ASK TOM WHY: September 2008 Archives

Dear Tom,
If a tornado is heading toward a city, why couldn't the Air Force shoot a missile into it
to mess up its circulation?
Bryan Roberts (age 11), Naperville, Ill.

Dear Bryan,

Tornado forecaster Roger Edwards at the Storm Prediction Center in Norman, Okla.,
which issues tornado watches for the United States) says, "Bad idea!"

He explains that a tornado is much larger than it looks. The "tornado" that we see
below the clouds actually extends many thousands of feet upward through the
thunderhead. Disrupting the tornado would require disrupting the entire thunderstorm.
Could an H-bomb do it? Possibly, but that would cause far more damage than the
tornado itself.

For more information, check out "Frequently Asked Questions About Tornadoes," an
excellent web site prepared by Edwards, at: http://www.spc.noaa.gov/faq/tornado/

Dear Tom,

Weather information is used in many ways. In your experience, what is the most
unusual use that it has been put to? Can you relate an interesting story?

Lynn Schneider

Dear Lynn,

Indeed I can. One of my co-workers here in the WGN-TV weather center spent his
childhood in southwest suburban Brookfield. He remembers that a neighbor, Mrs.
Woods, specialized in growing potted cacti, and she took great pride in her collection
of about 75 varieties of those plants. Her house was full of all kinds of cacti.

Mrs. Woods claimed that the secret to growing cacti successfully was water -- not too
much and not too little. Every day, without fail, she would check the weather listing in
the Chicago Daily News (which ceased publication in 1978) for Phoenix, Arizona. If and
only if it had rained in Phoenix did Mrs. Woods water her cacti.

Dear Tom,
You often mention Chicago's daily normal high and low temperatures. How are
they arrived at?
Jim Welch
Dear Jim,
The National Weather Service calculates normals for a location (like Midway
Airport) based on temperatures observed there during a 30-year period,
currently the period from 1971 through 2000. Those normals, to be used for
10 years, will next be updated for the period 1981 through 2010.
At a specific location, average high and low temperatures for a given day,
say Aug. 10, are obtained by averaging the 30 daily highs and 30 daily lows
observed on Aug. 10 in all years from 1971 through 2000, and the same is
done for all days of the year. The daily averages, which bounce up and down
a few degrees from one day to the next, are "smoothed," and those smoothed
values are adopted as the normal daily temperatures for that location.

Dear Tom,
We found a green fuzzy caterpillar in late July, the earliest we have ever
seen that kind before. Does this mean an early winter, or is it just a myth?
Dan Gazdziak, Chicago
Dear Dan,
It's an enduring myth. Your question is similar to many that we receive
concerning the ability of animals to predict the weather. There is no
documented evidence that robins or groundhogs (the usual suspects) -- or any
animals, including green fuzzy caterpillars -- have that ability.
Animal behavior, even when uncharacteristic or unusual, is a response to
past or current weather or the result of instinctive imperatives; it is not
based on predictive ability. That goes for plants, too. An exceptionally
great production of seeds, as with oak trees and acorns, or premature leaf
coloring in the autumn always result from past weather conditions or other
environmental factors, not from predictive ability.

Dear Tom,
Watching hourly weather reports, I notice the relative humidity is often not
100 percent when it is raining. Is this correct?
Simon Palmer
Dear Simon,
It's correct, and it can rain when the relative humidity is less than 100
percent. In fact, relative humidity values are in the range of 90 to 99
percent most of the time when it is raining.
Clouds form when invisible water vapor gas condenses into visible water
droplets, and rain occurs when cloud droplets grow too large to be supported
by currents of rising air within rain clouds. Condensation and raindrop
formation occur within clouds -- where the air is saturated and the relative
humidity is, indeed, 100 percent.
Most of the time, though, clouds lie above drier air at the ground, and
that's where we live. Relative humidity at the surface reflects the actual
moisture content of the air at ground level -- drier air into which rain is
falling.

Dear Tom,

Why do hurricanes cause coastal flooding?

