ASK TOM WHY: June 2008 Archives

Dear Tom,
You mentioned on the 9 p.m. newscast on WGN (on June 26) that telephone use
during a thunderstorm is dangerous. Does that include cell phones?
Carole Dussaussoy

Dear Carole,
It does not. Lightning's electric charge is conducted through the air and ground along
paths of least electrical resistance. If lightning strikes near a building (a tree next to a
house, for example), its electric charge can be conducted into the structure through
telephone and electric lines (even if they are buried) and underground water and gas
pipes. Using a corded telephone while a thunderstorm is in progress is discouraged
because the phone is physically connected by wires to the outside. A cell phone,
however, has no such physical connection and the electric current from a nearby
lightning strike cannot reach it. It is perfectly safe to use a cell phone during a
thunderstorm.

Dear Tom,
You recently corrected erroneous data that showed the record high and low
for June 5 occurring in the same year. Has that ever happened here?
John Dawson, Oak Park
Dear John,
For a city to establish a record high and record low on the same date is an
extremely rare event, one that has never occurred in Chicago. In most cases
extraordinary meteorological circumstances are required to accomplish this
feat. A dry atmosphere is almost always a prerequisite along with unlimited
sunshine, and clear skies and light winds at night. Such conditions were
present in August 2002 when a rash of same-day record highs and lows were
set in the western United States. Unbelievably, Park City, Utah, broke or
tied its record high/low three times in one week (Aug. 11, 13 and 15) and
Alamosa, Colo., did it on consecutive days (Aug. 25 and 26). A single
occurrence was also recorded on Aug. 25 at Safford, Ariz., northeast of
Tucson.

Dear Tom,
I hear meteorologists use the term "clear" skies and "fair" skies. What is the difference
between the two?
—Larry Jahn, Macomb

Dear Larry,
The term "clear" skies means exactly that: a near or total absence of clouds. It is a phrase
that most meteorologists like to reserve for nighttime forecasts, preferring to use an
unqualified "sunny" on a cloudless day without the need to modify the sunshine forecast
with the usual qualifiers of "mostly" or "partly." "Fair," on the other hand, isused to
describe pleasant weather conditions. Its use implies no precipitation, limited low-cloud
sky cover, good visibilities and light winds. Many forecasters rely on a forecast of "fair"
for the hours of darkness when thin, high-level cirrus clouds are present; the stars and
planets are still visible, but the presence of these high clouds precludes the use of "clear."

Dear Tom,
When we left Phoenix is was 105 degrees. Can it get that hot here?
Elise White Pearce
Dear Elise,
Triple-digit heat is common in Phoenix, occurring more than 100 days
annually, including 17 days when the mercury reaches at least 110. While
Chicago does experience 100-degree weather, it does so only on limited
occasions. Officially, the city's all-time hottest day was just 105 degrees
on July 24, 1934, but unofficially the city has recorded readings of 105
degrees or higher nine times at Midway Airport, most of them during the
torrid "Dust Bowl" summers of the 1930s. During the summer of 1934 the
thermometer reached at least 105 degrees five times, topped by a high of 109
degrees on July 23. The city's most recent encounter with extreme heat was
back on July 13, 1995, when Midway climbed to 106 degrees during a deadly
three-day episode of heat and humidity that claimed more than 750 lives.

Dear Tom,
"We will be vacationing in the Rocky Mountains in Colorado in early July and
camping out for several days, at an elevation of about 7,500 feet. What
effect does elevation have on sunburn times?
The Jansen Family, Chicago
Dear Jansen Family,
The atmosphere offers partial protection from ultraviolet (UV) light, the
component of sunlight that causes sunburn and other damage to our skin. It
stands to reason, therefore, that there is progressively less protection at
higher elevations.
Dermatologist Dr. Bryan Schultz (who prepares the sunburn times that appear
on this weather page) says, "UV intensity increases in an exponential
fashion as altitude increases, from 4-10 percent per 1,000 feet. With
Chicago at 620 feet, an extra 6,900 feet will increase UV over 50 percent.
Chicago's sunburn time at 1 p.m. on a clear July 5th would be close to 16
minutes, but under 10 minutes at 7,500 feet in Colorado."

Dear Tom,
Are there other countries that experience as many tornadoes as the United States
does?
Tom Skill, Peotone, Ill.
Dear Tom,
Nowhere in the world do more tornadoes occur than in the United States. The
latitude, geography and climate of this country provide a perfect breeding
ground for tornadoes unsurpassed anywhere in the world. As a result the U.S.
lays claim to nearly three-quarters of the world's twisters, the vast
majority occurring east of the Rockies.

