ASK TOM WHY: April 2008 Archives

Dear Tom,
When can I be assured the danger of frost and freezing
temperatures in Chicago is past for good?
Jeffrey Rayner, Chicago

Dear Jeffrey,

Light frost can form on plant leaves on a clear, calm night even when air
temperatures a few feet above the ground are as high as 38 degrees. A heavy, killing
frost usually requires a temperature of 32 or lower.

Because overnight temperatures on any given chilly spring night can, in
extreme cases, vary by 20 degrees across the Chicago area, it is difficult to pin down
an exact date. It really depends on your location.

Here are some guidelines: Near the lake, light frost (38 degrees) has never occurred
beyond May 31; heavy frost (32) never beyond May 14. Within about 2-6 miles inland,
the corresponding dates are June 4 and May 13; 7-15 miles inland, June 22 and May
29; farther inland, add seven days.

Dear Mr. Skilling,

How can a rainstorm or snowstorm travel thousands of miles, constantly depositing
moisture but never running out of it? Why does it not eventually lose steam and dry
up?
John H. Deppen, Portage, Mich.

Dear John,

Your questions rank high among the issues that weather forecasters contend with on
a daily basis. The precipitation-producing capability of a storm system changes
constantly—increasing, decreasing, sometimes ceasing.

The reasons are complex, but one factor stands out: Low-level winds, especially
southerly winds in advance of a low pressure system, constantly deliver "new air" to a
storm system as it moves along. If the moisture content of that new air varies, as it
usually does depending on its source region (like the Gulf of Mexico), the
precipitation capability of the storm system will also vary.

Dear Tom,
Why are severe thunderstorm warnings are never issued for lightning?
Jim Froemke, Chicago

The National Weather Service issues a severe thunderstorm warning when it believes
a storm will produce winds of at least 58 m.p.h. and/or hail at least three-quarters
inch in diameter or larger. Lightning ground strikes are not severe thunderstorm
criteria.

Meteorologist Rusty Kapela at the Milwaukee NWS says, "Any convective cloud that
manages to generate one lightning bolt is, by definition, a thunderstorm. If lightning
were a criterion, even a weak thunderstorm such as a garden-variety thundershower
would require the issuance of a warning."

That would greatly diminish the effectiveness of the warnings. Kapela advises that
any thunderstorm should be considered severe with respect to lightning.

Dear Tom,
How rare is it for Chicago to receive snow in May?
—Gladys Reynolds
Dear Gladys,
Snow during May in Chicago is not as rare as you might think. The city’s May snow
records date to 1885, and in those 123 Mays, snow has been recorded in 31 of them, a
frequency of once every four years. Of course, the vast majority of those May snows
were insignificant. But over the years the city has experienced a few noteworthy May
snows. In 1940 the city was surprised by a 2.2-inch snowfall on May 1-2 with some
northwest suburbs receiving as much as 5 inches. On May 3, 1907, 1.3 inches of snow
was logged. The city’s latest measurable May snow took place on May 11, 1966, when
0.2 inches fell. Chicago’s most recent encounter with snow in May took place three
years ago, when a trace was reported on May 2, 2005.

Dear Tom,
There is a family story that says my grandmother melted snow to give my aunt her
first bath. She was born Downstate in Flora, Ill., about halfway between St. Louis and
Vincennes, Ind.., on June 12, 1936. Could this be true?
Nancy Miller, Dyer, Ind.
Dear Nancy,
We are certain it did not snow in Flora, Ill., during June 1936, but we think we
may have found the basis of your family’s story. June 1936 was a hot month in
southern Illinois, with high temperatures reaching the 90s and low 100s on many
days, though on the date of your aunt’s birth the high was only 81 degrees. Flora was
hit by several thunderstorms that month, including some that produced hail and
damaging winds. One of these storms occurred on June 22, battering the area with
hail and 0.8 inches of rain. It is possible your grandmother collected and melted the
hail to give her 10-day-old daughter a bath.

Dear Tom,
I remember a very warm late April in Chicago when I was a boy in the early 1960s. Can
you tell me what year that was?
Robert Klein
Dear Robert,
We believe the period you remember occurred during late April 1962. After two warm
days in the lower 70s in March, April opened on a cool note, with the high temperature
reaching only the lower 40s on April 1 and 2. The month remained cool for the first
three weeks. Warm weather finally moved into the area April 20, when the mercury
soared to 80 degrees and remained entrenched for the rest of the month. The
temperature climbed to a summerlike 86 degrees April 25, followed by highs of 88 and
86 the next two days as persistent southwest winds pumped very warm air into the city.
After a two-day cool-down to around 70, readings surged back to a steamy 88 degrees
on the last day of the month.

