Dear Tom,
What is the most number of consecutive days of measurable snow we have ever had in
Chicago?
Paul Sarewich, Chicago
Dear Paul,
The answer is nine days, from January 29 through February 6 of 1902. That's the result
of a computer search of Chicago's daily snowfall records from the winter season of
1884-85 to the present. Measurable snow is defined as 0.1 inch or more, and every day
in that nine-day period nearly 107 years ago received at least that much. "Nine
consecutive days of snow" conjures up an image of a city paralyzed by an unrelenting
nine-day snowstorm, but don't let the statistics deceive you. In this instance, all that
occurred were periods of light snow and snow flurries. Daily snow totals Jan. 9, 1902,
onward were (in inches): 0.2, 0.6, 1.0, 0.1, 0.8, 0.2, 0.6, 0.3 and 1.2 for a nine-day total
of only 5.0 inches. The highest temperature through the period was 27 degrees, so the
snow did not melt.
ASK TOM WHY: December 2008 Archives
Dear Tom,
Recently your forecast said,"A frigid start to the day as temperatures begin in the single
digits," but then you show the low temperature as 18 degrees. My thermometer showed
8 degrees! How do you figure the low temperature?
Peggy Gudbrandsen
Dear Peggy,
We always present temperatures on this weather page in chronological order. The
expected high temperature on a given day appears first, followed by the expected low
temperature during the following nighttime period.
The situation that you described, "A frigid start to the day ...," refers to early-morning
temperatures expected before that day's forecast afternoon high temperature. The low
temperature is the predicted minimum temperature during the following night.
Minimum temperatures usually occur in the early morning, approximately at daybreak,
of the next calendar day.
Dear Tom,
With all the snow and cold we are getting, how can the advocates of global warming
justify their arguments?
Paul Nadick, Round Lake, Ill.
Dear Paul,
Actually, no justification is necessary because the trend of global temperatures speaks
for itself. Since the end of the last glacial maximum about 15,000 years ago, global
temperatures have risen approximately 11 degrees. The rate of warming accelerated
greatly with the onset of the Industrial Revolution, and the Earth's average surface
temperature has risen, conservatively, about 1.5 degrees since the mid 1800s.
The grand sweep of global temperatures is upward, but many factors -- some
understood, some not -- can temporarily reverse the rising trend. For example, global
temperatures declined from the 1940s through the 1970s. It is a probably a mistake to
interpret a harsh winter as a permanent reversal of the trend of rising global
temperatures.
Dear Tom,
What is the usual range of temperatures necessary for it to snow?
Beola Lenard, Bettie S., Patricia L., Evergreen Park
Dear Beola, Bettie and Patricia,
Ice crystals (that eventually clump into snowflakes) form in clouds only
when water vapor condenses in a below-freezing temperature environment.
Condensation in above-freezing temperatures results in water droplets and,
ultimately, rain. Air temperatures at ground level are sometimes a little
above freezing (but rarely above 40 degrees) when it is snowing because
snowflakes, falling from subfreezing air above, do not have time to melt
before they make it to the ground.
There really is no lower temperature limit at which snow can occur, though
Chicago's biggest snowstorms usually occur with readings in the 20s. Nearly
an inch of snow was measured at O'Hare Airport on Jan. 10, 1982, with the
temperature around 15 degrees below zero.
Dear Tom,
How can it rain at 26 degrees?
Jim O’Shea, Oak Lawn
Dear Jim,
The form in which precipitation reaches the ground is not determined by the surface
temperature, but by the temperature profile from the ground to cloud-level. Most winter
precipitation begins as snow. If it encounters a deep enough layer of above-freezing air as
it falls to Earth, it melts and becomes rain. The rain then refreezes when it makes contact
with the below-freezing ground and the result is a glaze-producing freezing rain. If the
rain falls through a thick enough layer of belowfreezing air before it hits the ground, it
refreezes into ice pellets (sleet). Forecasting winter precipitation types is difficult,
especially in the Chicago area, a region that is frequently close to the dividing line
between rain and snow.
Dear Tom,
Another active hurricane season has ended. As you have said many times,
hurricanes are fueled by warm, moist air, so why can't they be stopped by
chilling the eye with ice?