However, tornadoes can and do occur in other parts of the world with the exception of
the polar regions. Areas with significant tornadic activity include Europe, Australia,
Canada, Russia, Argentina, China and Japan. In Bangladesh, at least a dozen twisters
have killed more people than the U.S.'s deadliest storm, the 1925 Tri-State tornado with
695 fatalities.

Dear Tom,
What does a 30 percent chance of rain mean?
-Magee Amethyst
Dear Magee,
The percent chance (or probability) of rain is the forecaster's way of
expressing how certain he is that it will rain. Ideally, a forecaster would
prefer to issue a zero-percent forecast (it absolutely will not rain) or a
100-percent forecast (it absolutely will rain), but the present "state of
the art" does not give forecasters that ability.
The proper interpretation of a 30 percent chance of rain (assuming the
forecasts verify perfectly) is that you will have rain on your head three
out of ten times that you hear such a forecast.
Please note that the percent chance of rain does not address the duration of
rainfall (it might rain all day or only a few minutes) or the amount of rain
that comes down (unless the forecaster uses a qualifying term such as
"heavy" or "light").

Dear Tom,
I live in Beverly Shores, Indiana and on a few occasions around sunset I have seen a
view of the Chicago skyline that resembles a "bar code". Can you explain what is going
on?
-Patricia Kieta

Dear Patricia,
What you are seeing is a magnification of Chicago's shoreline resulting from
a temperature inversion where warm air is layered above colder air found
near the surface of the lake. This inversion bends rays of light from the
city back toward the colder, denser air above the lake creating a "towering"
effect where objects become magnified and stretched like high walls. This
type of mirage is sometimes called a Fata Morgana (Italian for Morgan the
fairy). The name comes from the fairy-like castle mirages that frequently
appear over the Strait of Messina separating Italy and Sicily. Morgana was
the mythical fairy who lived in a crystal palace beneath the waves.

Dear Tom,
I have heard infrared light referred to as "beyond the red." What does this
mean?
Dale Halvers
Dear Dale,
The answer can be found in the year 1800. British astronomer Sir William
Herschel (1738-1822), while investigating the nature of light, moved a
thermometer through the spectrum of colors obtained by passing light through
a prism. Herschel observed that the thermometer registered higher
temperatures as it absorbed heat energy contained in the light. That outcome
was expected.
Much to Herschel's surprise, however, the temperature continued to rise as
he moved the thermometer "beyond the red" portion of the spectrum and into
the darkness. Herschel had discovered a previously unknown form of radiant
energy: invisible infrared light whose wave length lies just "beyond the
red" wavelength.

Dear Tom,
While visiting Rocky Mountain National Park in Colorado, we came upon the source of
the Colorado River, and my daughter (age 9) asked how long it took water from there to
reach the Pacific Ocean.
-Steven Smith

Dear Steven,
From its source at Grand Lake in Rocky Mountain National Park, the Colorado River
winds its way 1,450 miles to the Gulf of California, an appendage bay to the Pacific
Ocean. The Bureau of Reclamation estimates that water leaving Grand Lake requires a
minimum of four to six years to complete its journey to the Gulf of California. Time
spent in several large reservoirs (such as Lake Mead behind Hoover Dam) accounts for
that extraordinarily lengthy trip. However, much of the water never arrives at the gulf. In
some recent years, all of the water as been drawn out for irrigation and municipal use.

Dear Tom,
Do tornadoes occur in the early morning hours? If so, why are there so few?
David Carli, Geneva High School
Dear David,
The vast majority of this nation's tornadoes occur during the late afternoon
and evening when the atmosphere is most unstable after a full day of solar
heating. After sunset the air cools and stabilizes, and the threat of
tornadoes diminishes. That being said, tornadoes have occurred at all times
of the day and night including the pre-dawn hours. At 6:10 a.m. on June 9,
1966, residents of Chicago's northwest suburbs were awakened by severe
thunderstorms that spawned an F2 tornado. The twister first touched down in
Hoffman Estates, then skipped along a 13-mile path through Arlington Heights
and Prospect Heights. The twister killed one and injured 23, and caused $4
million in property damage. Just 10 hours earlier, that same thunderstorm
complex produced a killer tornado in Topeka, Kan.

Dear Tom,
How many positive and negative lightning bolts are there?
Sharon Schutz
Dear Sharon,
We can separate lightning bolts into two broad categories: within-air flashes and
ground strikes. Meteorologist and former Chicagoan Dr. Walter Lyons estimates that in
the United States the ratio of flashes that do not strike the ground to those that do is
about 4-1; that is, 80 percent to 20 percent.