Dear Tom,
What is the difference between a "small tornado" and a "microburst"?
Paul Dunn, Bloomington, Ill.
Dear Paul,
A microburst is an area of strong and often damaging winds that occurs when rain-
chilled air, cooler and therefore heavier than its warmer surroundings,
surges down through a thunderstorm, then "splashes" violently outward when
it encounters the ground. By definition, a microburst affects an area less
than 2.5 miles in diameter.

Microburst winds, though limited in extent, can be strong. On June 29, 1990,
a microburst in Streamwood generated 150 m.p.h. winds that killed a person.

A tornado is a violently rotating column of air than extends from the base of a
thunderstorm down to the ground. The air within a tornado spirals upward in a
corkscrew fashion at speeds typically 100 to 300 m.p.h.

Dear Tom,
My job requires me to fly often. I have noticed that the "takeoff time"—the time
elapsed between when takeoff power is first applied and when the jet finally lifts off
—is greater during hot, humid weather. What is the explanation?
William Frederick

Dear William,
Air density is the issue. The wings of an aircraft generate lift that is

proportional to the density of the air through which the craft is flying, as well as its
forward speed. Reduce one and you must increase the other in order to compensate.

When attempting a takeoff in less-dense air, the pilot must bring the craft to a
greater speed to attain lift sufficient to get the craft airborne. This takes more time
on the runway.

Weather conditions that reduce air density are precisely the factors that you have
noticed: heat and humidity. Hot air is less dense than cold air and, although it seems
counter-intuitive, humid air is less dense than dry air.

Dear Tom,

Why was the moon shining blood red on the night of Sunday night?

Ken Boncela, Naperville, Ill

Dear Ken,

Many readers commented on the haunting shades of red and orange that the moon
displayed Sunday evening.

The moon does not generate any color of its own, but only reflects sunlight. Lunar
colors on Earth are a measure of our atmospheric clarity as moonlight passes through
it. Like sunlight, the atmosphere scatters the shorter wavelengths of moonlight,
leaving shades of red, orange and yellow. These colors are pronounced when the
moon is near the horizon and its light has to penetrate a greater distance of the
atmosphere. As the moon rises higher, the red/orange hues fade.

Because cloud-free, moonlit nights are often a result of high pressure, colors are
enhanced by haze or dust trapped under a high pressure’s inversion. On rare
occasions, warm colors are replaced by blue tints, a result of smoke from distant
forest fires.

Dear Tom,
What is the earliest date that Chicago has ever reached 90 degrees?
—Wayne Griffin
Dear Wayne,
Chicago’s earliest hot weather occurred nearly 80 years ago, with highs of 90 degrees
on April 10-11, 1930. Just two days earlier, with high pressure over the area, the
mercury dipped to 34 degrees. As the high moved east, strengthening southwest winds
delivered a flood of warm air. The mercury jumped to 70 on April 9, then soared to the
two record early-season 90s. Since 1871, the city has reached the 90-degree hot
weather benchmark only four times in April. The other two occurrences were a high of
90 degrees on April 30, 1942, and a 91 on April 22, 1980. The 1980 high of 91 was
notable because just a week earlier, the city was hit with nearly 4 inches of snow on
April 14-15, a testament to the huge temperature fluctuations Chicago can experience
in the spring.

Dear Tom,
Chicago’s temperatures are sometimes higher at night than during the day. How often
does this reversal of the normal temperature cycle occur?
—Thomas Rhiner
Dear Thomas,
Daily high and low temperatures are defined as the maximum and minimum that occur
during the 24-hour calendar day, local standard time. During the winter, Chicago’s
daily high occurs between 1 and 4 p.m. on 40 percent of the days. In the summer, the
percentage rises to 65 percent. On 13 percent of the days (one day in eight) the high
occurs about midnight. Frequently in winter, during a warm-up after a cold spell,
temperatures will continuously rise, with the day’s high being reached at midnight. Or
if a strong cold front passes the previous night, the day’s high will come at midnight at
the beginning of the calendar day, with temperatures falling the rest of the day.