Matthew Sabin
Dear Matthew,
Dr. Chris Lansdea, Director of the National Hurricane Center's Research
Division, has written extensively on the subject of hurricane modification.
He states categorically that "... Hurricane modification by any means is an
exercise in futility and impracticality. Consider the scale of what we are
talking about. The critical region in the hurricane for energy transfer is
under or near the eye wall," an area of about 2,000 square miles. Allowing
for storm movement and uncertainty in its track, the "cool patch" expands to
31,000 square miles. That's 55 percent of the area of Illinois, and it is
simply impossible to chill such a huge area.
Dear Tom,
I am a retired forecaster from the old Springfield, Ill., National Weather
Service office. Who originated the term, "Alberta clipper," and when?
Fred Snowden, Springfield, Ill.
Dear Fred,
"Alberta clipper" is a colorful term that appeared in meteorological jargon
in the early 1970s, courtesy of Rheinhart Harms. At that time, Harms was the
Meteorologist-in-Charge of the Milwaukee, Wis., Weather Bureau office, and
he authored a research paper titled, "Snow Forecasting for Southeastern
Wisconsin" in 1970.
Harms noted that a distinctive class of snow-producing low pressure systems
originated in or near Alberta Province, Canada, and moved rapidly southeast
across the Midwest, producing a belt of low water-content snow. He called
those storms "Alberta clippers," and staff members of the Milwaukee weather
office began using the term in the early 1970s.
Dear Tom,
Of the ten snowiest winters in Chicago history, how many had a white Christmas?
-Lt. Pat Byrne, Hoffman Estates, Ill.
Dear Pat,
A snowy winter in Chicago does not guarantee a white Christmas (at least one inch of
snow on the ground), but it does increase the odds. Historically, only four Christmases
in 10 have met the one-inch criterion
But Chicago weather historian Frank Wachowski found that six of the city's 10 snowiest
winters had white Christmases. Last winter was Chicago's eighth snowiest with 60.3
inches, yet the ground was bare on Christmas morning.
Chicago's snowiest winter (1978-79 with 89.7 inches) and the city's least-snowy winter
(1920-21 with 9.8 inches), barely qualified, each with only a one-inch Christmas
morning snow cover.
Dear Mr. Skilling,
Io, one of Jupiter's moons, has volcanoes but my friend Terry says I am wrong and he
won't listen to me. He says only Earth has volcanoes.
Robert Klyn, age 10
Dear Robert,
Your information is correct and we here in the WGN weather center congratulate you on
your knowledge of astrogeology.
Astronomers believe Io is the most volcanic body in the Solar System. Io revolves so
close to Jupiter (the largest planet in the Solar System) that Jupiter's powerful
gravitational field causes Io to bend and flex, heating its interior much like a paper clip
heats up when it is repeatedly flexed. That heat fuels massive volcanic eruptions on Io's
surface. NASA's Galileo spacecraft captured many high-resolution images of Io's
volcanoes in 1999 and 2000, and indicated lava at temperatures well above 2,600
degrees.
Earth's hottest lava: about 2,240 degrees.
Dear Tom,
Why do lowest average temperatures of the year occur in the weeks after the winter
solstice? It would seem to make sense that the most extreme temperatures would occur
when the sun is at its lowest angle in the sky.
Tim Arvidson
Dear Tim,
Chicago's normal high on Dec. 21 (the winter solstice) is 33 degrees; on Jan. 15, 29
degrees. On average, the winter's lowest temperatures occur about three weeks after
the winter solstice, and there are three good reasons why. First, the Earth's surface goes
on losing more heat than it gains long after the solstice. Second, the far north (the
source of our coldest air) receives no heat from the sun into mid January, and
temperatures there are still falling. Finally, the average continental snow field continues
to expand into early February. Snow surfaces effectively radiate heat into space and
generate cold air masses.
Dear Tom,
I've seen neighborhood children eating snow. Is that safe?
James Anderson
Dear James,
It's not a good idea because snow contains potentially harmful airborne
pollutants. It's not that snow is immediately toxic, but it can contain
chemicals that you don't want to put into your body.
