Dear Tom,
The air conditioning season is here again, and my co-workers will be arguing about thermostat settings in the office. What are your thoughts about ideal temperatures?
Greg Bolick
Dear Greg,
Indoor comfort levels are a matter of personal preference, and our "internal thermostats" are all set slightly differently. Air temperature is a big factor, but relative humidity and air movement are important too.
We checked with acknowledged experts in the field of indoor environmental comfort: the 55,000-member American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), headquartered in Atlanta. An ASHRAE study determined that a temperature of 74 to 76 degrees in an office environment is comfortable for most sedentary people when the relative humidity is 50 to 60 percent and there is a slight air movement (but not strong enough to be a draft).
Dear Mr. Skilling,
My teacher, Mrs. Denning, said Chicago sometimes gets very hot weather, and she said
it is a scorcher when it hits 100 degrees. How many days have been that hot, and what
is Chicago's hottest temperature?
---Ashley Jameson (5th grade)
Dear Ashley,
Few Chicagoans would disagree with Mrs. Denning when she calls a 100-degree day a scorcher. Since the start of Chicago's temperature records on Nov. 1, 1870,the city has had some real scorchers. Chicago has logged 61 days with temperatures of 100 degrees or higher, and the last time it happened was July 24, 2005, when it hit 102. Chicago's official highest temperature is 105(July 24, 1934), when the thermometer was at the University of Chicago. One day earlier it was 109 at Midway Airport, and that is Chicago's highest recognized, though unofficial, temperature.
Dear Tom,
I graduated high school in Chicago on June 21, 1950, in an outdoor ceremony
held on the school steps. I remember that it was so hot that some of the
girls didn't wear dresses under their robes. How hot did it get that day?
Ann Lee, Chicago
Dear Ann,
Your graduation day was probably not as hot as you remember, but it was warm
and rather humid with partly sunny skies. There were some showers, but
rainfall was light with most areas receiving nothing more than sprinkles.
Climatologist Frank Wachowski informed us that high temperatures across the
area that day were in the lower and middle 80s with dew points hovering in
the lower 60s. The city's official high observed at Midway Airport was
85 degrees. The next few days were even warmer, reaching 88 and 90. It
probably felt hotter than it actually was because of the heat absorbed by
the school building.
Dear Tom,
Living in Chicago during the summer of 1969 I seem to remember that it rained nearly every weekend. Am I correct?
-Mike Becker
Dear Mike,
Your question intrigued us and after checking it out with the help of Chicago climatologist Frank Wachowski we found that you are absolutely correct. Some rain was recorded on an astounding 11 of the 14 weekends (79 percent) of meteorological summer (June, July, August) in 1969---and 6 of those 11 weekends recorded rain on both days. Every weekend in June and July logged at least one rainy day. Only August brought some relief from the rainy weekend syndrome with three dry weekends and two having only one day with rain.
That summer's two rainiest weekends were June 7 and 8 when 2.10 inches fell and July 26 and 27 with 1.59 inches.
Dear Tom,
We're planning a big graduation party for June 6 or 13, preferably outdoors.
What is the weather likely to be?
Michael Krainas
Dear Michael
The ability to reliably predict details of the weather beyond seven days diminishes rapidly. Forecasts in the 7 to 16 day range are best at offering general insights into the weather pattern likely to prevail, such as the probable orientation of the weather-steering jet stream or the trend in precipitation. Weather forecasts, as we move into summer, become especially tricky because of thunderstorms. These storms can be notoriously troublesome for forecasters because of the cool outflow of air they produce which all too often alters other features of the large scale weather pattern. Because thunderstorms can be short-lived and comparatively small, predicting the specifics of their evolution is difficult at longer time ranges.
Climatologically, the chance of rain on any given day during the first half of June is one in three. Bear in mind, though, that a thunderstorm usually passes in an hour or two, and rarely is a thundery day a total washout. Even on a "rainy" summer day, most of the hours are rain-free.
Dear Mr. Skilling,
We are studying weather and I want to ask you how many days in a year we have precipitation. How many days do we have heavy precipitation?
-Robert, Wilmette
Dear Robert,
The Chicago area receives precipitation (like rain, snow, drizzle and sleet) quite often. That comes as no surprise, considering that far above-normal precipitation has been drenching us since last summer. In 138 years (1871 through 2008, or 50,404 days) measurable precipitation (0.01 inch or more) has fallen on 17,153 days, or an average of 124 days per year. In addition, a trace of precipitation (an amount too small to measure, like just a few drops of rain or a few flakes of snow) has fallen on 8,292 days, or 60 days per year. On average, at least some precipitation (trace or measurable) falls on 184 days, or half of the year's 365 days. Heavy precipitation (1.00 inch or more) arrives on seven days per year.
