ASK TOM WHY: April 2009 Archives

Dear Mr. Skilling,
We are learning about weather in first grade. What kind of clouds do we see the most?
Raquelle Blentlinger, Forest Glen School, Glen Ellyn, Ill.

Dear Raquelle,

Weather people have identified more than 70 different kinds of clouds, but they all fall
into three general groups that depend on the height of the cloud bases above the
ground. Those groups are low clouds (with bases below 6,000 feet), middle
clouds (6,000 to 18,000 feet) and high clouds (above 18,000 feet).

Most of the various kinds of clouds can be seen at one time or other in the Chicago
area, but the most common are the cumulus clouds--the flat-bottomed, puffy, "dob of
cotton" clouds seen on sunny days. Some cumulus clouds (like cumulonimbus -- the
thunderhead) build upward to 35 thousand feet or more but, with bases below 6,000
feet, are classed as low clouds.

Dear Tom,
Has Chicago's average temperature changed through the years?
John Mannos Chicago

Dear John,
Chicago's long-term climate record that dates back to late 1870 is difficult to analyze
because of the numerous changes in the location of the city's official thermometer.

Through June 1942 readings were taken in an urban environment close to Lake
Michigan. From July, 1942 through mid-January 1980, the readings were logged at
Midway Airport, still an urban environment but not near the lake. Since then the official
site has been at O'Hare International Airport, a more rural, inland location. Despite all
these site variances, there has been little change. The city's earliest average
temperature, from 1871-1910 was 48.8 degrees and the most recent, from 1971-2000
is 49.1 degrees.

Dear Tom,
Can you recommend a book or website where I could find precipitation totals
broken down by day for Chicago's Official readings at O'Hare, going back
about three years? I am a big fan of both your work and of meteorology
itself.
Mike O'Connor

Dear Mike,

We receive many requests -- too many to answer individually -- for Chicago's
official daily weather statistics from O'Hare International Airport. Those
data (temperatures, precipitation, snowfall and wind) are available from
1998 to the present, free of charge, on the Chicago National Weather Service
web site www.crh.noaa.gov/lot

Once there, follow these steps: search down the left side for Climate and click on word
"Local"; click on the button for "Preliminary Monthly Climate Data (CF6)" under the
header, "1. Product"; click on the button for "Archived Data", under the "3 timeframe"
header; select the desired month and year, then click "Go".

Dear Tom,
What was the deadliest tornado ever to hit Illinois?
Pat Beranek, Downers Grove

Dear Pat,
Illinois' deadliest tornado and this nation's deadliest are one in the same; the Tri-State
Tornado of March 18, 1925. That storm took a record 695 lives with 613 of the deaths
occurring in Illinois, as it cut a continuous 219-mile-long path of F4-F5 destruction
from southeast Missouri to southwest Indiana. The town of Murphysboro, in Downstate
Illinois, suffered 234 of the fatalities, the most ever in a single community.

The storm began around 1 p.m. near Ellington in southeast Missouri and dissipated 3.5
hours later near Petersburg in southwest Indiana. The twister's average speed was 62
m.p.h. and at one point reached a record 73 m.p.h. between Gorham and Murphysboro
as it sped across southern Illinois.

Dear Tom,
When can I safely put my young tomato plants out without a chance of frost
killing them?
Charles Heidgen, Batavia
Dear Charles,
Noting that you live in far west suburban Batavia, we would caution to not
put out your tomato plants without protecting them on chilly nights until at
least the middle of May. If you lived in the city -- especially near Lake
Michigan -- it would be a totally different story; it would probably be safe
to plant them now. With Lake Michigan water temperatures already in the
middle and upper 40s, a combination of the lake's warming influence and the
urban heat island effect make the threat of frost negligible there after
about April 15. Further inland it is a far different story, and frost can
form on clear, calm nights well into May. Typically, areas west of the Fox
River don't cross the 90 percent safe-to-plant threshold until about May 20.