Here are the opinions of two experts. University of Toronto environmental
chemist Dr. Frank Wania reports that the atmosphere is exceedingly efficient
at transporting pollutants -- so efficient, in fact, that industrial
pollutants released into the atmosphere in India could be found in snow in
northern Canada only five days later. Argonne National Laboratory's Dr. Jeff
Gaffney is more specific. He says snowflakes can contain anything that
floats in the air: the chemicals that fall in acid rain, bacteria, sulfates,
nitrates and even lead from areas in the world that still burn leaded
gasoline.
Dear Tom,
What is the most unusual question that you have ever answered?
-Jim Oosner
Dear Jim,
This one from Dave Nelson in Evanston stands out: While enjoying a bright blue sky the
other day with puffy white cumulus clouds, I wondered: Am I looking at a sky and cloud
scape that a dinosaur might have observed 100 million years ago? The reply: Dinosaurs
lived from 245 million to 65 million years ago, and the same kinds of cumulus clouds
that you observe in our present-day summer sky assuredly populated the dinosaurs’
sky as well. The laws of atmospheric physics are immutable. Even assuming the
gaseous composition of the Earth's atmosphere was somewhat different during the age
of the dinosaurs, the nature of water vapor and the atmospheric processes that produce
cumulus clouds have not changed with time.
Dear Tom,
When people claim, "Back in my time, the winters were more severe," it is
usually a case of the weather being exaggerated in their memories. Well, I
grew up in the 1980s and winters were more severe then, but a neighbor who
moved here from Seattle thinks I'm wrong when I say I can remember many days
with temperatures 20 below zero. Help me convince him.
John Potempka, Chicago
Dear John,
You are absolutely correct. A computer sweep of the entire data set of
Chicago's official temperatures from Nov. 1, 1870, to the present supports
your recollections. In that period, Chicago recorded 15 days on which the
minimum temperature sank to 20 degrees below zero or lower. Five of those
days occurred before 1900 and nine of them in the winters from 1982 through
1985. The most recent occurrence was 21 below on Jan. 18, 1994.
Dear Tom,
Where would you have to go in the United States to be absolutely sure of a
white Christmas?
Kenneth Gutman
Dear Kenneth,
"Absolutely sure" is a powerful term, and with that in mind, Canadian
meteorologist Dr. George Kimble has written, with some hyperbole, "The
regrettable truth of the matter is that over fully three-quarters of the
United States of America the odds on your waking up to a white Christmas are
so long as to be incapable of calculation."
Historically, Chicagoans can expect at least one inch of snow on the ground
on Christmas morning on four Christmases out of 10. Elsewhere in the lower
48 states, a 100 percent certainty of snow-covered ground on Dec. 25 exists
only on our western mountains, on a few of the Appalachian peaks (such as
Mount Washington) in New England, and in the Lake Superior snow belt of
northern Wisconsin and Upper Michigan.
Dear Tom,
I know that sometimes the smaller Great Lakes like Erie freeze over
completely, but has that ever happened to Michigan, Superior or Huron?
Ed Berling, Lockport
Dear Ed,
Environment Canada and the U.S. National Oceanic and Atmospheric
Administration monitor ice development on the Great Lakes during the winter.
Their data indicate that three of the Great Lakes (Superior, Huron and Erie)
have totally frozen over in a few of the very harshest winters since 1900,
but Michigan and Ontario have never achieved complete ice coverage.
Ice development usually begins on all five lakes in January and attains its
maximum extent in late February or early March. Ice covered 90 to 95 percent
of Lakes Michigan and Ontario in the severe winters of 1903-04, 1976-77 and
1978-79, the maximum coverage ever reported for those lakes.
Dear Tom,
Recently, it was snowing at my place but the radar showed only green
(indicating all rain) over my area. Why would the radar become confused like
that?
Randy Walker, Portage, Ind.
Dear Randy,
Displays of precipitation "echoes" from National Weather Service Doppler
radars are artificially colorized images of the strength of the radar energy
that is reflected from precipitation particles back to the radar. Echo
strength is closely related to precipitation intensity.
