ASK TOM WHY: May 2008 Archives

Dear Tom,

Is it a myth to count the number of seconds between lightning and thunder to find the
distance of the lighting? It was not true between 2 and 3 a.m. Sunday, May 25. Thunder
followed some bolts, but other bolts had no thunder.
--Tom Litke

Dear Tom,
The “lightning rule of five” that you are referring to is this: Count the number of seconds
between flash and crash, and divide by five. That gives the distance, in miles, between you
and the closest portion of the lightning bolt. That rule of thumb is as valid as it always
has been because the physics behind the transfer of sound waves through the atmosphere
has not changed. However, practical application of the rule can be difficult. Be sure to
make a correct link between a given flash and its thunder. Also, thunder rarely carries
beyond 15 miles, but lightning is often visible far beyond that.

Dear Tom,
I am in the third grade, and our class is studying about weather. My teacher
said I should ask you if it ever got to 100 degrees in Chicago.
Zain Mohammed
Dear Zain,
Chicago's temperature has indeed risen to 100 degrees, though not very
often. Chicago weather historian Frank Wachowski reports that the
thermometer at Midway Airport has registered at least one day at or above
100 degrees in 30 of the 80 years (1928-2007) for which we have data at that
location.
And one (but only one) of those super-hot days occurred in May, and right at
this time of the year. On May 31, 1934, the Midway temperature zoomed to
102. Chicago's all-time high temperature was also recorded at Midway
Airport: a scorching 109 degrees on July 23, 1934.
Chicago's most recent 100-degree day occurred three years ago: 104 on July
24, 2005.

Dear Tom,
Chicago's temperatures jump all over the place, but what daily high
temperature occurs most often? And what about the lows?
Pam Tellock

Dear Janice,
To find the answer, we scanned the entire Midway Airport temperature data
set from Jan. 1, 1929, through Dec. 31, 2007 -- a period of 79 years
made up of 28,854 days. The results might surprise you.
As determined by the Midway data, Chicagoans experience daily high
temperatures in 80s more frequently than any other 10-degree interval, and
nighttime low temperatures most frequently in the 30s.
The most commonly occurring specific high temperature is 80 degrees
(registered on 569 days) followed by 78 degrees (533 days), and the most
frequently occurring low is 32 degrees (634 days) followed by 31 degrees
(602 days).

Dear Tom,
The recent big temperature drop prompts me to ask what our all-time largest 24-hour
temperature drop might be.

Tim Bonham, Chicago

Dear Tim,
Huge temperature fluctuations are among the defining characteristics of Chicago's
vigorous climate. From Monday into Tuesday area residents experienced a 24-hour
temperature collapse from the low 80s to the low 40s. The city's largest-ever
temperature decline dwarfs that 40-degree tumble, however. At 4 p.m. on Nov. 11,
1911, Chicago's temperature peaked at 74 degrees, a record high for the date. Then,
readings plunged as arctic air surged in. By 12:30 p.m. on the 12th -- 20 1/2 hours
later -- the temperature was 13 degrees.

From one afternoon to the next, Chicago's temperature had crashed 61 degrees, a
tumble that still stands as the city's largest 24-hour temperature change.

Dear Tom,
On May 26 on the news the weather report mentioned a "pneumonia front"
coming through the Chicago area. What is a pneumonia front?

Michelle Kramer, Elmhurst, Ill.

Dear Michelle,
It's not a technical meteorological term and it has nothing to do with
pneumonia, but a "pneumonia front" refers to a strong
northeast-to-southwest-moving cold front occurring on the western shore of
Lake Michigan in the spring or summer.

It is accompanied by the sudden onset of gusty northeast winds and a sharp
temperature drop at the lake shore, with readings sometimes plummeting from
the 70s into the 40s in less than an hour. Temperature changes are less
abrupt farther inland.

The term was first used by the Milwaukee Weather Bureau Office in the 1960s
and was probably coined by then Meteorologist-in-Charge Rheinhart Harms (who
is also credited with "panhandle hook" and "Alberta clipper").