Dear Tom,
We live near San Antonio and just heard a report of teacup-size hail. How big is that?
-Ann and Chuck Keeling

Dear Ann and Chuck,

Severe weather season always brings a rash of hail reports along with a load of descriptive
size adjectives. Most hail identifiers can be associated with a specific diameter, like pea
size (one-fourth of an inch) or golf-ball size (1 3⁄ 4 inches), giving an accurate mental
image of the hailstone. Other terms such as teacup, hen egg and grapefruit, items which
can vary in size, are less easily visualized. Most hail charts list teacup hail at about 3
inches in diameter. Other effective size-related terms in common use include penny (
three fourths of an inch), quarter (1 inch), baseball (2 3⁄ 4 inches), tennis ball (2 1⁄ 2 inches)
and softball (4 1⁄ 2 inches).

Dear Tom,
The "change of seasons" is often cited as the explanation for severe weather
in the spring. So why is spring traditionally the tornado season and not the
fall?
Randall George
Dear Randall,
The explanation is not just the "change of seasons"; it's the kind of
change, and it has to do with the stability of the atmosphere.
Powerful currents of warm, rising air -- so-called "updrafts" -- provide the
energy that feeds thunderstorms. When the lowest few thousand feet of the
atmosphere is warm and it is much colder aloft, the warm air gives birth to
updrafts, and the atmosphere is said to be "unstable." That's thunderstorm
weather.
The atmosphere is thunderstorm-prone (unstable) in the spring because the
air aloft is still cold with the lingering chill of winter, but
strengthening spring sun is rapidly warming the surface layer. It's the
opposite (stable) in the fall. Air aloft still retains summer heat and the
surface layer is cooling.

Dear Tom,
I remember back in the 1950s when several fisherman perished in a seiche. Can you
provide some information on this?
Kathleen Madden, Chicago

Dear Kathleen,
It was back on June 26, 1954 when a killer 8-10 foot wall of water called a seiche
(pronounced saysh) swept eight unsuspecting fishermen off the Montrose Harbor pier
to their deaths. The seiche was caused by an outflow of cold air rushing out of a
fast-moving line of thunderstorms speeding southeast across southern Lake Michigan
in excess of 60 m.p.h. The cold outflow caused a rapid rise in air pressure that pushed
a slowly-building bulge of water toward the southeast shore of the lake. As a result,
water levels dropped on the Chicago shore and rose on the Michigan/Indiana side. The
surge then moved back to the Chicago shore as a large wave, taking nearly 90 minutes
to make the return trip.

Dear Tom,
Your column about rainbows sparked a lively argument with a friend.
He says rainbows are shaped like arches like the St. Louis Arch.
I say they are circular.
Susan Taylor

Dear Susan,

The arc of a rainbow is the same for all rainbows, and the arc is always circular. Triton
College astronomer Dan Joyce explains, "Most of the time, because we see only a
fraction of the full rainbow, it is tempting to perceive what we see as a parabola or
some other non-spherical conic section -- but it's a circle."

Because a rainbow always appears at an angle of 138 degrees from the sun, it is seen
on the opposite side of the sky from the sun. The best view of a rainbow's spectacular
color display is afforded not from the ground, however, but from high-flying aircraft.
Occupants of jet planes can sometimes see a rainbow's entire full circular sweep from
top to bottom.

Dear Tom,
When you present "normal" spring temperatures, are these really the averages gathered
over all of the years of record, or are they obtained by taking some low point in winter
and a high point summer and plotting a rising line from one to the other?
Ralph Kravis, La Grange, Ill.

Dear Ralph,

Normals for a location are derived from temperatures observed there during a 30-year
period, currently the years from 1971 through 2000.

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. 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.