Charles Kesman, Chicago

Dear Charles,

It's complicated. First, there is wave setup. A hurricane's huge breaking waves transport
so much water to shore so rapidly that it cannot all flow back before the next wave
arrives. Water literally piles up and the sea level rises as much as 3 feet along the coast.

Then there is the soda-straw effect. Lower air pressure at a hurricane's center causes
the sea there to rise about 1 foot for every 1-inch difference in air pressure. The rise
can be three feet. Finally, there is storm surge. A hurricane's counterclockwise winds
spiral inward toward the eye, pushing water with it. At the storm's center, the water
sinks and flows outward. But as a hurricane approaches land and shallow water, the
water cannot sink; the sea mounds up as much as 20 feet.

Additional complicating factors: storm speed and strength, coastline configuration and
near-shore water depth.

Dear Tom,

After the Sept. 12-14 deluge across the Chicago area, I'm wondering how much snow
would have fallen had it been mid January instead. I'm guessing since the ground would
be frozen, it would greatly exceed the eight to ten inches of rain that hit most areas.
Rick, Crown Point, Ind.

Dear Rick,

"Greatly exceed" is quite an understatement. A typical snow-to-water ratio in big
snowstorms around here is 12-1 (12 inches of snow will melt down to one inch of
water). The ratio varies greatly because it is strongly affected by air temperature, wind
speed and snowflake size during the storm. The state of the ground in the winter,
frozen or unfrozen, isn't too important. The 12-1 ratio works well for temperatures in
the upper 20s with moderate wind with little drifting and a mix of snowflake sizes. That
said, 8 to 10 inches of rain would yield 96 to 120 inches of snow.

Dear Tom,

People always talk about the "Blizzard of January 1967".
What snowstorm did they talk about before that one?

David Jackson Wheaton

Dear David,

It ranks as the city's fourth largest snowstorm, but prior to the 23 inch Big Snow of
January 26-27, 1967, Chicago's premier snow event was a 19.2-inch blizzard that
paralyzed the city on March 25-26, 1930. Snow fell for 43 consecutive hours, and
strong northeast winds created 4-5 foot drifts that made travel nearly impossible. Parts
of the city received even more snow than the official University of Chicago station.
Fledgling Midway Airport measured 22.3 inches of snow, putting it right up there with
the 1967 storm. However, this time the snow melted however under the strengthening
late-March sun and two weeks later the mercury soared to 90 degrees on April 10 and
11, the city's earliest 90-degree days on record.

Dear Tom,

Do hurricanes form in the South Atlantic? It seems that we only hear about storms in
the North Atlantic. What about storms in the Indian and Pacific oceans?

Rebecca Jeffery

Dear Rebecca,

We never hear about hurricanes in the South Atlantic because, with only one
documented exception, they just do not occur there. Cool ocean currents, strong winds
aloft and a lack of trade wind convergence all combine to provide an inhospitable
environment for tropical cyclone formation. The meteorological community was truly
surprised when unprecedented hurricane dubbed "Catarina" struck coastal Brazil on
March 28, 2004.

Numerous tropical cyclones roam both the northern and southern portions of the
Pacific and Indian oceans each year. Earlier this year a devastating storm known as
Cyclone Nargis struck Myanmar in early May, with the death toll estimated as high as
150,000.

Dear Tom,
We just broke the record for most rain in a calendar day. What is the city's
record for rainfall in any 24-hour period?
Mark Kristoff, Lincolnwood
Dear Mark,
Chicago's record 24-hour rainfall of 9.35 inches was established during the
historic deluge that swamped the O'Hare area on Aug. 13-14, 1987. The rain
began at 9:16 p.m. on Aug. 13 and finally ended 17 hours and 29 minutes
later at 2:45 p.m. on Aug. 14. The flooding was so severe the airport was
turned into an inaccessible island. As devastating as this rainfall episode
was, it was dwarfed by Illinois' 24-hour rainfall record of 16.91 inches set
at Aurora on July 17-18, 1996. The U.S. 24-hour rainfall record is 43 inches
at Alvin, Texas, on July 25-26, 1979, while the world record is an
incredible 73.62 inches measured on March 15-16, 1952 on Reunion Island at
Cilaos, about 500 miles east of Madagascar.

