ASK TOM WHY: July 2008 Archives

Dear Tom,

My grandson is eight years old and has a terrible fear of the weather. Is there something
we should do or someone we can contact?
Judi
Dear Judi,

The violent aspects of weather can be especially frightening to children. Here are a few
tips to deal with your grandson's excessive fears:
- Communicate with him; don't deny his fears, but explain that storms are natural
events and you and he can take steps to thwart the threats they pose.
- Explain that he will be safe indoors. Coming inside when rough
weather threatens is a simple yet effective action.
- Research coping techniques. Many web sites contain useful information prepared by
professionals, and help books are available from libraries.
- Finally, consultation with professionals might be warranted if your grandson's
excessive fears cannot be overcome.

Dear Tom,
I remember a very stormy period a few summers ago marked by a lot of hail. What
summer was that?

Marge Wallace

Dear Marge,
It was five years ago during July and August of 2003 that Chicago was bombarded with
hail. Waves of severe thunderstorms swept the area on 16 days between July 4 and
Aug. 3. Some communities were hit with hail on five or six days.

The hail was large, ranging from penny size (3/4-inch) to tennis ball (2.5 inches,) and the
extensive damage was estimated in excess of $90 million. Automobiles and rooftops
took the brunt of the damage, but trees also were stripped of their leaves and gardens
flattened.

Damaging wind and torrential rainfall also accompanied the thunderstorms. The storms
on Aug. 3 brought as much as 4 inches of rain to Chicago's Northwest Side, flooding
streets and viaducts and shutting down the Kennedy Expressway.

Dear Tom,
Nevada!?!? I thought Arizona was the driest state with 7.5 inches of rain annually.

Elise White Pearce
Dear Elise,
You are referring to our statement on July 21 that, "Nevada, with a scant 9 inches [of
rain] annually, is the driest [state]."

The annual total (7.5 inches) that you suggested for Arizona is representative only of
central Arizona. Other areas of the Grand Canyon State, especially the higher elevations
of the north, receive up to 22 inches. The state as a whole gets 14 inches.

Nevada lies in the "rain shadow" of the Sierra Nevada Mountains to its west and it is
sheltered from precipitation accompanying storms from the Pacific Ocean.

In addition, Arizona experiences a summer monsoon when thunderstorm-triggering
moisture from the Gulf of California, just to the south, works its way across the state.
Nevada, farther northwest, receives little of that moisture.

Dear Tom,
What is the proper placement for the sensor element for a wireless
thermometer? Mine, located on the house above the patio, reads much hotter
than the reported temperature. Is there a general rule to follow when
selecting a location?
Fred Kloster, Naperville
Dear Fred,
Proper thermometer placement is not easily accomplished in an urban
environment.
The temperature-sensing element must be protected from exposure to direct
sunlight, from precipitation (like rain) and from condensation (like dew),
while at the same time be given adequate ventilation. It must also be
located well away from mechanical heat sources (such as air conditioners)
and from concrete surfaces that are warmed by sunlight.
Ideally, the sensor is best placed in a wooden instrument shelter with
louvered sides, painted white, four feet off the ground, over grass.

Dear Tom,
Are the weather events described in the book "Devil in the White City"
correct? Did a funnel cloud really blow through the Midway?
Michael Maher, Arlington Heights
Dear Michael,
Author Erik Larson described the weather conditions during Chicago's 1893
Columbian Exposition correctly; there was indeed a "Funnel in the White
City."
The storm took place the evening of July 9. A sunny, warm afternoon with
scarcely a breeze turned dark as night as a fast-moving squall approached
from the north. The storm brought strong northwest winds that did
considerable damage to the fairgrounds blowing out windows in Jackson Park.
A funnel-shaped cloud destroyed a large hot-air balloon that was tethered to
the ground. Yachts capsized on Lake Michigan, drowning at least four. The
storm dumped 0.42 inches of rain in less than an hour. Peak wind gusts were
not available because the Weather Bureau's wind equipment was damaged during
the storm.