Dear Tom,
Can the relative humidity ever be greater than 100 percent?
S. Wahrman

Dear S.,
Surprisingly, yes, and the condition is known as supersaturation. At any given
temperature and air pressure, a specific maximum amount of water vapor present in the
air will produce a relative humidity of 100 percent, and air in that state is said to be
saturated. Supersaturated air literally contains more water vapor than is needed to
cause saturation.

Airborne water vapor begins to condense onto impurities in the air (such as salt
particles and dust) as the relative humidity of moist air approaches 100 percent; a
cloud (or fog) forms. In absolutely clean air devoid of impurities that ordinarily serve as
condensation surfaces, the humidity can climb to incredible levels of supersaturation
-- 400 to 800 percent -- before condensation begins. Realistically, though, air is never
totally clean.

Dear Tom,
Do rainbows only come out in the summer (when it is warm)?
Sam Fortuna (age 5)
Dear Sam,
Rainbows can appear in the sky at any time of the year because temperatures
are not involved in their formation. Unfortunately, the sky conditions that
are necessary to produce rainbows rarely occur during the coldest part of
the year, and so we almost never see them in the period from November
through February.
A rainbow can form only when bright sunlight shines directly on water
droplets (like raindrops) and the observer is in the proper place to see it.
But in the winter, the usual situation when it is raining is a gray, gloomy
and solidly overcast sky with no direct sunlight. Breaks in the clouds that
let sunlight shine directly onto raindrops usually occur only with showery
rain such as thunderstorms, which are mainly warm-season weather events.

Dear Tom,
I can get pink skin from the sun, even in the winter. Does this not mean that my skin is
producing vitamin D?
Laurie Black

Dear Laurie,

With regard to exposure to sunlight during the winter, Dr. Bryan Schultz, an Oak Park
dermatologist, tells us, "With Chicago's winter sun at about 10 percent of summer's
intensity ... one is much less likely to get adequate vitamin D that way." Schultz explains
that other factors are also at work. “Skin may turn pink from cold or wind, but this does
not produce vitamin D." In addition, shorter winter days and winter cloudiness greatly
limit exposure to the sun, and winter clothing reduces the amount of skin area exposed.
However, Schultz cautions that prolonged exposure to winter sun "may still cause the skin
to turn pink from UV [rays], especially with reflection from snow which may double the
dose."

Dear Tom,
Has it ever officially snowed in June in the Chicago area?
James A. Ulrich, Riverside
Dear James,
In the 138 Junes of Chicago weather records dating back to 1871, only once has this city recorded an official June snowfall, and that occurred on June 2, 1910. It was a very un-June-like day with a high temperature of only 55 degrees and a low of 43. Chicago weather historian Frank Wachowski said that during a thunderstorm rain, hail and wet snowflakes were observed. Rainfall that day totaled 0.41 inches, but the snowfall was just a trace. It's been a long time, but other traces of snow have fallen here in late May including May 25, 1924, and May 26, 1889. The city's latest measurable snowfall was 0.2 inches on May 11, 1966. Snow has fallen in Chicago in 10 months of the year; only July and August remain totally snow-free.

Dear Tom,
What is a "landspout"?
Dennis Gilliland

Dear Dennis,

It's a colloquial term for a tornado, usually rather weak as tornadoes go, produced by a
thunderstorm that is still in its initial stages of intensification (during which time it
rarely produces severe weather). Landspouts are so-named because, in appearance,
they resemble weak Florida Keys waterspouts over land.

Most tornadoes are produced by a special breed of thunderstorms known as "supercell
thunderstorms" -- enormous, severe, rotating (in the sense that air in the 10-50 mile
wind field in which supercells are embedded spirals inward) and, most uniquely, they
persist for hours. Most thunderstorms move through a life cycle of an hour or less,
then die away. Landspouts are the products of non-rotating thunderstorms that, in
other respects, are not severe.