Dear Tom,
When did the science of meteorology begin? Can you give an approximate year?
Fred Fredericks, Belle Plaine, Iowa
Dear Fred,
The earliest documents about weather, preserved on fragments of papyrus and clay, come
to us from ancient Egypt and Babylonia, and date from about 2500-2000 B.C. At about the
same time, ancient Chinese scholars had begun keeping descriptive weather journals by
about 2200 B.C., and during the Shang Dynasty in 1600 B.C., scholars attempted to
analyze weather in 10-day blocks. Later, in ancient Greece, Aristotle formalized the study
in his “Meteorologica” (340 B.C.). As you might suspect, the term “meteorology” comes to
us from Aristotle’s work. At the time, a “meteor” referred to hail, rain, clouds,
rainbows—basically any substance suspended in the air or that fell from the sky.

Dear Tom,

Spring means more thunderstorms and that always causes controversy at outdoor
sports events like baseball and soccer. I maintain that such events should be stopped
as soon as thunder is heard, but many disagree with me. What is your opinion?

Samuel Widder

Dear Samuel,

I will defer to the experts on this one, and without exception they all say that outdoor
activities should cease immediately when thunder is heard. Thunder is a by-product of
lightning and if thunder is heard, lighting is also present.

Brian Bennett, a principal author of the NCAA's official lightning safety rules, says
thunder should prompt an orderly evacuation of the field and that the refuge of a safe
shelter should be sought. Furthermore, he recommends that outdoor activities not be
resumed until 30 minutes after the last rumble of thunder. It's unfortunate, but
lightning safety is very inconvenient.

Dear Tom,
My parents gave me a picture that was taken on our farm in Manteno, Ill. after a mid-
April 1961 snowstorm. Do you have any record of this storm?

Janet Sorensen, Wilmington, Ill.

Dear Janet,
Your picture, which has been posted on our weather blog at www.wgntv.com, was
taken after one of the heaviest late-season snowstorms to ever hit northeast Illinois.
The storm began late April 15 and continued into the early morning hours of April
17, bringing the area 5-8 inches of heavy, wet snow. Strong winds piled the snow
into 6 foot high drifts. The Chicago area measured 6.8 inches of snow while south of
the city near Manteno, Kankakee received 5.6 inches. Like all late-season snows, this
one did not last long. Temperatures quickly rebounded into the 60s and 70s and the
snow was gone in a day or two.

Dear Tom,
What kind of spring did Chicago experience in 1967
prior to the Oak Lawn tornado?
Veronica Michaels

Dear Veronica,
The Oak Lawn tornado, the deadliest to ever hit the Chicago area, killed 33 people as
it raced through that southwest suburb on Friday evening, April 21, 1967. Before the
tornado, the city had been experiencing a typical spring.

March remained chilly until the middle of the month, when the last vestiges of
January’s paralyzing 23-inch “Big Snow” finally melted. March closed with several
days in the 60s and 70s. The mild weather continued through mid-April, with eight
more days in the 70s, including a 74- degree high on the day of the tornado. The
weather turned cold in the wake of the twister. Three inches of snow blanketed the
city on April 23, followed by the city’s first below freezing temperatures since March 22.

Dear Tom,

Why do the sun and moon look larger as they rise and set and are close to the
ground?

Barry Baxter, Ottawa, Ill.

Dear Barry,
Both the sun and moon seem to be larger when they are positioned near the horizon,
but it is entirely an illusion.

It's such a compelling illusion, though, that it has a name: the "moon illusion."

No single explanation sufficiently accounts for the illusion, but it probably results
from the varying ways our minds relate the size and distance of the sun and moon
when they are viewed by themselves (high in the sky) or against a backdrop of other
objects (near the horizon). The sky at the horizon seems more distant than the sky
overhead. When viewed against the "more distant" horizon sky, the sun and moon
appear larger than when seen against the "closer" overhead sky.

Dear Tom,
When I was a kid, we couldn't count all the stars in the sky. Now I have to look hard to
see one. How come the change?
—J. Roberts, Batavia
Dear J. Roberts,
Triton College astronomer Dan Joyce says you can blame metropolitan lighting. Here's
Joyce's answer: "Back in 1978, I sent a 'real telescope' to 'Star Wars' creator George
Lucas, claiming that with it he could detect galaxies over 300 million lightyears away.
But it was with a caveat: I warned him that proximity to city lights would spoil the view.
"Imagine the disappointment of those of us in astronomy who seek to show off the
marvels of the cosmos, understanding that the galactic light has traversed space so
vast, only to be obscured in the last fifty-thousandth of a second as it encounters the
glare of metropolitan lighting wasted on the night sky."