Dear Tom,
I was surprised to read that a tornado occurred in the mountains of Colorado
last May. Is there any place in the continental U.S. where tornadoes do not
occur?
Tim Rasmussen
Dear Tim,
Tornadoes have occurred in every U.S. state including Alaska and Hawaii, but
of course they are much more common in portions of the Plains, Midwest and
South in the area known as Tornado Alley. Twisters occur with a much lower
frequency in the West, especially in the chilly coastal region of the
Pacific Northwest. Mountain tornadoes have been documented as high as the
12,000-foot level in California's Sequoia National Park. Roger Edwards of
NOAA's Storm Prediction Center notes that the most violent high-altitude
twister was an F3 storm that struck at the 11,000-foot level in Utah's Unita
Mountains on Aug. 11, 1993. Many meteorologists feel that the number of
mountain twisters is under-reported, going undetected in the remote areas of
mountainous West.

Dear Tom,
You are always telling us how high the thunderstorm tops are. What is the
highest top ever recorded in the Chicago area and in the world?
Bill Matthews, Schaumburg
Dear Bill,
As a general rule of thumb, the higher the thunderstorm top the more severe
the thunderstorm is likely to be. Storms with the highest tops tend to form
in areas of greatest lift where the atmosphere is most volatile. In the
Chicago area, garden-variety summer thunderstorms develop to heights between
35,000-45,000 feet, but the tops of severe thunderstorms here can approach
60,000 feet and in extreme cases 70,000 feet. The top of the thunderstorm
that produced the Plainfield tornado on Aug. 28, 1990, towered to 65,000
feet. The tallest thunderstorms on Earth have been documented in the tropics
where tops have been measured to about 75,000 feet, building more than 14
miles up into the atmosphere.

Dear Tom,
I was planning on having lightning rods installed on my house to ward off lightning,
but someone told me they attract lightning. Now I'm having second thoughts. What is
the real story?

Ron White

Dear Ron,
Lightning rod protection is a wise investment.

It is commonly believed lightning rods will prevent a lightning strike by discharging the
overhead thundercloud or by deflecting the bolt. Both thoughts are incorrect. A
lightning rod protects the structure on which it is mounted by attracting to itself
the descending lightning bolt and then conducting its electric charge harmlessly into
the ground via a suitable wire system.

A downward-striking bolt is not affected by surface-based objects like buildings until
it is within less than 300 feet of them, and only then will it usually, but not always,
strike the highest object.

Dear Tom,
I read the article about rip currents in the Sunday Chicago Tribune. Is this the same as
what we used to call "undertow" when I was a kid?

Judith Allen, Chicago

Dear Judith,

Rip currents and undertow are two entirely different water transport features that occur
in the shore area of oceans and large lakes like Lake Michigan. Both move water
brought to the shore by waves back into the lake (or ocean).

A rip current, flowing at speeds up to 4 m.p.h., is a surface flow of water, usually 20 to
100 feet in width, flowing outward from the shore. It won't pull a swimmer beneath the
waves, but it can carry him a few hundred feet out. It's literally a "river in the lake."

Unlike the powerful, localized and dangerous surface rip current, undertow is a brisk
bottom flow in shallow water (2-4 feet deep). It transports water carried onto the beach
by breaking waves, and is a lesser threat.

Dear Tom,
I am in charge of planning an annual summer party and it has rained the last few
summers on the weekend chosen. Historically, which weekends in July and August is it
least likely to rain?

Bob Haza, Oak Brook, Ill.

Dear Bob,

When it comes to avoiding a rainy weekend, it doesn't matter which dates in July and
August you select. The chance of rain is the same on any given day in those months: 31
or 32 percent. That's the result of a computer scan of 137 years of Chicago's daily
rainfall records. You can attribute your rainouts in the last few years to bad luck rather
than to poor planning. But consider this: Thunderstorms account for most of Chicago's
mid- and late-summer rainfall, and August thunderstorms tend to be of shorter
duration. That suggests an August outing is slightly preferable. The tradeoff is that
August storms, on average, produce more intense (but more brief) rain.