Dear Tom,
What are the longest and shortest intervals between Chicago's first flakes in the
autumn and the last ones in spring?
Matt Mills

Dear Matt,
The period you describe is sometimes referred to as the snowing season and in Chicago
it typically extends for 174 days from Oct. 30 to April 21. The city's shortest snowing
season on record spanned only 109 days from Nov. 21, 1945 to March 9, 1946.

Chicago's longest snow season, recorded in the winter of 1909-10, covered 235 days
from Oct. 11 to June 2, a date that marks the city's latest-in the-season snowfall.

The last snowflakes recorded in Chicago this season fell April 6, and if no more snow
falls the rest of this spring, the 2008-09 snow season will enter the books at a lower-
than-average 163 days from Oct. 26-April 6.

Dear Tom:

So far this season we have had 56 days where the temperature did not reach 32
degrees. How does that compare with an "average" winter season?
Bob Matthei, Lake Bluff, Ill.

Dear Bob,

Chicago normally experiences 41 days per winter season on which the temperature fails
to climb to 32 degrees. That's the result of a computer sweep of the city's entire
temperature data base, 1871 to the present. (And by "winter season" we are referring to
the period from October through April, rather than the precise but more limited
definition of meteorological winter, December through February.)

We've logged 56 sub-32 degree days this winter, or 36 percent more than average -- a
statistic that will come as no surprise to winter-weary Chicagoans awaiting genuine
spring warmth. Only 22 winters out of 138 have produced more sub-32 days.

Dear Tom,
Please settle this dispute between me and my brother. Has Chicago ever had a July day
with a high below 60 degrees?
Patrick O'Heath

Dear Patrick,

Chicago has experienced some chilly days in July but never one with a high less than
60. The chilliest July maximum on record here is 60 degrees observed on July 8, 1883.
In those days the city's official temperature site was downtown near Madison and
LaSalle Streets and the brisk north-northeast winds off Lake Michigan that prevailed in
the wake of a cold front's passage contributed to the day's anemic high. There have
been sub-60 highs at the end of June, most notably the three-day string in 1902 of 58
degrees June 28 and a pair of 59s on the 29th and 30th. The only August day with a
high below 60 was Aug. 30, 1915 with a maximum temperature of 58.

Dear Tom,
With the Cubs home opener today, I wonder: What kind of weather has the team
encountered over the years?
Derrick Jabczynski
Dear Derrick,
Chicago Cubs home openers are typically cold-weather affairs, a combination
of the early April date and Wrigley Field's proximity to chilly Lake
Michigan. Today's opener will be no exception with game-time temperatures
expected in the lower 40s combined with stiff east winds and a cold rain. A check
of Cubs home openers dating back to 1954 shows that 56 percent of the
games have been played with temperatures in the 30s or 40s. The team's
coldest opener took place on April 8, 2003, with the mercury at an icy 32
degrees. There have been some warm exceptions. On April 22, 1960, against
the San Francisco Giants, the first-pitch temperature was a summery 80
degrees with a stiff 25 m.p.h. south wind blowing out to left field.

Dear Tom,
Last week you reported that when Chicago recorded the same high (50 degrees) three
days in a row (April 2-4), it was only the 193rd such occurrence. Has Chicago ever had
the same high four days in a row?
-Chris Goebel, Aurora

Dear Chris,

Since Nov. 1, 1870, the city has recorded the same high temperature on four
consecutive days 17 times—-a frequency of once about every eight years. The last time
this happened was Feb. 21-24, 2005, with a string of four 36-degree days; before that
four highs of 37 degrees were recorded Dec. 20-23, 1997.

Only once has the city achieved the same high five days in a row, and that was March
15-19, 1877, when the maximum temperature got stuck on the 30-degree mark. It may
happen again someday, but for now the city’s string of consecutive identical maximum
temperatures ends at five.