Dear Tom,
In late May 1947, Wisconsin was hit by a snowstorm. What was the weather
like in Chicago?
Bruce Tatge, Janesville, Wis.
Dear Bruce,
It seems incredible, but an unprecedented late-spring snowstorm did strike
portions of the Midwest from northern Iowa to eastern Upper Michigan with as
much as 10 inches of snow on May 28-29, 1947. Hardest-hit Wisconsin areas
were in the southwest portion of the state where 10 inches of snow fell just
south of La Crosse. The weight of the heavy snow caused severe damage to
power and telephone lines and the already-leafed-out vegetation. The
responsible storm center passed south of Chicago, bringing the city a cold
rain and some thunderstorms, highs in the 50s and strong winds. The storm
produced high swells and high waves on Lake Michigan from Kenosha to
Milwaukee, damaging waterfront property. Chicago did set a record low of 38
degrees on May 29, 1947, but that was eclipsed by a 37 degree low in 1984.

Dear Tom,
How often do we reach 90 degrees or higher on Memorial Day?
Nick Recchia, River Grove
Dear Nick,
With its late-May occurrence, Memorial Day is celebrated before Chicagoas
weather usually turns hot. Prior to 1971 it was observed on May 30 and since
then on the last Monday in May. Since 1871 the official Chicago temperature
has reached the 90-degree mark on Memorial Day in only eight years, most
recently in 2006 when it climbed to 91 degrees on May 29. The high
temperature on the holiday has never been higher than 93 degrees, a reading
recorded twice, first in 1942 and again in 1953. In reality, many Memorial
Days in Chicago are quite chilly, putting a damper on holiday picnics and
outdoor celebrations. The high temperature has failed to reach 60 degrees on
22 Memorial Days including two very chilly holidays (42 degrees in 1889 and
46 degrees in 1894) when the mercury failed to get out of the 40s.

Dear Tom,

Dear Tom,
Earlier this month, video clips were shown of Mark Grace's "hit-for-the-cycle" game on
May 9, 1993. The ivy was bare then, but this year on about the same date the ivy was
quite full. Was it a much cooler spring 15 years ago?

Art Schlicht, Sugar Grove

Dear Art,

There may be other factors in play that influence the emergence of the Wrigley Field ivy,
but based solely on temperature, it appears that a cool April may have been the culprit.
April 1993 was quite cool in Chicago with the mercury topping 70 only twice. In
contrast, this April produced eight days of 70 degrees or higher, including a warm
82-degree high on the 25th. It finally did warm up in early May 1993 with seven days
topping the 70 mark including a summerlike high of 87 degrees on May 9, the day
Grace hit for the cycle--and I’m sure the ivy fully emerged by mid or late May.

Dear Tom,
Is there any meteorological explanation for all the violent weather
occurring around the world -- cyclones, earthquakes, tornadoes, etc.?
Barbara Hampton, Chicago
Dear Barbara,
Worldwide, increases in reported severe weather occurrences result from far
larger populations in weather-vulnerable areas and better communication of
the events rather than a greater frequency of severe weather.
Consider the U.S. Gulf Coast. Prior to the 1950s, it was sparsely populated,
and landfalling hurricanes often struck between population centers.
Also, note that some phenomena (such as volcanoes, earthquakes and tsunamis)
are geological events, not meteorological. Their occurrence is independent
of the atmosphere and the weather. The U.S. Geological Survey says the
frequency of huge earthquakes (magnitude 7.0 or greater) has remained fairly
constant.

Dear Tom,
Has Chicago ever had exactly the same high and low temperatures on
consecutive days, or even three days in a row?
Ed Kozak, Burbank, Ill.
Dear Ed,
Chicago's weather is in flux almost all the time. Occasionally, though, when
weather systems are hardly moving, with a little luck the city can register
identical high and low temperatures on two consecutive days.
A computer scan of 137 years of Chicago temperatures (1871-2007, a period
encompassing 50,038 days) reveals that 287 pairs of days logged identical
highs and lows -- an average of two such occurrences per year; most recently
Aug. 26-27, 2007, with highs/lows of 79/60 degrees.
Three days in a row? Exceedingly rare: only four events; Aug. 11-13, 1879
(82/65); Aug. 26-28, 1882 (74/70); Sept. 5-7, 1910 (81/68); and Nov. 22-24,
1920 (42/37). It has never occurred four days in a row.