Dear Tom,
Where does Illinois rank among the states in annual rainfall? Who's the
wettest and who's the driest?

Karen Samuelson

Dear Karen,
Average precipitation (rain plus the water equivalent of melted snow) across
the lower 48 states is about 30 inches per year. If none of it evaporated,
soaked in or drained off, a 30-inch layer of water would flood the country
after one year.
The nation's average annual precipitation is not distributed evenly, of
course. The land surface of the United States, which ranges from barren
deserts to lush rain forests, attests to that. And even within many states,
especially those with mountains, precipitation varies greatly.
On average, though, Louisiana, with 55 inches, is our wettest state; Nevada,
with a scant 9 inches annually, is the driest. Illinois, with 37 inches,
ranks 27th.

Dear Tom,
In a recent column you said that the dog days of summer are here. I thought
those were in August when the dog star is seen in the sky?
Rose Hedman
Dear Rose,
The term "dog days" of summer refers to periods of hot and humid weather
that typically occur in many parts of the Northern Hemisphere during July
and August. The term's origin dates back to ancient times when the Egyptians
believed that the appearance of Sirius, the Dog Star located in the
constellation Canis Major (The Great Dog) in the early morning sky added its
heat to that of the sun to produce exceptionally hot weather. Astronomically
the Egyptians calculated the dog days from July 3-August 11. In more modern
times, the term has been applied to extended periods of late-summer hot
weather lasting up to a week or more, usually in August.

Dear Tom,
I thought that if a storm develops into a hurricane its name is retired. I remember
another Hurricane Bertha that was threatening Ft. Lauderdale in 1996. What gives?

—Fred Zagone, Milwaukee

Dear Fred,
The list of Atlantic Basin tropical cyclone names is repeated every six years,
and Category 3 Hurricane Bertha, with top winds of 115 m.p.h., did indeed threaten
Florida before making a North Carolina landfall on July 13, 1996. Tropical Storm Bertha
made a Louisiana landfall back in 2002, and in 1990 Hurricane Bertha brushed the
Canadian Maritimes. Hurricane names are retired only if a storm is deemed historically
noteworthy, usually in terms of damage or loss of life. Previous storms named Bertha
did not merit retirement status and unless this season’s Bertha becomes memorable,
there will be another Bertha in 2014.

Dear readers: Yesterday's column contained an arithmetic error. Here is a
correction.

Dear Tom,
You indicated the water level in Lake Michigan has risen seven inches since
July 2007, and that rise added 2.73 trillion gallons of water. What happens
at the evaporation end? How much water evaporates from Lake Michigan?
Carl Gaines
Dear Carl,
Atmospheric processes operate on a scale that boggles the mind, and the
evaporation of water from Lake Michigan into the atmosphere is no exception.
Lake Michigan's 22,300 square miles of surface area loses 29.5 inches of
water annually via evaporation. That loss works out to a staggering 11.51
trillion gallons of water per year, or a daily average of 31.5 billion
gallons.
Here's the actual number: 31,500,000,000 gallons of water lost to the
atmosphere from Lake Michigan daily via evaporation.

Dear Tom,
You frequently point out that heating from sun during the day leads to afternoon and
early evening thunderstorms, but why do we have so many storms late at night when
there is no heating?
Malcolm Vye
Dear Malcolm,
While thunderstorm activity does peak during the late afternoon when
atmospheric heat reaches its highest level, Chicago experiences a "secondary
thunderstorm maximum" between 2 and 5 a.m.

Retired National Weather Service research meteorologist Robert Johns tells us:

"The energy of wind that is lost in friction with the ground during the day, when
it is windy, remains in the atmosphere at night when surface air cools and ground-level
winds die away. With less energy lost to the ground, winds in the 1-4 thousand foot
level actually increase at night, and can more effectively transport heat and moisture to
generate thunderstorms."