Dear Tom,
When was the city's coldest April? Has April ever been colder than March?
Anna P., Wilmette
Dear Anna,
At a time of year when Chicago's average temperatures are undergoing their
fastest increase, it would be rare to have an April colder than March, and
the city's temperature archives bear that out. Dating back to 1871 there has
been only one such occurrence, and that was more than a century ago in 1907.
In that year March, buoyed by nine days of 60 degrees or higher including
five 70s and even one 80, averaged 42.6 degrees; while April, chilled by 10
days in the 30s only two 60s and just one 70, averaged only 39.8 degrees --
making it the second coldest April in history. The city's coldest April
logged way back in 1884 averaged a chilly 38.8 degrees, but that year March
logged 36.4 degrees.

Dear Tom,
The level of Lake Michigan is up 13 inches from last year. That's great, but could you
express that in gallons of water?
Dan Fridley

Dear Dan,

The quantity of water that circulates through the Lake Michigan hydrologic system is
truly staggering. And expressing that volume in units as miniscule as gallons yields
numbers that are so huge as to be practically incomprehensible, but here it goes.
A 13-inch increase in the level of Lake Michigan's 22,300 square miles amounts to
5.044 trillion gallons of additional water (5,044,000,000,000 gallons). And that's not
all. Lake Michigan and Lake Huron are essentially one lake; their water levels rise and
fall in tandem. Thirteen inches of water added to the level of Lake Michigan means 13
inches added to the 23,000 square miles of Lake Huron as well, and that amounts to an
additional 5.202 trillion gallons (5,202,000,000,000 gallons).

Dear Tom,
What is meant by the term "carbon sequestration"?
Dorothy Manning

Dear Dorothy,

Carbon sequestration originally referred to the removal of carbon (in the form of carbon
dioxide gas, CO2) from the atmosphere by natural processes, mainly the absorption of
CO2 by the world's oceans and by photosynthesis in plants.

The tiny shells of ocean-dwelling animals contain carbon (chemically captured from
CO2 dissolved in ocean water). When they die, their shells and their store of carbon
sink to the ocean floor. Green plants use atmospheric CO2 during photosynthesis and
chemically store the carbon in their tissues.

The definition of carbon sequestration has expanded in recent years to include the
capture and storage or disposal of CO2 or other carbon compounds produced by
industrial processes in order to prevent their release into the atmosphere.

Dear Tom,
Everyone's been talking about the White Sox opening games being snowed out in 1982.
But I remember the "second" opener on April 16 was also postponed. Can you confirm
this?
Eric

Dear Eric,
You are correct. After the first three attempts to open the home season were thwarted
by snow and cold, the Sox hoped to finally play baseball in Chicago on April 16. This
time it was heavy rain, not snow and cold, that caused the game to be canceled. There
was slight hope for baseball that night when the sun briefly broke through the clouds
around 5 p.m., but then the heavens opened and torrents of rain soaked the field. The
White Sox huddled helplessly in the clubhouse for more than two hours before the
game was finally called. The Sox finally began their 1982 home season the following
day by sweeping the Baltimore Orioles in a double header.

Dear Tom,
I remember a tornado that swept through Bourbonnais when I was a small boy
around 1960. Do you have any details on that storm?
Tim Guimond, Evanston
Dear Tim,
The tornado that you remember was an F4 storm that killed one and injured 70
on April 17, 1963, as it carved out a nearly 70-mile long damage path from
near Essex in Kankakee County to Medaryville, Ind., northeast of Rensselaer.
Some of the damage in Illinois was rated close to F5 strength as several
homes literally vanished. In Bourbonnais, the twister smashed the five-story
stone administration building at Olivet Nazarene University, damaged the
village hall, tore up a trailer park and partially unroofed the Maternity
Church. Earlier, the same storm complex brought hail and high winds to the
Chicago area with golf-ball-sized hail reported in Hinsdale and wind gusts
to 55 m.p.h. at Aurora.