Dear Tom,

The Earth's surface is always split 50-50, the part receiving direct sunlight and the part
not receiving it. That's common sense, but a friend claims there is always more daylight
than darkness (excluding twilight). Please shoot him down.

-Ken Nathanson

Dear Ken,

Reality sometimes defies common sense. Your friend is correct: There is more sun than no
sun. Here’s why: The sun is not a point in space. Triton College astronomer Dan Joyce
says direct sunlight first reaches a given spot the moment the top of the sun’s disk pokes
above the horizon at sunrise; at sunset, sunlight continues until the top of the disk sinks
below the horizon. The sunrise/sunset minutes when only a portion of the solar disk is
above the horizon add to daylight time. Also atmospheric refraction makes the sun appear
higher in the sky, lengthening daylight by a few minutes.

Dear Tom,
How do winds affect Lake Michigan water temperatures? Is the lake still warm
enough for one more family outing at the St. Joseph, Mich., beach?
Mark Tieszen
Dear Mark,
Current Lake Michigan water temperatures are available at this web address:
http://www.coastwatch.msu.edu/twomichigans.html
Right now (Sept. 19), waters at St. Joseph are in the upper 60s.
During the warmer months of the year, a layer of mild water at the surface
of Lake Michigan floats over much cooler water beneath it. Winds push the
surface waters around and stormy weather causes colder water to mix upward.
The highest surface water temperatures therefore occur during periods of
sunny, tranquil weather. Onshore winds will push the warmest surface water
to the beaches, whereas offshore winds cause cold-water upwelling at the
beaches.

Dear Mr. Skilling,

Was the "record rain" this past weekend really a record? I remember more than nine
inches of rain back in mid August in 1987.

Steve Platko, Prospect Heights, Ill.

Dear Steve,

Although your memory is correct, a record was established. Chicago logged 9.35 inches
of rain Aug. 13-14, 1987 --2.86 inches the 13th and 6.49 inches the 14th -- and that
two-day event stands as the city's greatest rain event.

Sept. 12-13-14, 2008, brought less rain: 8.45 inches officially (0.37 of an inch the
12th, 6.64 inches the 13th and 1.44 inches the 14th). But the 6.64 inches on Sept. 13
established a calendar-day rainfall record by surpassing the August 14, 1987, total.

The recent rain and resultant flooding was far more widespread, affecting all of
metropolitan Chicago (including northwest Indiana), whereas the 1987 event affected
only extreme northwest Chicago and the northwest suburbs.

Dear Tom,

What tropical cyclone hold the record for longevity?

Ken Bockelmann

Dear Ken,

The tropical cyclone longevity award goes to Hurricane/Typhoon John, which roamed
the Pacific Ocean for 31 days back in 1994. John formed off southern Mexico on August
11 and gradually strengthened to a Category 5 monster with 175 m.p.h. winds By Aug.
23.

As it traveled west it eventually crossed the dateline and was reclassified as Typhoon
John. The storm then turned and headed east where it again crossed the dateline
regaining its hurricane title. John finally dissipated in the cooler waters of the North
Pacific on September 11.

In the Atlantic Basin, two storms lasted about 28 days, the San Ciriaco Hurricane, which
affected Puerto Rico, the Bahamas and North Carolina in 1899 and Hurricane Ginger that
made a North Carolina landfall in 1971.

Dear Tom,

The symbols for cold fronts and warm fronts on weather maps are used everywhere.
Who dreamed up those symbols?

Barry Phillips

Dear Barry,

In the 1920s, an energetic group of researchers in Bergen, Norway, was making
significant progress in theoretical and applied meteorology. At the time, the group was
advancing the concept of "fronts," the boundaries between different air masses.

On their maps, the researchers used blue lines to indicate cold fronts and red lines for
warm fronts, but color maps were expensive to print.