Dear Tom,
Why can you hear thunder so long after a lightning bolt? I have timed it to go on for
more than 40 seconds.

Dave Coleman, Naperville, Ill.

Dear Dave,

The phenomenal heating (about 50,000 degrees) brought to bear on a column of
air exposed to a lightning spark produces an explosive expansion and subsequent
contraction of the column—a process that sets up the shock waves we hear as thunder.

Rather than originating from a single point, these sound waves, propagating at about
1,100 feet per second, are generated along the entire length of a lightning stroke. That
length is often in excess of 10 miles. If a lightning bolt were oriented such that the
distance between its nearest and farthest points from you were 10 miles, the train of
sound waves thus generated would require 48 seconds to pass your ears. You would
hear thunder from that bolt for 48 seconds.

Dear Tom,
What is the rainiest month of the year in the Chicago area?
Frank Koral

Dear Frank,
The title for Chicago's rainiest month has been held by August since 1991, but it hasn't
always been the city's precipitation leader. Through much of Chicago's climate history,
August ranked only as the city's 4th or 5th wettest month, with June or July leading the
year. However, since around 1980 the city has become a magnet for heavy August
cloudbursts. The month's precipitation normals have skyrocketed from around 3.15
inches using data before 1970 to its current 4.62-inch total based on the latest
1971-2000 climatological normal period. Climate normals are recalculated every 10
years using the latest 30 years of data to reveal a "moving" climate picture of an area.
Dating back to 1871, seven of the city's 10 wettest Augusts have been logged since
1970, lead by August 1987 with 17.10 inches.

Dear Tom,
Why does wet skin feel so cold?
Robert Allen
Dear Robert,
It's due to the behavior of water molecules: They're in constant motion and
the warmer they are, the faster they're moving. We sense that motion as
heat.
In liquid form, water molecules are vibrating and tumbling around because
they're not bonded together the way they are in ice. In order for a
within-liquid water molecule to escape from the liquid into the air -- to
evaporate -- it must obtain greater speed, and this can happen through
random collisions with other molecules in the liquid (which thereby lose
some of their speed).
If, after a collision, a particular molecule already at the water's surface
has sufficient speed to break into the air, it takes its energy (its heat)
with it. That's heat lost from the liquid, which therefore cools, and that's
why water evaporating from our skin gets cooler.

Dear Tom,
July 5 was sunny, warm and calm at our house. Out of nowhere, the wind kicked up and
our neighbor's flag, launched it from its holder,and carried it slowly upward perhaps
100 feet. It traveled north for a minute and fluttered down 100 yards from where it took
off, then the calm winds returned. Any thoughts on this?

-Dave Kircher, Orland Park

Dear Dave,

Your neighbor's flag had a close encounter with a dust devil--but one
apparently containing no dust. Dust devils are vigorous, short-lived whirlwinds that
form on quiet days when warm air at the ground,heated by the sun, surges upward in a
tight corkscrew spiral. It's almost like hot air rising in a chimney. The whirls, usually
made visible by dust in their 20 to 40 m.p.h. winds, can grow to several yards across
and a few hundred feet in height before dying just as suddenly as they appeared.

Dear Tom,
During a recent thunderstorm with strong winds, cars on the Eisenhower
Expressway were stopping in the underpasses. Is this safe? It sure messed up
the traffic.
Richard Waller
Dear Richard,
Your question comes up frequently. The answer: It's a very dangerous thing
to do.
Contrary to popular belief, expressway underpasses do not offer safe shelter
from the high winds of 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, Kan., on April
26, 1991.

Dear Tom,
Because rain falls so far from the clouds, why doesn't it damage whatever it
hits? For example, you would think raindrops would break my car windshield.
Is it more a physics question as opposed to a weather question?

Tami Stevenson, Elgin, Ill.