Dear Tom,
It rarely snows in May but almost always snows in April. Are there more
Aprils without snow or Mays with snow?
Bill White
Dear Bill,
You raised an intriguing question that we immediately put Chicago weather
historian Frank Wachowski to work on. Wachowski found only 12 snowless
Aprils since 1885, the most recent occurring in 2004. Over the same time
period Wachowski noted 31 Mays with at least a trace of snow on the books,
the last May snow event occurring four years ago on May 2, 2005. The vast
majority of the 31 Mays with snow were just brief, one-day trace events --
but there were a few significant snowfalls. In 1903 the city recorded 1.3
inches on May 2, and 2.2 inches of snow fell on May 1-2 in 1940, a storm
that brought as much as 5 inches of snow to northwest suburban areas from
Elgin to Marengo, and up to 1 inch to areas as far south as Joliet.

Dear Tom,
I recall a troublesome snowstorm in Chicago on April 2, 1975, but my husband is
positive it was at the end of March. I remember the date because it followed April Fool’s
Day. What do your records show?
-Mrs. L. Mazur

Dear Mrs. Mazur,
Chicago weather historian Frank Wachowski cracked open the record books, and he
confirms that your memory is correct. However, Wachowski commented
that "troublesome" is hardly a suitable description for the snowstorm that lashed the area
April 2, 1975. The storm put down 9.8 inches of heavy, wet snow that was mixed at
times with freezing rain and sleetand accompanied by howling northeast winds gusting
at 40 m.p.h. With temperatures mostly in the lower 30s, the snow stuck to everything
and power outages were widespread. It was officially the city's second-heaviest April
snowstorm.

Dear Tom,
Okay, so we have had 50 inches of snow this winter, but now it's April. Dare
I hope the snow is now just a memory?
Jerry Donohue
Dear Jerry,
The likelihood of snow grows slimmer with each passing day, but Chicago's
snowfall records suggest it's a bit too soon to dismiss the possibility of
snow. On average (1885-2008), Chicago's normal snow season (October through
April) delivers 36.8 inches, and 98 percent of it has come down by April 4.
That means two percent, or 0.7 inches, is yet to occur.
April has produced measurable snow (at least 0.1 inch) in 66 years out of
124, or 53 percent of the Aprils. One-tenth inch of snow isn't very much, so
we'll arbitrarily define a "significant snow" as 3 inches or more. In April,
the chance of a storm with at least that much snow is one in four, and it's
one in 20 for a storm of 6 inches or more.

Dear Tom,
Is it true that girls have less chance of getting struck by lightning than boys?
Melanie Timms (6th grade), St.Rita School, Rockford, IL

Dear Melanie,
Yes, it is true. Ronald Holle and Raul E. Lopez of the National Severe Storms Laboratory
and E. Brian Curran of the National Weather Service conducted a comprehensive study
of lightning deaths and injuries in the United States and they found that boys and men
are struck by lightning about fours times as often as girls and women. Their statistics
indicated that males account for 84 percent of lightning fatalities and 82 percent of
lightning injuries.

It's not because lightning prefers to strike males. Rather, a much greater proportion of
men have outdoor jobs that put them in jeopardy (construction workers, farmers,
linemen, emergency responders). The study also found that men engage in
miscellaneous outdoor activities more often than women.

Dear Tom,
Has Lake Michigan ever frozen over enough to enable travel from Michigan to Illinois?
Jack Brooks Orlando, Florida

Dear Jack,
Though constant wind and wave action combined with the vast reservoir of heat
contained in the lake prevents it from freezing completely, it has reached between
90-95 percent ice-coverage in this area's coldest winters including 1903-04, 1976-77
and 1978-79.

However, in very cold winters ice will form and bank up solidly along shore areas,
making it possible to travel that entire route over ice. An article sent to the Chicago
Tribune by Delicia Jane Lane in 1959 documents the severe cold that gripped the
Midwest in early 1899. "Lake Michigan froze as solidly as Lake Baikal. Horse drawn
cutters made trips to Michigan over the ice which remained firm until Feb. 22."