Dear Tom,

I am tired of Chicago's temperature extremes and would like to move someplace
where readings remain relatively stable and on the cooler side. Any suggestions?
--John Mannos, Chicago

Dear John,

We don’t want to lose you. But to answer your question, the Pacific Coast of
California and Oregon will come closest to satisfying your desire for a relatively stable
and cool temperature environment. Bathed by moderately cool air that prevailing
westerly winds carry in from the Ocean, the coastal zones of those states rarely
experience extreme temperatures.

Daily normal high/low temperatures on the San Diego coast, for example, range from
61/48 degrees in January to 72/58 in August. Coastal northern California and coastal
Oregon run a few degrees cooler.

On rare occasions, summer heat waves of a few days duration can send temperatures
into the 90s, even along the coast.

Dear Tom,
Which wind direction brings Chicago its highest temperatures in the summer?
Paul Sarewich, Chicago

Dear Paul,

Southwest winds (southwest-to-northeast-blowing winds) bring Chicago its highest
temperatures.

The southern Great Plains--the area from Kansas across Oklahoma into central Texas--
routinely experiences the nation's highest non-desert temperatures in the summer.
Chicago sits to the northeast of that area, and so it takes southwesterly winds to
transport hot air from that region to the city. But Chicago's highest summer
temperatures can occur only when southwest winds overcome the cooling effect of Lake
Michigan. That usually happens when high pressure is centered over the Southeastern
U.S. and a stationary or slowly-moving cold front stretches from the Dakotas to Lake
Superior.

Dear Tom,
While sailing on Lake Michigan, the sun is both overhead and reflecting off the water
on a sunny day. Does the addition of the reflecting sun change the exposure time for
getting a sunburn on water versus on land?

Gerald Migely, Chicago

Dear Gerald,
"The higher incidence of skin cancers in sailors and farmers was one of the first
clinical clues that ultraviolet (UV) radiation is a carcinogen." Said Dr. Bryan Schultz, an
Oak dermatologist. "Most of the risk for sailors is in the long exposure times, but the
percent of UV reflected is also a factor."

Water reflects only about five percent of light when the sun is high, but the shiny
surface of a boat may approach 80 percent. "This does increase one's UV dose
significantly," Schultz said. UV reflection increases as the sun drops below 45º, but
those rays, filtered through more atmosphere, are much weaker.

Dear Tom,
I have observed high-altitude jets dispensing heavy, non-dissipating trails behind them,
unusual in that they expand across the entire sky, turning a once-blue sky into a
complete cover of milky white. Tom, these are not condensation trails.
--Jeff Blondell
Dear Jeff,
Ah, but they are. Water vapor is one of the gaseous products of combustion and, when
added to the air in the exhaust of jet engines, condenses into visible clouds of ice
crystals--the wispy, cirruslike clouds that we see as condensation trails (contrails)
behind high-flying jets. Temperatures and humidity at flight altitude determine whether
contrails form and how long they last. In dry air, contrails will not form, or they will be
small and evaporate in a few seconds. In moist air, they persist for hours, often
"seeding" their own further growth.

Dear Tom,

What might be the relationship between clouds and fog? Is low-lying fog actually a
cloud?

—Leon J. Hoffman, Chicago

Dear Leon,
Physically, clouds and fog are identical. Both are visible aggregates of water droplets
(or ice crystals) so small that they are nearly suspended in the air. Clouds differ from
fog only in that, by definition, fog is either at the ground or within several feet of it,
whereas the bases of clouds are aloft. It’s a convenient shorthand to say that cloud
and fog droplets are “suspended” in the air, but that is technically incorrect:
Thedroplets do fall. The fall rate is so slow—generally only 1 foot per minute, or less
—that, for all practical purposes, clouds and fog appear to be suspended. A final
distinction: Meteorologists also consider a visible concentration of smoke or dust
aloft to be a cloud.