Tor Bergeron (1891-1977), a member of the group, proposed a solution. While on a trip,
he mailed a postcard dated Jan. 8, 1924, to Bergen. On the postcard he sketched a cold
front with sharp, triangular barbs and a warm front with rounded, semicircular pips --
and a new meteorological tradition was born.

Dear Tom,

Have we ever been hit by hurricane remnants twice in one season let alone one month
like we've have with Gustav and Ike?

Judy Serwatka Valparaiso and Mary King

Dear Judy and Mary,

Since 1900 there have been 17 low pressure centers that were once hurricanes that have
passed within about 100 miles of Chicago. Some, like Carla's remnants in 1961, have
brought Chicago torrential rain and strong winds, while others have passed bearing
only light showers

Before this month, the last hurricane remnant to affect Chicago was Rita's on September
25-26, 2005, when a meager 0.14 of an inch of rain fell. While Chicago has
undoubtedly received rainfall associated with moisture from a hurricane more than
once in a season, this month's close encounters with the remains of both Hurricane
Gustav and Hurricane Ike appear to be a first for the city.

Dear Tom,
The storm surge at Galveston did not reach the 20-foot level that would have
been disastrous, but the surge close to New Orleans topped some levees in
that area and was higher than observed with Katrina. How could this be, when
Ike tracked right over Galveston?
Diane, western suburbs
Dear Diane,
The top of the sea wall at Galveston stands at 17 feet above mean sea level,
and a 20-foot rise there would have inundated the city. Hurricane Ike was an
unusually large storm and it was rather slow-moving -- both factors (long
fetch for a long time) allowed it to generate excessively high waves. Buoy
data indicated 25-35 feet.
At New Orleans, Katrina's winds mainly blew offshore (the hurricane made
landfall slightly to the east on the city), whereas Ike's winds blew onshore
(Ike made landfall 285 miles west of New Orleans). Surge is always higher
where winds blow onshore.

Dear Tom,

This year Chicago’s hottest day occurred after Labor Day. Has this ever happened before?
James Cook, Chicago

Dear James,

In Chicago, the occurrence of the year’s highest temperature after Labor Day is not
without precedent, but it is rare: only two times in 80 years at Midway Airport.
(Note:Labor Day is observed on the first Monday in September and the holiday can land
on any day from Sept. 1 through Sept. 7. It fell on Sept. 1 this year and the year’s
highest temperature, 94 degrees, occurred on Sept. 2.) In 1939, the year’s highest
temperature, 102 degrees, was on Sept. 15. That reading was a record high for the date
and also the latest-occurring date for an annual high. The year 1978 stands as the only
other year whose highest temperature occurred after Labor Day: 95 degrees on Sept. 8.

Dear Tom,
News reports Friday morning said water at the shore of the Gulf of Mexico
had already risen three feet. In a video from Galveston, it was sunny, a
gentle breeze was blowing and water a couple feet deep covered roads and was
into houses -- 30 hours before Ike was due to arrive. How can this be?
John Wilkerson
Dear John,
Dr. Bob Sheets, past director of the National Hurricane Center, says it's
called "wave set-up." Abnormally large waves generated by hurricane winds
bring to the shore abnormally large amounts of water in the tops of breaking
waves -- too much water to be sent back into deeper water before the next
wave arrives. Incoming and outgoing water are not in balance. All that water
pushing into the coastal zone has no place to go but up, and the sea
literally rises along the coast. The rise at a given spot depends on the
depth of water near shore and the configuration of the shoreline.

Dear Tom,

Who decides to sound tornado sirens in the Chicago area?

Mary Harris Russell, Chicago

Dear Mary,

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.

"Procedures (to sound sirens) vary from community to community." says meteorologist
Jim Allsopp of the weather service . "The sirens can be set off at the request of village
officials such as mayor, police or fire chief or emergency management coordinator." A
standard criterion for sounding sirens is the report of a tornado or funnel cloud, usually
within five to ten miles of the town. Allsopp advises that, "NOAA weather radio is the
best way to get warnings direct from the NWS," Allsopp says.