Dear Tami,

It's a question of physics -- the physics of raindrop stability. As a large
raindrop falls faster and faster through the air, the friction of its
passage through the air causes it to flatten, then to bulge upward in the
middle rather like an inverted paper bag, and finally to shatter into
smaller drops that fall more slowly.

It turns out the maximum size that a raindrop can attain before shattering
is 0.2 of an inch in diameter. The top fall speed of the largest "shatter-proof"
raindrops occurs when frictional drag equals the weight of the drop, and
that's 21 m.p.h. Fortunately, raindrops 0.2 of an inch across falling at 21 m.p.h.
don't cause damage.

Dear Tom,
I know that a dew point of 70 degrees or higher is a benchmark for being
uncomfortable. What is Chicago's highest dew point on record?
Jim Birkett Chicago

Dear Jim,
Many people begin to feel uncomfortable when the dew point reaches the
middle or upper 60s, but almost everyone is affected when the 70 degree dew
point threshold is crossed. In Chicago, the dew point typically climbs to 70
or higher about 30 times a year, most often in July and August. However, on
rare occasions dew points in Chicago can reach 80, a sweltering level most
common in summer along the Gulf Coast. The city has recorded an official 80
degree or higher dew point only six times, all in the month of July. Chicago's
highest-ever dew point was a stifling 83 degrees on July 30, 1999, and the most
recent steamy encounters with an 80 dew point were on July 7 and July 23 in 2001.

Dear Tom,
If hurricanes are named alphabetically why is the season's first named storm Bertha and
not an "A" name?
Ed and Barb Wargo

Dear Ed and Barb,
Bertha is actually the season's second storm and that is why its name starts with the
letter "B". The first storm of the season, Tropical Storm Arthur came and went so fast
that many people missed it. Arthur formed inland about 45 miles northwest of Belize
City, Belize on May 31, moved west across the southern Yucatan and quickly weakened
to a tropical depression by the evening of June 1 near the Guatemala-Mexico border.
During its short lifespan Arthur could only muster top winds of 40 m.p.h., but it did
produce extensive flooding in Belize from up to 15 inches of rain. Five people perished
there and damage was estimated at nearly 80 million dollars. As the 2008 Atlantic Basin
hurricane season progresses, the next names to be used will be Cristobal, Dolly,
Edouard and Fay.

Dear Tom,
Having observed the weather in Chicago for many years, I have concluded that
thunderstorms heading for the city dissipate or weaken before getting here,
then strengthen or reform east of the city; or they always pass north or
south of the city. Why is this?
Doug Mann

Dear Doug,
It might sometimes seem that thunderstorms occur less frequently and with
less vigor in Chicago than at surrounding locations, but area weather
records do not support that contention.

Historical data from National Weather Service and from the
observations of cooperative weather observers show that, over time,
thunderstorm frequency and intensity are quite uniform across northern
Illinois, northwest Indiana and southern Wisconsin.
Locations that thunderstorms repeatedly spare for weeks, even months, always
catch up when the weather patterns change--and eventually they do change.

Dear Tom,
Does a very snowy winter mean a very wet summer?
Meredith Graf, West Chicago
Dear Meredith,
It does not. Broadly speaking, Chicago experiences its wettest weather --
heavy snow in the winter and heavy rain in the summer -- when upper-level
winds repeatedly direct low pressure systems and their attendant
precipitation from the southern Plains northeast into the Midwest. Storm
systems moving on that general path tap moisture from the Gulf of Mexico and
tend to produce copious precipitation at Chicago.
However, such weather patterns usually do not persist for months and months
at a time, that is, from the winter through the spring and summer.
We examined rainfall in the summers following Chicago's 10 snowiest winters
and did not find a snowy winter-rainy summer link. A very rainy summer
followed only two of the city's 10 snowiest winters.

Dear Tom,
Why are the Weather Service radars in Romeoville and elsewhere "down for
maintenance" so often? It’s troubling, especially during inclement weather.