The installation of a nationwide lightning detection network in the early 1990s enabled
researchers to count lightning bolts accurately, and the numbers are stunning. Just in
the U.S., the tally is 100 to 125 million bolts annually, of which about 22
million are ground strikes.

Of those 22 million, 90 percent are termed "negative," meaning the flow of electric
current is from cloud to ground, and 10 percent are "positive," current flowing from
ground to cloud.

Dear Tom,
Do bursts of heavy rain called "rain gushes" follow thunder?
Richard Samall
Dear Richard,
University of Florida lightning expert Dr. Martin Uman says, "Observers of
thunderstorms, from antiquity to the present, have noted that a heavy gush of rain
often reaches ground a minute or two after a lightning flash and its accompanying
thunder." He explains that questions about the relationship between lightning, thunder
and rain gushes were finally resolved in the 1960s, when radar observations of
precipitation in clouds before and after lightning confirmed that, in some cases, a gush
followed the flash.

Researchers postulate that cloud droplets near a lightning channel and its many
branches acquire such intense charge that they repel each other, flying outward and
colliding with non-charged drops, thereby growing large and falling as a rain gush.

Dear Tom,
I love hot weather. Has the city even recorded back-to-back 100 degree days?
John Cosgrove
Dear John,
Chicago does not experience a lot of 100 degree days but the city's official
station has recorded consecutive 100 degree days on 12 occasions since
records began in late 1870, including three 100s in a row in 1911 and 1947.

However, these numbers don't tell the entire story because during the
intensely hot "Dust Bowl" summers of the 1930s the city's official
thermometer was located very close to the cooling breezes of Lake Michigan.

In the summer of 1934, inland Midway Airport logged six consecutive days
above 100 degrees from July 20-25, including a torrid 109 degree high on July 23. Two
years later, the summer of 1936 produced eight straight days in the 100s from July
7-14. The city's last back-to-back 100s were July 13-14, 1995 during a deadly heat
wave that killed more than 750 people.

Dear Tom,
The other day when I was out running, it began to drizzle and I noticed a strong smell
of rain in the air. What causes that odor?
DeeDee Rossman

Dear DeeDee,
There are a number of explanations for the unique smell of rain. The vegetation of an
area appears to contribute, with the reaction between moisture and high humidity on
certain substances in plants releasing odors into the air. For example, pine forests
release terpenes, a substance that is found in medicine and perfumes. Moisture,
warmth and lowering pressure, all forerunners of rain, hasten the release of fragrant
molecules from plants. As rainwater soaks into soil particles, it pushes aromatic gases
produced by streptomyces, a soil bacteria, into the air, producing an earthy smell.
Falling raindrops also kick up particles from blacktop and concrete that add to the
potpourri of odors that we like to describe as the smell of rain.

Dear Tom,
Someone told me you actually predicted the New Orleans disaster with
Hurricane Katrina here in this weather column. Is this true?
Anton Smithson
Dear Anton,
It is not true, but this column did speak of the vulnerability of New
Orleans to the ravages of hurricanes two years before Katrina devastated the
city. The following information appeared in this column on Oct. 4, 2003:
"The National Hurricane Center ... advises that New Orleans is considered to
be the U.S. city most vulnerable to hurricane storm surge."
Katrina came ashore on the Louisiana and Mississippi coast on Aug. 29, 2005,
as a strong Category 3 hurricane (on the 5-point hurricane intensity scale).
It brought 127-m.p.h. sustained winds and an unprecedented storm surge of 15
to 20 feet on the Louisiana coast, and up to 28 feet on the Mississippi
coast.

Dear Tom,
We winter in Naples,Fla., and love viewing the beautiful sunsets over the Gulf of Mexico.
Where in the world is the sun rising when we see it set?

Ted and Diane Olsen, Lake Forest, Ill.

Dear Ted and Diane,

We posed your question to Dan Joyce, astronomer at Triton College's Cernan Earth and
Space Center. He informed us that when the sun is setting in Naples, it is rising in the
eastern Indian Ocean, west of Australia, at a point just as far south of the equator as
Naples is north of the equator (26 degrees). The exact point is precisely halfway around
the globe from Naples (180 degrees longitude away).

When the sun is setting in Chicago (longitude 88 degrees west, latitude 42 degrees
north), it is rising at a point in the southern Indian Ocean about 1,500 miles southwest
of Perth, Australia. That point lies at longitude 92 degrees east, latitude 42 degrees
south).

