Tom,
The National Weather Service website provides two short-range radar images: base and composite reflectivity. What is the difference?
Dave Makarski, Arlington Heights
Dear Dave,
Radars transmit a beam of energy, some of which is reflected back to the radar site when the beam strikes an object (such as a raindrop). Reflectivity, a measure of the amount of energy that returns to the radar, is greatest when raindrops and cloud particles are both large and numerous.
The radar beam is first transmitted at 0.5 degree above the horizontal, making a full circular sweep, followed by additional sweeps at progressively greater tilt angles. Base reflectivity refers to reflectivity when the beam is elevated 0.5 degree; its data indicate rain reaching the ground. Composite reflectivity displays in a single picture the highest reflectivity values from all beam tilt angles; it describes the storm's total water content.
Dear Tom,
On average, how many days a year does Chicago experience fog?
Alice H.
Dear Alice,
On many cool, calm mornings around sunrise, parts of the Chicago area, especially in the suburbs, will experience ground fog as the air cools to saturation. The fog usually burns off quickly, creating only brief inconvenience. There are many other days where light fog is present from a variety of causes resulting in reduced visibility. However, there are only an average of about 12 days each year when Chicago is socked in with dense fog (visibility one-quarter mile or less) that seriously impacts transportation. Dense fog is most common here during the cold season, frequently occurring when warm, moist air passes over cold snow-covered ground. December through March hosts a majority of the area's dense fog occurrences with each month averaging about two days.
Dear Tom,
Why does the air sometimes smell like "rain"?
Laura Farr, Berwyn
Dear Laura,
Theories abound about the "smell of rain", but so far there is no definitive answer. Many feel that the dominant vegetation of a region contributes to the odor. The reaction between the moisture in the air and certain volatile substances on plants appears to be a partial explanation. Pine forests, for example, release terpenes (substances found in perfumes or medicines) into the air. Some olfactory specialists believe that moisture, warmth and low pressure, all harbingers of rain, enhance our sensitivity to smell while hastening the release of fragrant molecules from plants. Once the rain begins, the drops can kick up ground particles that add to the aroma. Some suggest that raindrops push earthy smelling gases produced by streptomyces bacteria out of the soil.
Dear Tom,
I remember a very chilly 4th of July in the middle or late 1960s. At our picnic we were all huddled in blankets. Can you pinpoint the year?
C.J. Phillips, Chicago
Dear C.J.,
The year was 1967 and Chicago's high that day was only 64 degrees, a value more typical of late April or early May. It remains the fourth coldest Independence Day on record, out-chilled only by a 62-degree high in 1920, and 63-degree highs in 1882 and 1909. The day was generally dry, though a few sprinkles were noted. Skies were gray and overcast, and cool winds blew from the north and northwest. The month opened hot in typical July fashion with highs around 90 degrees, but the mercury plunged following the passage of a cold front. July 5 was equally chilly with the official high climbing only one degree to 65. Summer weather finally returned by July 8 with readings rebounding into the 80s.
Dear Tom,
What is "aphelion"?
Susan Bosserman Fairview, NC
Dear Susan,
Our starry-eyed sky guru, Triton College astronomer Dan Joyce, informs us that today is Aphelion Day, the one day in the year on which the Earth passes farthest from the sun. That's right: farthest. Surprising, but true. We're farther from the sun during summer than during winter (but remember, it's winter in the Southern Hemisphere now).
The average Earth-sun separation is about 93.2 million miles, but the path the Earth traces as it orbits the sun is that of an ellipse, not a circle. When the Earth arrives at the end of the ellipse most distant from the sun, as it does today at about 9 p.m. CDT, the sun will be 94.4 million miles distant. Joyce says that point, in the parlance of astronomy, is aphelion. At perihelion, the point of closest approach (Jan. 3), the Earth-sun separation is 91.3 million miles.