Dear Tom,
Standing on a 75-foot-high bluff just south of Saugatuck, Mich., we saw what appeared to be the Milwaukee skyline across Lake Michigan. Was it a "Fata Morgana" type of mirage?
Bonnie Wheaton
Dear Bonnie,
What you saw was a "superior mirage" sometimes called a Fata Morgana, caused by a temperature inversion with a layer of cold, dense air over the lake and warmer air above. The inversion bends the light rays from the city, creating stretched or "towering" images sometimes resembling bar codes back down towards the chilled air near the lake's surface. Fata Morgana is Italian for Morgan the fairy, a mythical character that supposedly lived in a crystal palace beneath the waves. The name came from the fairylike castle mirages that frequently appear over the Strait of Messina separating Sicily and Italy.
Dear Tom,
How often has it been 90 degrees or higher on Memorial Day? Has the temperature ever failed to reach 70 degrees?
Nick Recchia, River Grove
Dear Nick,
Memorial Day, originally known as Decoration Day, is a national day of remembrance of U.S. men and women who died while in military service. Until 1970, Memorial Day was always observed on May 30, but since 1971 it has been observed on the last Monday of May (and the date varies from May 25 through May 31).
On average, the Memorial Day temperature climbs to 90 degrees or higher on one day out of eight. However, Lake Michigan, whose water temperature off Chicago is often in the 40s in late May, greatly reduces the occurrence of 90-degree days to about one in 70 at the lakeshore. High temperatures fail to reach 70 degrees quite often: one day in three inland, and one day in two at the lakeshore.
Dear Tom,
Why aren't there hurricanes in California if it is by an ocean?
-Ellen Szostak, age 10
Dear Ellen,
The reason why California doesn't get many hurricanes is because the ocean water there is just too cold. Tropical cyclones generally need a water temperature of 80 degrees or higher for formation, and the waters off the California coast usually range from the middle 70s in the far south portion of the state to the very chilly upper 40s near the Oregon border. The eastern Pacific Ocean gets its share of hurricanes, but most develop in the warmer tropical waters from Central America north to Baja California. These storms sometimes strike the west coast of Mexico, but most tend to move west into the open Pacific and dissipate. On rare occasions, one of these storms will move north and affect the southwestern United States.
Dear Tom,
I often hear the term "degree day units" and it has me stumped. What are
they and what do they mean?
Mike Matuk
Dear Mike,
Heating and cooling degree days are measures that indicate the departure of
a day's average temperature from 65 degrees. When the average temperature
(sum of the high and low divided by two) equals 65, engineers have
determined that neither heating nor cooling is needed to maintain a
comfortable indoor environment. When the day's average temperature is above
65, each degree of departure is one cooling degree day (CDD) and when below
65 one heating degree day (HDD). Running totals are kept for both HDD and
CDD giving a quick one-number snapshot to compare seasonal heating and
cooling costs. The HDD season runs from July 1-June 30 to capture the entire
heating season while CDD totals are kept on a calendar year basis.
Dear Tom,
Is there such a thing as lake-effect rain?
John Lustrup Glen Ellyn
Dear John,
While we most often hear about lake-effect snow, lake-effect rain does occur
over Lake Michigan and the other Great Lakes, but is it a rare phenomenon
usually seen in autumn. In order for lake-effect precipitation to form, the
temperature of the air about a mile above the lake surface must be at least
20-25 degrees colder than the water. That magnitude of temperature
differential is seldom realized in summer so we rarely experience
lake-effect rain. When these conditions do occur, they are frequently in
conjunction with potent early autumn cold outbreaks packing air sufficiently
chilly to generate lake-effect rain. The very cold air over the warm lake
creates a very unstable atmosphere and the lake-effect rain is often
accompanied by thunder and lightning.
Dear Tom,
What is the largest number of consecutive 100-degree days we have ever had
in Chicago?
Paul Sarewich, Chicago
Dear Paul,
In official temperature records dating from 1871, Chicago has logged three
consecutive 100-degree days on two occasions: July 3-5, 1911 (100, 102, 102
respectively) and August 4-6, 1947 (100, 100, 101). Those are Chicago's
longest strings of 100-degree days, but they are hardly a true
representation of the blistering heat that has occurred in the city.