The radars cannot determine precipitation type (such as rain or snow), but
certain kinds of precipitation (like hail) produce characteristic radar
images from which its type can sometimes be deduced.
Artificially colorized displays of precipitation type (green for rain, pink
for "ice" and white or blue for snow) are computer-generated, taking into
consideration weather observations and other non-radar data.
Dear Tom,
Growing up in Chicago in the 1980s, I remember some brutally cold mornings
with the temperature lower than minus 20. When was the last time the city
was that cold?
Greg Patterson
Dear Greg,
The city's last encounter with truly cold weather was nearly 15 years ago
back on Jan. 18, 1994, when the mercury dipped to 21 below zero and the high
for the day was only minus 11. Your frigid memories cover some of Chicago's
historically coldest days. Who can ever forget the "frozen" Christmas of
1983 when temperatures plunged to minus 25 on Christmas Eve and 17 below on
Christmas Day? It was even colder in January 1985 when Chicago recorded its
all-time low of 27 degrees below zero on Jan. 20. In recent years, the
city's arctic blasts have been more modest. Since 2000 the lowest official
recorded temperature here has been 10 below zero on Feb. 5, 2007.
Dear Tom,
Would you please explain what constitutes a blizzard versus a snowstorm?
Robin Rembelski, Des Plaines
Dear Robin,
Despite the fact that the National Weather Service has strict guidelines for
qualifying a storm as a blizzard, the term is often erroneously applied to
snowstorms that don't fit the bill. The two key factors in categorizing a
storm as a blizzard are wind and visibility. Winds need to reach sustained
speeds or frequent gusts of 35 m.p.h. or greater, and visibilities have to
be repeatedly reduced to 1/4 mile or less by falling and/or blowing snow for
a period of at least three hours. A blizzard can actually occur with clear
skies if the visibility reduction is caused by blowing snow. While there are
no temperature requirements for a blizzard, the combination of strong winds
and below-freezing temperatures will drop windchills at least into the
single digits.
Dear Tom,
When was Chicago's warmest Christmas?
Adam Bednarek, Dyer, Ind.
Dear Adam,
It was nearly 26 years ago in 1982 when Chicago experienced a Christmas that
was warmer than most Easters. The afternoon temperature peaked at a balmy 64
degrees, twice the day's normal 32-degree high. While most Chicagoans
celebrated a traditional holiday, some people were washing their cars,
barbequing and even playing golf. Excited children got an unexpected chance
to ride their new bikes and scooters outdoors. The very next year was
payback time as the city shivered through its coldest Christmas. After a
frigid Christmas Eve when the mercury dipped to minus 25, Christmas Day 1983
dawned with a low of 17 below and could only muster a high of minus 5. The
back-to-back Christmases had an incredible temperature spread of 81 degrees,
a true testament to Chicago's vigorous and volatile climate.
Dear Tom,
I remember a frigid period in the 1980s when temperatures were below zero
for nearly two weeks. Can you confirm my recollections?
Todd Theurer
Dear Todd,
Chicago winters in the late 1970s and early 1980s were infamous for severe
cold, but we believe you are referring to the benchmark 1982 cold spell that
besieged Chicago Jan. 7-17. This 11-day period was marked by unprecedented
cold, ground blizzards and whiteouts, and dangerous windchills (based on the
old windchill formula) to minus 80 degrees. Temperatures fell below zero on
10 of the 11 days with the lowest reading crashing to minus 26 degrees on
Jan. 10. It was the coldest 11-day period in Chicago's history, averaging
0.6 degrees below zero. During this brutal cold wave, the city established
five new record minimums and logged four days of minus 19 or lower.
Dear Tom,
People are talking about how cold it was at last Sunday's Bears game with temperatures
in the high teens. I remember a much colder game against Green Bay in December 1982.
As I right?
Vince Kerner
Dear Vince,
Not quite. The Bears and Packers did play in frigid conditions in Chicago in the early
1980s. But it was on Dec. 18, 1983 with the Bears winning 23-21. That game
goes down as the second coldest Bears home game on record with the game-time
temperature near 5 degrees. The Bears coldest home game was played at Wrigley Field
on Dec. 16, 1951 against the Chicago Cardinals with the mercury peaking at just 2
degrees. Cold weather does seem to give the Bears a decided advantage. We conducted
an in-house study focusing on home games played with temperatures in the 20s or
lower that showed the Bears winning more than 60 percent of them, giving credence to
the term "Bear weather."