Dear Tom,

I always assumed the moon came up at dark and set at dawn, just the opposite of the
sun, but now I find I was wrong. On Aug. 1, both the moon and sun rose and set within
a few minutes of each other. What's the theory of this?
Jane Kuntz, Arlington Heights

Dear Jane,

Triton College astronomer Dan Joyce explains:

The moon orbits the Earth on roughly a monthly basis in the same direction the Earth
rotates on its axis, thereby permitting the moon to appear either close to or far from the
sun. When near the sun, the moon's far side is illuminated and we cannot see it, the
phase called new moon.

Halfway to becoming full, we see a half-lit moon (1st quarter), which rises near noon
and sets near midnight. The full moon rises at sunset and sets at sunrise . The phase
halfway back to new moon from full, also half-lit (3rd quarter) it rises at midnight and
sets at noon.

Dear Tom,

How did meteorologists track hurricanes before satellites?
Mike Schaeffer

Dear Mike,
Today, meteorologists have the luxury of continuous global satellite coverage that
makes it impossible for a tropical cyclone to escape detection. Before the advent of
aircraft and satellite surveillance, forecasters had to rely on a smattering of ship reports
and weather transmissions from the islands to learn about approaching storms. In the
era before radio communication, when an island was devastated by a hurricane, there
was no way to spread the warning to areas in the storm's path.

Detailed Atlantic Basin tropical cyclone records date back to 1851, but it is almost a
certainty that these records are incomplete with some storms that never made landfall
going undetected. In many instances, hurricanes struck with little advance warning,
other than a rapid increase in wind and rain and a rapidly falling barometer.

Dear Tom,

Who put out the first hurricane forecast, and when?
-Kathleen Samaras

Dear Kathleen,

The answer will surprise you: July, 1502, by Christopher Columbus. In the several years
since his epic voyage to the New World in 1492, Columbus had learned to read the
signs of an approaching hurricane in the behavior of the clouds and sea. In July 1502,
he saw those signs again and requested the Spanish governor of Hispaniola permission
to harbor his ships there. Permission denied, Columbus rode out the hurricane in a less
favorable port, but 21 of 30 Spanish ships that sailed from Hispaniola against his
advice sank, with more than 500 lives lost.

The incensed Columbus wrote, "What man ever born, not excepting Job, who would not
have died of despair when in such weather, seeking safety for ... my shipmates and
myself, we were forbidden the land and the harbors that I, by God's will and sweating
blood, had won for Spain?"

Dear Tom,
Recently you talked about disturbances near the Cape Verde Islands that turn
into hurricanes. Can you elaborate?
Rich Gardner
Dear Rich,
Cape Verde hurricanes are Atlantic Basin tropical cyclones that develop from
"easterly waves" that move west across Africa into the Atlantic near the
Cape Verde Islands off the coast of Senegal and Mauritania. The
disturbances, usually clusters of thunderstorms, travel west across the
tropical Atlantic along the southern periphery of the Atlantic Subtropical
Ridge, gradually gaining strength and circulation from the warm ocean. On
average two Cape Verde storms form each season, usually in August and
September. Some notable Cape Verde hurricane include the 1900 Galveston
Hurricane, Donna (1960), Hugo (1989) and Andrew (1992). The current
Hurricane Ike is also a Cape Verde type.

Dear Tom,
Please tell me a few things about the “eye” of a hurricane.
-Leecie Pearce, Delavan, Wis.

Dear Leecie,

The “eye” is a roughly circular area of rather quiet weather found at the center of a
hurricane. Eyes generally are 20 to 40 miles across, but they range in size from about 5
miles to (rarely) more than 120 miles. Winds are light or calm at the very center of an
eye, but strong winds can sometimes extend into its periphery. There is little or no rain,
and sometimes the sky is partially clear, but the eye is surrounded by a circular ring of
towering cumulonimbus clouds— the “eyewall”—that often extends upward more than
30,000 feet. A hurricane’s lowest air pressure is found in its eye, and its highest winds
occur in the eyewall. Temperatures in the eye are only slightly warmer than in the
surrounding storm.