Dear Tom,
I read a book about the 1930s Dust Bowl, “The Worst Hard Time.”
It mentions a dust storm that impacted Chicago.
What exactly happened here?
—Mike Rehor

Dear Mike,

During the 1930s, Chicago along with the Plains and Midwest were in the
grasp of a combination of heat and drought known as the “Dust Bowl.” These
summers were characterized by searing heat and blowing dust, and it was
during this period that Chicago recorded many of its highest temperatures.
The dust storm of May 10-11, 1934, was remarkable. With northwest winds
gusting to more than 50 m.p.h. and relative humidity levels at a record low
13 percent, clouds of dust swept into Chicago, reducing the Visibility to
less than one mile in many areas. Officially, seven days in the 90s were
logged in May 1934 including a scorching 98 degree high on May 31.

Dear Tom,
How can animals detect weather before something happens?
Paul See, Schaumburg
Dear Paul,
Your question is representative of many that we receive concerning the ability of
animals to predict the weather. But contrary to popular belief, there is no documented
evidence that animals have that ability. Authorities from a range of disciplines agree
that animal behavior, even when it is uncharacteristic, is a response to past or current
weather, or the result of instinctive imperatives and is not based on predictive ability.
That goes for plants, too. Such conditions as an exceptionally great (or poor)
production of seeds, as with oak trees and acorns, or premature leaf coloring in the
autumn invariably result from past weather conditions or other environmental factors
and not from any predictive ability that the organism might possess.

Dear Tom,

What is the origin of the phrase, “dizzying heights”? A friend believes it refers to the
sensation you can get when you are at the edge of a precipice. I think it has to do with
the atmosphere.

William Johnson

Dear William,

Manned, hot-air ballooning got its start on Nov. 21, 1783, when brothers Joseph and
Etienne Montgolfier launched a balloon in Paris and were airborne for 20 minutes.
Technological advances followed quickly and by the early 1800s balloon pioneers were
able to ascend high enough that the amount of oxygen in the air was dangerously thin,
giving rise to the phrase“dizzying heights.” The atmospheric content of oxygen—21
percent of dry air by volume—does not change as we go higher, but the amount of
oxygen that we can inhale diminishes because the density of air decreases with
increasing height.

Dear Tom, I saw your statistic about the large number of days that have passed without a 70-degree temperature here in Chicago. Could you expound? John Meyers Dear John, Chicagoans are growing increasingly anxious for at least a hint of spring warmth. And it’s no wonder: The city’s temperature last reached 70 degrees on Oct. 21 — 172 days ago — and the interval since that warmweather benchmark continues to expand as one chilly day follows another in a spring characterized, so far at least, by subnormal temperatures. On average, Chicago experiences its first 70-degree day on March 26 (although that occurrence is delayed until April 9 in the immediate vicinity of Lake Michigan). The average number of days between autumn’s last and spring’s first 70-degree day is 143, and the record interval is 185 days (Oct. 21-April 22, 1943-44).

Dear Tom,
It’s snowing and thundering here
in late March. How is this possible?

Crystal Hepner, Gary, Ind.

Dear Crystal,
Lightning does sometimes occur during
snowstorms and meteorologists have a
colloquial name for the phenomenon:
thundersnow.

It’s a rare event, occurring in the Chicago
area only a few times in a given 10-year
period. Lightning injuries have been
recorded in the area during snowstorms.
Typically during snowstorms, moist and
relatively stable air lows gently upward
over colder air; it cools as it ascends and
its moisture condenses into snow.
However, on rare occasions when a
vigorous low-pressure system is passing
across the region and it has drawn
unstable air, usually of Gulf origin, into
its wind system, the moist air will surge
into powerful rising currents that build
into bona fide thunderstorms.

Write to: ASK TOM WHY,
2501 W. Bradley Pl.,
Chicago, IL 60618
or: asktomwhy@wgntv.com

Dear readers,
In January we introduced a new weather page in the Tribune. While some readers liked the new look, others were disappointed to see the disappearance of such features as forecast maps for all seven days, expanded U.S. city forecasts, planet watch and Lake Michigan conditions.
Because of your passionate responses, we have been working on determining the best way to bring these features back. We are happy to report that, starting today, all four of the features listed above have returned. As we head into summer, we will also bring you daily pollen counts provided by Dr. Joseph Leija and sunburn times from Dr. Bryan Schultz.
We hope you like these changes. We welcome your comments as we try to provide our readers with not only the most informative weather page in the nation but one that meets all your meteorological needs.
--Tom Skilling, WGN-TV Chief Meteorologist