Dear Tom,
My son, age 7, has become deathly fearful of thunderstorms. Short of professional help, can you offer any suggestions?
James Smith, Madison, Wis.
Dear James,
The high winds, beating rain, lightning and crashing thunder of violent storms prove humbling to all of us, but they can be especially frightening to children. Here are a few tips that will allow you to confront and ease children's storm fears: Communicate with them—don't deny their fears. Let them know it is normal to feel afraid and explain that the storm will end. Remain calm and cool. Children take behavioral cues from parents and react accordingly. Shelter family pets during a storm so children will know they are safe. Assure children that lost or damaged toys, articles of clothing, etc., can be replaced; explain that it is more important that the family is safe.

Dear Tom,

Years ago, I remember hearing seiche (pronounced saysh) warnings mentioned for
Chicago's lakefront. Could you please explain that phenomenon?

Royse Cramton, Oak Park, Ill.

Dear Royse,

In the Great Lakes Region, any sudden rise in the water level in a harbor or on the
shore of one of the Great Lakes is known as a seiche. In extreme cases, the rise,
occurring within a minute or less, can be several feet. Such dramatic events obviously
present great danger to people at the lakeshore.

The Chicago seiche situation is unique. In rare cases, an air-pressure jump in
advance of southeastward-moving thunderstorms can push a low surge of water
from Chicago across Lake Michigan. Upon striking the Michigan/Indiana shore, the
surge reflects back to, and focuses upon, Chicago's lakeshore,initiating a sudden,
temporary rise in the lake level.

Dear Tom,
Why do we name hurricanes and typhoons but not tornadoes?
Stan, 11 and Ted, 9, Jatczak, La Grange Park

Dear Stan and Ted,
Tropical cyclones are named to enable better communication between meteorologists
and the public. Frequently multiple storms are active and naming the storms reduces
confusion and helps the public keep the storms straight.

In 1950, previously anonymous Atlantic Basin tropical cyclones were named using the
phonetic alphabet (Able, Baker, Charlie, etc.). From 1953 until 1978 only women’s
names were used, but starting in 1979 lists of alternating women’s and men’s names
were developed, a system still in use today.

It also makes sense to name tropical cyclones because they can roam the oceans for
days or weeks, while most tornadoes last for just a few minutes.

However, significant tornadoes or major tornado outbreaks are named after the fact,
like the Oak Lawn tornado or the Palm Sunday tornadoes.

Dear Tom,

I'm old fashioned and I like to hang my wash outside to dry, but I only do this during
the summer. Would clothes still dry even in colder winter temperatures? Would it be
worth trying?
-Marlene Forst

Dear Marlene,

A sunny day is conducive to hanging your wash out anytime, but you will be
disappointed during the colder months.

Drying (evaporation) rates are a function of the air's ability to hold moisture, and this
is determined by temperature. Cold air has a very small capacity to hold moisture,
and a full day on the line on a cold day might still result in damp or frozen clothes.

Sunshine itself has a minor effect, raising the temperature of the clothes and adjacent
air molecules a few degrees.

Indirectly, though, sunny skies usually indicate a dry air mass and, hence, increased
evaporation.

Dear Tom,
Are tornado warnings ever issued for funnel clouds?
-Patricia Nesbit

Dear Patricia,
A meteorologist would not hesitate to issue a tornado warning based on a
funnel cloud if the report appeared to be reliable and could be associated with a
Doppler radar that shows a rotation in the clouds in the area.
A funnel cloud becomes a tornado when it hits the ground. During periods of severe
weather, it could only be a matter of seconds before a funnel reaches the ground and
starts causing damage.

Storm spotters and weather professionals often use the redundant phrase “tornado on
the ground” to convey a sense of urgency and danger to the public about an
approaching twister. The redundancy makes it clear that the storm is an actual tornado
and poses an immediate threat.