Ray Deremo, La Grange Park

Dear Ray,
It's a matter of routine maintenance and computer software upgrades,
according to meteorologist Mark Ratzer at the Romeoville office of the Chicago
National Weather Service. Please note, though, that the surveillance patterns of
individual radars overlap. When a radar is out of service, storms within its primary
scanning area continue to be monitored by adjacent radars. Ratzer says the
Romeoville radar was occasionally down for maintenance this spring so panels in the
radar dome could be replaced, and also for an upgrade to the computer software that
processes radar data. That upgrade is being installed at all radars across the country.

Dear Tom,
I just returned from Costa Rica, and the people there boasted about never
having been hit by a hurricane. Can any of our Gulf or Atlantic states make
this claim?
Paul Brown
Dear Paul,
Because hurricanes seldom occur within 10 degrees latitude of the equator,
it is true that low-latitude Costa Rica has never sustained a direct
hurricane strike. (The country was hit head-on by a tropical storm in
December 1887.) However, Costa Rica can experience deadly consequences when
hurricanes strike areas farther north in Central America. Portions of the
country were devastated from floods and mudslides when Hurricane Cesar made
a landfall in Nicaragua in 1996. Hurricanes Mitch (1998) and Stan (2005)
also produced flooding in Costa Rica. In this country, no states on the Gulf
or Atlantic coasts are immune from hurricanes, with all of them recording
multiple landfalls dating back to 1850.

Dear Tom,
I remember that severe thunderstorms struck the Taste of Chicago in the early 1990s.
Was that a derecho?
Jim Smetana Arlington Heights

Dear Jim,
A derecho, a long-lived windstorm associated with a band of rapidly-moving
thunderstorms, did indeed sweep the Chicago area on July 2, 1992. The storm which
formed in the Plains the previous evening reached Chicago by mid afternoon, blasted
the area with wind gusts as high as 86 m.p.h. The storm killed one and injured several
dozen, capsized many sailboats on Lake Michigan, knocked down walls, unroofed
homes and devastated trees and power lines leaving more than 200,000 without power.
At the Taste at least six food booths and stages were overturned and the event shut
down for the day. The storm, accompanied by hail and torrential rain, also stopped
racing at Arlington Park and halted the Western Open golf tournament at Cog Hill in
Lemont.

Dear Tom,
I remember a cold 4th of July in Chicago back in the 1960s. We were at a picnic and
everyone was huddled in blankets. What year was that?
Vanessa Sandler

Dear Vanessa,
The date was July 4, 1967 and the high in Chicago was an unseasonably chilly 64
degrees, just two degrees from the holiday's 62 degree record lowest maximum
established in 1920. The day was overcast with a brisk north wind adding to the chill.
Normally packed beaches and pools were all but deserted and many parties had to be
moved indoors. The next day was just as chilly with a high of 65, a reading that did
establish a record daytime chill for July 5. The city has not experienced a colder 4th of
July since then, though it was almost as cool in 1997 when the mercury topped out at
66. In contrast, the city's hottest July 4th took place in 1911 when the temperature
soared to 102 degrees.

Dear Tom,
Why is the study of weather termed meteorology? You don't study meteors, so why not
call it weatherology?
Stew Cherlin Park Ridge

Dear Stew,
The term meteorology dates back to early Greece (340 B.C.) when Aristotle wrote his
treatise "Meteorologica", a classic work that not only covered that era's knowledge of
weather and climate but also subjects relating to geology, astronomy and
oceanography. In Aristotle's time anything that was suspended or fell from the sky was
called a "meteor" including rain, snow, hail, and rainbows and meteoroids. Because
most of Aristotle's work dealt with weather, the term meteorology became linked with
weather and atmospheric studies and meteoroids become part of astronomy. Thus, in
the context of meteorology's contemporary definition, a meteorologist is a scientist
that studies weather and the Earth's atmosphere, not meteors.