Dear Tom,
As a second grader in 1965 I vividly remember going to bed on Christmas Eve with the
ground bare and waking to a white Christmas. Can you check this out for me?
-Eric Field
Dear Eric,
Your recall is perfect. Christmas Eve, 1965, was mild and rainy with a high of 51
degrees. In fact it was Chicago's wettest Christmas Eve, with precipitation totaling 2.61
inches, accompanied by thunder and flooding. However, colder air moved in during the
evening as a cold front passed through the area and temperatures plunged into the 30s.
Rain turned to sleet and then snow with half an inch falling before midnight. At 6 a.m.
on Christmas, 2 inches of snow covered the ground at the then official site at Midway
Airport, and 1965 was officially declared a white Christmas. The snow cover remained
through Dec. 29 then vanished as the mercury climbed to 56 degrees on Dec. 30 and to
60 degrees on New Year's Eve.
Dear Mr. Skilling,
Your recent graph of Chicago's winter temperatures reminded me of the harsh winters
that Chicago had in my childhood years from 1975 to 1985. Because of my name, I took
a lot of ribbing. How cold were those winters?
Jeff Winters, Chicago
Dear Jeff,
The answer will surprise you. We averaged Chicago's winter temperatures (December
through February) for each of the 11 years in the period that you mentioned,
1975-1985, and arrived at 23.0 degrees.
A similar averaging calculation for every set of 11 consecutive winters from 1870 to the
present yielded this startling fact: The 11 winters from 1975-76 through 1985-86 --
the period that constituted your childhood years -- were the absolute coldest in
Chicago weather history. No other period of 11 consecutive Chicago winters was
colder. The second coldest: 1976-77 through 1986-87 with 23.1 degrees.
Dear Tom,
Why doesn't the shortest day of the year have the earliest sunset?
James Stahr Sr. Wheaton
Dear James,
This year, Chicago's earliest sunset occurred on Dec. 8 at 4:19:46 p.m. Monday.
By the winter solstice Dec. 21, sunset will occur several minutes later, with the sun
dipping below the horizon at 4:23 p.m.
Despite this apparent inconsistency, the shortest amount of daylight does occur on the
winter solstice because of a later sunrise. Astronomer Dan Joyce from Triton College's
Cernan Space Center explains these discrepancies are related to the eccentricity of the
Earth's orbit around the sun. Joyce also points out that if we define afternoon to be the
time from solar noon, the moment of the sun's highest ascent each day (11:43 a.m. on
Dec. 21) to sunset, then the year's shortest afternoon does occur on Dec. 21.
Dear Tom,
I am a big fan of winter temperature records. When did Chicago have its
highest and lowest December temperatures?
Maximino Garza, Jr., Chicago
Dear Maximino,
By any standard, December is a genuinely wintry month in Chicago. It's the
city's third-coldest month (behind January and February), but it also has
surprisingly mild moments.
In 138 years of records dating from 1870, December temperature extremes here
have ranged 96 degrees from an all-time high of 71 degrees (Dec. 3, 1970,
and Dec. 2, 1982) to 25 below zero (Dec. 24, 1983 -- Christmas Eve).
Averaging 43.4 degrees, December 1877, was the city's mildest, and many
Chicagoans will remember the bitter December of 1983, the coldest, with an
average of 14.3 degrees. Incredibly, each day in December 1983, when
averaged through the month, was 29.1 degrees colder than each day in
December 1877.
Dear Tom,
You recently said the world’s wettest spot is in South America.
Where was that, and what about the driest spot?
Jim Lottman
Dear Jim,
South America is host to the world’s wettest and driest places: Lloro, Colombia, and
Arica, Chile. Located 440 miles north of the Equator, Llororeceives an annual deluge of
523.60 inches of rain (14.4 times Chicago’s average of 36.27 inches), and that makes it
the world’s wettest spot. Prevailing west winds bathe Lloro in moist tropical air from the
Pacific Ocean. The world’s driest location, the Atacama Desert extending 975 miles
along Chile’s Pacific Coast. Some locations have never recorded rain, though fog is
common. Arica, 1,500 miles south of the Equator, lies in the northern part of the desert
and registers annual rainfall of 0.02 inches, though some years arerainless.