Dear Tom,
Who activates tornado sirens in a town?
Sandra Browning, Niles
Dear Sandra,
In the Chicago area, the decision to sound tornado sirens is made by
individual communities. The Chicago National Weather Service (NWS), which
issues tornado warnings in northeast Illinois, has no control.
NWS Warning Coordination Meteorologist Jim Allsopp says, "Procedures to
sound sirens vary from community to community. The sirens can be set off at
the request of village officials such as mayor, police or fire chief, or an
emergency management coordinator."
A standard criterion for sounding sirens is the report of a tornado or
funnel cloud from a reliable source such as a trained spotter, police
officer or firefighter, and usually within a few miles of the town. Allsopp
advises that, "NOAA weather radio is the best way to get warnings direct
from the NWS."

Dear Tom,
Is it possible for a drought to happen in the winter? Has it happened?
Katie Stevens (age 10), Tinley Park, Ill.

Dear Katie,

Drought (extreme dryness due to lack of precipitation) can occur at any time of the year
-- winter included. But its effects are most noticeable during the period from April
through September when actively growing plants need water.

A drought that began in January of 2007 in the southeastern United States continues
today. The Chicago area suffered through harsh winter droughts in 1919-20, 1930-31
and 1962-63.

A winter drought usually coincides with a lack of stormy weather, which urban
residents welcome. But they fail to realize that winter precipitation is vital because it
recharges soil moisture in preparation for the next growing season.

Dear Tom,
Do thunderstorms that grow higher than usual also tend to be more severe?

Ronny Nauta

Dear Ronny,

A general relationship between thunderstorm height and thunderstorm severity does
indeed exist: The higher the cloud top relative to other thunderstorms in the air mass,
the greater is the storm's severity. We measure thunderstorm severity by the strength of
its straight-line winds, the size of its hail and its ability to generate tornadoes.

Thunderstorm top height is the single most telling factor in storm severity, but speed
is also critical. Fast-moving thunderstorms (40 m.p.h. or greater) tend to be more
severe than slower-moving storms. Note that lightning production and heavy rain, even
flood-producing rain, are not defined to be severe criteria even though such aspects of
thunderstorm weather are dangerous and potentially deadly.

Dear Tom,
I've read about the "green flash" being seen in southern Florida. Fact or fiction?
Helen Baker, Naperville, Ill.

Dear Helen,
The "green flash" is real. It is a momentary flash of green light emanating from the
upper rim of the rising or setting sun at the moment it crosses the horizon.

Widely thought to be visible only in the tropics, the green flash can be seen anywhere in
the world, and most easily from a vantage point with an unimpeded view of the horizon,
such as over a large lake or ocean.

Sunlight passing through the atmosphere is bent upward, and the bending is greatest
when the sun is on the horizon. The shortest wavelengths of light (blue and green) are
bent the most, so they are the first colors seen on the rising sun and the last as it sets.
Blue wavelengths are removed by atmospheric scattering, leaving the green
wavelengths visible for the flash.

Dear Tom,
My wife and I were married on June 10, 1967 during terrible thunderstorms. City
viaducts were flooded and the church lost power. What exactly happened that
afternoon?

Thanks, Mary Jane and Pat Clifford, Chicago

Dear Mary Jane and Pat,

Your wedding took place during one of the most intense rain events in Chicago weather
history. A record 5.20-inch deluge swamped the Loop in just a two-hour period that
afternoon, and the aftermath was widespread major flooding. Water flowed six inches
deep through Loop streets; many basements flooded. The total precipitation from the
storm was 6.09 inches downtown. O'Hare International and Midway Airports registered
about 3 inches. High water closed city expressways and thousands lost power. In
addition to the heavy rainfall, tornadoes tore the roofs off houses in Romeoville and
caused damage at the Galt Airport in McHenry County. The storms were responsible for
two deaths in the Chicago area.

Dear Tom,
Can you provide proof that it snowed in the Chicago area on the 4th of July
in the early 1970s? I remember watching fireworks at Deerfield High School
and it snowed. My mom remembers it, too.
Cheryl Fayne
Dear Cheryl,
The proof that you are looking for does not exist because snow, even a few
wet snowflakes, has never been observed in the Chicago area on July 4, or on
any day in July or August.
That fact is borne out by unofficial weather records assembled from several
observation sites in northeast Illinois during the period 1830-1870 and from
Chicago's official weather data from 1871 to the present. Those records
constitute a period of 178 years of absolutely snow-free Julys and Augusts!
July 4th, 1972, was downright chilly (high/low temperatures of 66/49
degrees), but dry: no precipitation at all. Sometimes our memories play
tricks on us.

Dear Tom,
I have been keeping records of the pollen count for many years, and the pollen level has
been increasing. What weather factors are causing this?