Dear Tom,
What is the date of Chicago's latest freeze?
--Shirley Bell
Dear Shirley,
Chicago's latest freeze occurred 16 years ago when the mercury fell to 32 degrees at
O'Hare International Airport on May 25, 1992. The previous morning was also chilly with
a frosty low of 34 degrees, and back-to-back minimums of 33 followed May 27-28.
Just six years ago, the city experienced its second-latest freeze when the thermometer
registered 31 degrees May 21, 2002. These freezes stand out because they are a full
week later than the previous latest freeze set May 14, 1895, with a low of 32. The two
latest freezes were established at O'Hare's more rural environment, which fosters
somewhat lower minimum temperatures than the city's earlier official thermometer
sites, which were near the lake until 1942 and then at Midway Airport through 1979.

Dear Tom,
What is the difference between a hurricane, a cyclone and a typhoon?
—Jessica Vega

Dear Jessica,

There is no difference. Unfortunately, meteorological jargon and popular usage
variously apply different or overlapping meanings to those three words. In
meteorological parlance, a cyclone is a lowpressure system. Hurricanes and typhoons,
too, are low-pressure systems, but they are a specific kind of cyclone—tropical
cyclones (logically, cyclones that form in the world’s tropical and subtropical zones).
Hurricanes (in the Atlantic Ocean), typhoons (western Pacific Ocean) and cyclones
(Indian Ocean and Australia) are different names for the same type of storm. In
popular usage in the United States, “cyclone” is applied loosely to tornadoes,
waterspouts, dust storms, hurricanes and even to any strong wind.

Dear Tom,
A lot of Chicago's moisture comes from the saltwater Gulf of Mexico, so why isn't our
rain salty?
Bob Johnson, Oak Park
Dear Bob,
Since the beginning of time, the saltwater oceans have been the reservoir of about 97
percent of this planet's water supply. The water is constantly being recycled,
evaporating into the atmosphere and returning to Earth as rain or snow in an ongoing
hydrologic cycle. Sea water is indeed salty, with about 35,000 parts of salt per million
parts of water. However, the salt in sea water is only dissolved in it and not chemically
bonded, so it is left behind when the water evaporates, like when a pot of saltwater is
boiled dry on a stove. That is the reason sea air is salty when the ocean's spray
evaporates, and also why precipitation falls as fresh water. When salty ocean water
surges inland as in a hurricane storm surge, it can kill vegetation.

Dear Tom,

The Myanmar cyclone disaster is horrific. What could account for such a catastrophe?
Steve Bartik

Dear Steve,
The disaster in Myanmar, formerly known as Burma, is staggering. The official death
toll, now at more the 20,000, could climb to 100,000 as a worst-case possibility, said
Shari Villarosa, the U.S. chargé d’affaires in Yangon.

On the evening of May 2, Cyclone Nargis moved from west to east along Myanmar's
southern coast. It brought sustained winds of 125 m.p.h. and a 12-foot storm surge
that swept across the flat, low-lying and densely populated delta of the Irrawaddy
River.

The river branches into several large channels that drain south into the Indian Ocean.
Unfortunately, the north/south orientation of those channels allowed the cyclone's
onshore winds to push storm surge many miles inland.

Dear Tom,
Please define the "growing season." When does it start and end?
Gregg Dubin, Huntley, Ill.

Dear Gregg,

No single definition of "growing season" is satisfactory for all situations.

Generally speaking, the growing season is the period of the year during which air
temperatures remain continuously high enough for the growth of cultivated plants.

A commonly accepted standard is the number of days between the average dates of
the last spring and first fall occurrences of a temperature of 32 degrees. By that
measure, the growing season varies from 191 days (April 17 to Oct. 24) along
Chicago's lakefront to 162 days (May 1 to Oct. 9) in outlying suburban locations.

Ideally, the growing season should be defined biologically rather than
meteorologically because plant species vary greatly in their tolerance to cold
temperatures.

Dear Tom,
Weather-knowledgeable friends say large hail always comes before a tornado, but I
wonder about that. Is it true?
Peter Micham, southwest suburbs of Chicago
Dear Peter,
It is not true. Only the wind fields within and in the area several miles around a severe
thunderstorm provide reliable clues as to the storm's tornado-producing capabilities.
Unfortunately, those telltale signs are rarely apparent to an observer. Only Doppler
radars can detect them.