Dear Tom,
Why does the Atlantic Ocean get more hurricane activity than the Pacific?
Scott Okun
Dear Scott,
With all of the recent hurricane activity in the Atlantic Basin it may seem
that way, but the truth is the Pacific wins the tropical cyclone competition
hands down. The Atlantic Basin averages about 10 named storms each year
while the Eastern Pacific typically logs 16 storms. Because of the potential
threat to the United States, the Atlantic storms get a lot of publicity
while most of the eastern Pacific storms get little press as they pose a
threat only to shipping. In addition, each year the Northwest Pacific
averages 20 typhoons and the Southwest Pacific waters generate as many as 10
hurricanes. In contrast, because of colder water and strong winds aloft
there are almost no hurricanes in the South Atlantic.

Dear Tom,

I lived in Liberia for seven years and we experienced dust storms called harmattans. Do
these winds eventually cause the hurricanes that reach the U.S.
-Patrick McKeen Buffalo Grove

Dear Patrick,

Hurricanes that affect United States often do develop from westward-moving
disturbances the move out of Africa into the Atlantic near the Cape Verde Islands.
However, these dust storms are not associated with hurricanes. Instead, the harmattan
winds are dry, dust-carrying winds from the northeast or east that blow in West Africa
especially from late November until mid-March. In summer, an onshore flow called the
southwest monsoon brings cooler air inland to West Africa, undercutting the
harmattan. The harmattan continue to blow aloft in a layer from about 3,000-6000 feet
carrying dust out into the Atlantic.

Dear Tom,

While watching the morning news on Sept. 1 regarding Hurricane Gustav as it was
making landfall, the videos showed a complete lack of lightning. Was this trick
photography?

Peter Gottstein

Dear Peter,

There were no tricks. Despite widespread belief to the contrary, hurricanes are
notoriously lacking in lightning. Hurricane Gustav, like most hurricanes, contained very
little of it. What lightning there was occurred not in the storm's warm and violent
interior but in its cooler outlying rain bands.

Dr. John Hallett, Director of the Ice Physics Laboratory at the Desert Research Institute
in Reno, Nevada, explains that electrification of clouds requires the presence of large
numbers of ice particles held aloft by violent updrafts. Hurricanes, though, are warm
systems usually lacking in the powerful localized updrafts found in "standard"
thunderstorms. As a result, lightning production in hurricanes is minimal.

Dear Tom,
This morning there are four named storms from Gustav to Josephine in
existence. Is this a record?
Ruth Patton

Dear Ruth,
Not quite. We checked the tropical cyclone archives and according to Dr.
Christopher Landsea of the Tropical Prediction Center this morning's four
named storms: Gustav, Hanna, Ike and Josephine fell just short of record of
five named tropical cyclones that coexisted during the period from Sept.
10-12, 1971 when Edith, Fern, Ginger, Heidi and Irene all roamed the
Atlantic Basin.

Landsea noted two other instances with four concurrent storms, the first was in August
1893 (before storms were named) and more recently from Sept. 25-27, 1998, with the
tropical quartet of Georges, Ivan, Jeanne and Karl. One of the main reasons that
meteorologists began naming tropical cyclones in 1950 was to help the public avoid
confusion when referring to multiple storms which used to be referenced only by
number.

Dear Tom,
I don't think we've hit a 100-degree high yet this year. What is the
likelihood we might still get one?
Amy Chatfield
Dear Amy,
Not only haven't we hit 100 degrees yet this summer, but the city's warmest
day so far has been only 91, something that has never happened here since
Midway Airport weather records began in 1928. Since then, a 92-degree high
in 1951, 1979 and 2004 reigns as the year's lowest maximum temperature. The
odds are very much against getting 100 yet this year. Chicago has logged
only six September 100-degree days since 1871, and none since it reached 100
on Sept. 7, 1960. The latest 100 on record here is Sept. 15, 1939, when the
mercury peaked at 102. The city's last encounter with triple-digit heat was
July 24, 2005, when it hit 102 at O'Hare and 104 at Midway.