Dear Tom,
I remember as a child my dad telling me that it had to be above freezing in
order for it to snow. I'd like to trust his wisdom, but this seems wrong.
Was he right?
Alycia Pirc
Dear Alycia,
Snow can occur at temperatures above and below freezing (32 degrees), but it
forms in the clouds only in a below-freezing temperature environment.
Clouds develop when airborne water vapor condenses into visible particles --
water droplets when condensation occurs at or above 32 degrees and ice
crystals below 32 degrees. When cloud particles grow too large and heavy to
be supported by currents of rising air in the clouds, they fall to the
ground as precipitation (rain or snow).
If air at ground level is above freezing (but generally not warmer than
about 40 degrees) and the warm layer is not too deep, snowflakes falling out
of sub-32 degree air above can make it to the ground without melting.
Dear Tom,
We're having a discussion here at work regarding winter humidity versus summer
humidity. Tom (at work) insists there is more humidity in the winter than in the
summer. The rest of us question this. Help!
Sharon Plucinski
Dear Sharon,
Relative humidity (RH) is the percent saturation of air. In Chicago's low winter
temperatures, the RH is generally quite high, averaging 72 percent in January, because
as the temperature drops, so does the amount of moisture the air can hold. A very
small amount of water at low temperatures may come close to saturating the air and
yield a high RH.
The RH is usually lower in Chicago's warm summer temperatures, averaging 68 percent
in July, because much more water is necessary to give high RH values. However, the
average amount of water in Chicago's July air is seven times greater than in its January
air.
Dear Tom,
I love sunshine and Chicago's never-ending cloudy days of winter depress me. Where is
the best place to get my "sunshine-fix"?
Barbara Allen
Dear Barbara,
Millions of Americans suffer to some extent from seasonal affective disorder (SAD), a
form of depression associated with the reduction of daylight that occurs during the
winter months. Often called the "winter blues", this type of depression is much more
common at higher latitudes where long dark nights and prolonged strings of cloudy
days are most prevalent. December is Chicago's cloudiest month, typically logging only
39 percent of the possible sunshine in a month where daylight is limited to a little more
than 9 hours each day. Areas receiving a great deal of winter sunshine include Arizona,
Nevada, Florida, Mexico and the Caribbean which makes them popular winter vacation
destinations.
Dear Tom,
What is the smallest range of temperatures in a single day in Chicago?
Frank Fleming, Chicago
Dear Frank,
Chicago's weather is always in a state of flux and rarely do temperatures hover at
exactly the same value for more than two or three hours. That's the reality we live with.
On very rare occasions, though, when the progression of weather systems across the
Midwest has greatly slowed and Chicago finds itself with high humidity, a foggy and
densely overcast sky and practically no wind, the temperature can remain stationary.
The answer to your question is, surprisingly, zero degrees of temperature change.
Chicago's temperature sat at 35 degrees for a record 30 hours from 9 p.m., Feb. 5,
1942, through Feb. 6, to 3 a.m. on the 7th. It happened on two other days as well,
though not for 30 hours: March 24, 1891 (32 degrees) and March 13, 1878 (44 degrees).
Dear Tom,
As I look out at [Sunday's] snowfall, I noted that most of it melted as soon as it hit the
ground. How does weather observer Frank Wachowski measure the snowfall with any
accuracy?
William J Hopkinson, Maywood
Dear William,
Snow depth measurements are based strictly on accumulated snowfall. When falling
snow melts upon contact with the ground, snow depth is reported as "Trace -- melted
as it fell" and, if no measurable snow (0.1 inch or more) occurs during the calendar day,
a trace of snow enters the record books for that day.
Snow accumulation is measured on a designated surface, usually a snow board -- a
slab of wood, painted white, and placed where snow accumulation is representative of
the area. The water content of the snow (and all frozen precipitation such as sleet and
hail) is also measured and recorded.