Severe storms forecaster Roger Edwards of the National Weather Service's Storm
Prediction Center in Norman, Okla., says: "Does hail always come before a tornado?
Rain? Lightning? Utter silence? High winds? Not necessarily for any of those. Hail can
indicate the presence of an unusually dangerous thunderstorm, but it is not a reliable
predictor of tornado threat."

ATW GRAPHIC HEADER LINK:

Dear Tom,

I've noticed that sounds carry better at night than during the day. Could you provide
an explanation?

Louise, Park Forest

Dear Louise,

Sounds do carry better and farther at night. Because it tends to be quieter then,
individual sounds are easier to distinguish. By far the most important factor, however,
is a difference in the temperature structure of the lower few hundred feet of the
atmosphere between day and night.

Sound travels through cold, dense air more slowly than through warmer, less dense
air. When air temperatures change on the path along which sound waves are
traveling, the waves always bend toward the colder air.

By day, it's warmer near the ground and colder above; sound bends up and away from
the ground (and you). At night, it's colder near the ground and warmer above; sound
waves bend down.

Dear Tom,
On a visit to the Black Hills of South Dakota, my family and I noticed outdoor neon signs
had screens over them for protection from hail. Is hail that frequent there?
--James Walker, Madison, Wis.
Dear James,
It is. Large hail is massively damaging. It flattens crops, shatters windows, dents cars and
kills birds and small mammals. Chicago experiences an average of two days per year with
hail. Most of it is small and causes no damage. But the frequency of hail, and especially
the frequency of large, damaging hail, increases steadily westward from Chicago. Hail is a
costly problem in the Great Plains, especially across Wyoming and Colorado. Cheyenne,
Wyo., with nine hail days annually, is the "hail capital" of the nation. The Black Hills, 200
miles northeast of Cheyenne, logs six hail days per year.

Dear Tom,
What U.S. city has the most sunny days in a calendar year?
—Paul Pincawis, Elgin.

Dear Paul,
Few weather and sky phenomena lift our spirits more than a bright, sunny day,
especially after a cold, cloudy and snowy winter such as the one we have just put
behind us. Unfortunately, sunny days are often at a premium in Chicago and across
the Great Lakes region, and the area’s residents will not be surprised to learn that
they dwell in one of the nation’s cloudier areas; only New England and the Pacific
Northwest are cloudier. On average, Chicago logs 84 sunny days per year, well ahead
of cloudy cities like Seattle or Burlington, Vt., (58 days at both cities), but trailing far
behind cities in the nation’s desert Southwest. Yuma, Ariz., heads the list with 242
sunny days per year, followed by Phoenix (211) and Las Vegas (210).

Dear Tom,
I love hot weather. When does Chicago usually record its first 90-degree day? When
was our last 100-degree day?
--Don Grelecki
Dear Don,
Chicago climate records peg the average date of the city’s first 90-degree day
around May 29 based on 80 years of data from Midway Airport. The earliest 90-
degree day ever recorded here occurred back-to- back on April 10-11, 1930.
In 1875, there were no 90s at all, the only year the city failed to reach that
benchmark, according to other records.
Last year Chicago recorded its first 90-degree day on May 14 and in 2006
it occurred on May 28.
Days with highs in the 100s here are not as common. Our last 100-degree
day was July 24, 2005, when the thermometer officially peaked at 102
degrees. Recent highs in the 100s occurred in 1999 and in 1995, when a deadly
heat wave caused more than 750 fatalities.

Dear Tom,
A friend in Iraq has told me that, besides a lot of heat and dust, they had a lightning
storm—no rain, no thunder—just lightning. How can you have lightning and no
thunder?

Mary Abuja, San Diego

Dear Mary,

Thunder is always a by-product of lightning, but sometimes we are too far away to
hear it.

A lightning spark can heat the air through which it passes to 54,000 degrees
practically instantaneously. Such extreme heating causes air to expand explosively,
followed immediately by cooling and rapid contraction. That physical process sets up
sound waves that blast outward, and we hear them as thunder.

However, thunder dampens quickly as it spreads and it rarely carries more than 15
miles from its source, whereas lightning bolts are often visible at distances beyond
50 miles if intervening clouds are not present.