Chicago's official thermometers were sited near Lake Michigan (in the Loop
or at the University of Chicago) from 1871 to 1942 and stifling heat waves
during that period were blunted by the city's well-known lake breezes. Data
from Midway Airport tell the real story. Eight consecutive days of
100-degree temperatures roasted the city on July 7-14, 1936: 102, 106, 100,
106, 107, 100, 102 and 104.
Dear Tom,
Growing up in Mendota, Ill. In the 1950s I remember one very hot decade with
only a small fan to cool me off. Just how hot were those summers?
Terry Guilfoyle, Chicago
Dear Terry,
The 1950s brought Chicago and the surrounding Midwest some of its hottest
summers on record. Based on average temperature, five of the city's
11 warmest summers (June, July, August) occurred during that decade,
including 1955, the warmest summer on record with an average temperature of 76.4
degrees. Days in the 90s were rampant with 1955 leading the pack with 46.
Other summers in the 1950s with an excess of 90s were 1953 (42), 1959 (39),
1952 (38) and 1954 (36). Based on Midway Airport data dating back to 1928,
the 1950s logged 276 days of 90 degrees or higher, second only to the "Dust
Bowl" summers of the 1930s with 343 such days.
Dear Tom,
How often does it get to 100 degrees in Chicago? Has it ever reached 105 degree?
John Behrens Glenview
Dear John,
The official Chicago thermometer has cracked the 100-degree mark just 61 times in the city's nearly 139 years of weather records, a frequency of about one every 2.25 years.
It's been nearly four years since the city's last 100-degree day: July 24, 2005 when the high reached 102. Officially the city's hottest reading is 105 degrees and that occurred just once, on July 24, 1934.
Unofficially that temperature has been topped nine times at Midway Airport, mostly during the torrid summers of the 1930s, including a scorching109 on July 23, 1934. During the city's killer heat wave in July 1995, Midway peaked at 106 degrees on July 13, while the official reading at O'Hare International Airport topped out at 104 degrees.
Dear Tom,
A relative who lives in Denver believes that in winter Chicago's higher
outdoor humidity makes one feel colder than in Denver's drier air at the
same outdoor temperature. It's the opposite of humidity's effect in the
summer. Is this correct?
Rob Whitecotton, Hoffman Estates
Dear Rob,
Probably, but the explanation is as much psychological as it is physical.
Winter days with high humidity are often cloudy, gray and gloomy, and that's
the kind of weather that creates an impression of coldness. In the winter,
Chicago experiences many more of those dreary days than does Denver.
Here's a compelling physical explanation: Thermal conductivity -- the rate
at which heat moves from warmer to colder places -- increases as the
relative humidity rises. Even when we're wearing heavy winter clothing, our
bodies lose heat more quickly in humid air than in dry air, and we feel
cold.
Dear Tom,
What the heck is "fetch"?
-Don Schlax, Deerfield
Dear Don,
Like so many words in our language, fetch has a wide variety of meanings. In meteorology,
oceanography, the nautical sciences and related fields, fetch usually refers to the
uninterrupted distance that wind of uniform direction blows across a body of water for the
generation of waves. That's the meaning we use on this weather page. A north wind (air
moving from north to south) blowing down the full length of Lake Michigan traverses 307
miles of water; it has a wave-generating fetch of 307 miles. A west wind blowing across the
lake's greatest width has a fetch of 118 miles. Air-water temperature difference, wind speed
and fetch are the primary factors that determine wave heights, and an accurate wave forecast
depends upon a correct estimate of all three.
Dear Tom,
"Global warming" must be a recent phrase because I don't recall hearing it
in the 1970s when I was in college. When did it first get started?
Jeremy Johnson
Dear Jeremy,
Global warming as a distinct phrase has been around for 34 years; it first
appeared in 1975 -- and it has a Chicago connection.
Dr. Wallace Broecker, the world's premier authority on abrupt global climate
change and a long-time researcher at Columbia University in New York, is a
native of Oak Park. We asked Broecker if he originated the term and his
humble response was, "I guess I did, but it was inadvertent."
In 1975, worldwide temperatures had been falling for 20 years (Chicago's
too) and the concern was that a new ice age was imminent. Broecker dismissed
that concern and, in 1975, published a paper titled, "Climate Change: Are We
on the Brink of a Pronounced Global Warming?" The phrase caught on.