--William Sanders, Chicago

Dear William,

It's not the weather but rather higher levels of carbon dioxide in the atmosphere that
have markedly increased plant growth rates, including ragweed (the source of pollen
that triggers hay fever). Since 1900, atmospheric carbon dioxide has increased 30
percent, from 294 parts per million (ppm) to 380 ppm now. In response, ragweed has
doubled its pollen production since 1900. Dr. Paul Epstein of Harvard University's Center
for Health and the Global Environment says, "This carbon dioxide stimulation of plants
has a side effect for public health. The hyper-production of pollen can cause hay fever,
conjunctivitis and can exacerbate asthma."

Dear Tom,
I have heard you say, "Afternoon highs ran 17 degrees below normal." Those
temperature swings are normal for the Chicago region. The correct comment is
that "Afternoon highs ran 17 degrees below average."
Charles Rosenfelder, Glen Ellyn
Audrey Roen, Oak Park
Dear Charles and Audrey,
Meteorologists (and others in the atmospheric sciences) 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 a 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,
What technology was available to meteorologists that helped them prepare
weather forecasts for the D-Day invasion on June 6, 1944?

Brian O'Carroll, Glenview, Ill.

Dear Brian,
World War II meteorologists did not have the tools that are so important to
today's forecasters -- computer models, satellite information, Doppler radar
data and remote weather observations. They relied instead on available
surface and upper-air observations to draw their weather maps, and they used
meteorological expertise to forecast the movement, development and
dissipation of the weather systems.

WW II forecasters correlated current weather systems with analogous past
situations and often based their forecasts on the outcome of those past
events. Using such techniques, Allied forecasters accurately predicted a short
period of clearing skies that allowed the successful invasion of the
Normandy beaches.

Dear Mr. Skilling,
You once ran a piece about the number of daylight hours being the same
everywhere on Earth. Could you please repeat that information?

John Windgrove

Dear John,
The number of daylight hours in a 365-day year is always 4,380 and it
doesn't matter where you might call home. Arctic polar bears experience the
same number of hours of daylight annually as African elephants and Antarctic
penguins. Cloudiness aside, the sun shines for an equal amount of time on
every square inch of the Earth's surface.

The complication is that, because the Earth's axis of rotation is tilted
23.45 degrees from the plane described by its revolution around the sun, the
daily hours of light, planet-wide, are distributed unequally through the
year. At the poles, daylight comes in six-month blocks; at the Equator it is
always within a few minutes of 12 hours a day.

Dear Tom,
While driving across Nebraska in May and watching distant thunderstorms, the
lightning bolts streaking through the sky were immense. Just how long can
lightning bolts be?

Dominik Majerski

Dear Dominik,
The Vaisala Group, headquartered in Vantaa, Finland, has been conducting
extensive lightning studies in the United States. Lightning researcher Ron
Holle of Vaisala says the company has developed a very high resolution
three-dimensional lightning detection system that is in operation in the Dallas-Ft.
Worth area. The network continuously measures the emissions of lightning flashes in
the very high frequency range.

"It has measured quite a few flashes over 100 miles long,” Holle said. “Our current
record length (as of 2006) is 120 miles from a flash. I don't think we would have
expected this, but there they are!"

Dear Tom,
On a recent trip to Playa Del Carmen, Mexico, I noticed a perfect rainbow
around the sun when it was almost directly overhead. What was this
phenomenon?
Bob Arnold, Mokena, Ill.

Dear Bob,
What you saw was a solar halo, a fairly common occurrence produced when
sunlight passes through a thin veil of cirrus clouds (clouds that are
composed of ice crystals).
The colors of such halos are often dim, but they can be vivid when the
atmosphere is exceptionally clean. A full spectrum of colors is visible in
those instances, with the colors ranging from red to blue. Most halos form in a
22-degree ring around the sun, and fainter versions, usually whitish, also occur around
the moon.

In weather folklore, halos are said to foretell precipitation because a veil
of cirrus is often the precursor to approaching storms.

Dear Tom,
Why does the weather change?
Dorrease Williams
Dear Dorrease,
The unequal application of heat energy to the Earth's atmosphere drives
weather changes, and almost all the energy that fuels the weather comes
ultimately from heat energy contained in sunlight.
Because the Earth is a sphere whose polar axis is approximately
perpendicular to the sun's rays, its equatorial regions receive much more
heat from sunlight than do its polar regions. Therefore, the Earth's
equatorial regions are warm and the polar regions are cold.
It's crucial to the explanation to realize that warm air is less dense,
relative to cold air. Air density differences established by temperature
contrasts drive global wind patterns that attempt to equalize atmospheric
density differences and reduce temperature contrasts. The ongoing interplay
of warm and cool air masses causes what we refer to as "the weather."