Recently in ASK TOM WHY Category

Dear Tom,  
During the full moon on Jan. 7, the moon was completely surrounded by a very large white circle. What was that?
Jim, Orland Park

Dear Jim,

You observed a halo, formed in this case when moonlight penetrated a thin layer of cirrostratus clouds -- high-level clouds, generally above 20 thousand feet, composed of ice crystals. In penetrating the cloud layer, moonlight was refracted through the ice crystals and also reflected from their surfaces.

Many kinds of halos are possible, depending on the type of ice crystals, the orientation of the crystal surfaces relative to the observer and the height above the horizon of the light source (the moon, in this case; halos can also form around the sun). Two kinds of halos predominate: a halo of 46 degrees (a circle with an angular radius of about 46 degrees, centered on the light source) and the more common halo of 22 degrees.
 

Dear Tom,
What is the latest date that Chicago's temperature dropped below zero?
--Richard L. Ettlinger, Highland Park

Dear Richard,
While the heart of Chicago's below-zero season runs from late December to mid-February, the city has logged subzero weather as early as Nov. 23, 1950, (minus 1) and as late as March 22, 1888, (minus 1). Subzero weather in March is rare having occurred only 15 times in 141 years dating back to 1871, with all but three of the occurrences falling between March 1 and March 8. During the final stages of the winter of 2001-02, when it appeared that the city was headed for its first winter without subzero temperatures since 1982-83, frigid air arrived in the wake of a 10-inch snowstorm March 1-3 and the mercury plunged to minus 7 on March 4 ---the season's lone subzero day.

Dear Tom,

A friend in North Pole, Alaska, (260 miles north of Anchorage), indicated the temperature was 51 degrees below zero and at the same time a weather observation at nearby Eielson Air Force Base reported light drizzle. Can drizzle occur at that temperature? Can that observation be correct?

--Marty Becker, Schaumburg

Dear Marty,

It was an error. Drizzle occurring at 51 degrees below zero should have been reported as freezing drizzle (tiny liquid droplets that freeze upon contact with exposed surfaces). The larger issue is this: Is drizzle even possible at that temperature? The answer, surprisingly, is yes. The surface tension that exists in tiny water droplets is so great that it can prevent the formation of ice crystals even when the droplet is cooled far below the usual freezing point of water (32 degrees) if the droplet is in a calm, undisturbed environment. However, 50 below is pushing the theoretical limit.

Dear Tom,

How soon can we expect our first 60-degree day of the season on average, and what is the earliest and latest date for such an occurrence?

--Nick Recchia, River Grove

Dear Nick,

Your question suggests that even in a winter as relatively mild as this one has been to date, anticipation is building for the return of spring warmth. A search for 60-degree days in Chicago winters reveals they can occur at any time from December through February, though not often. A computer sweep of the city's 141-year winter temperature data set, beginning on Jan. 1 in each year, reveals the average date for the first occurrence of a temperature of 60 degrees or higher is March 2.

But averages don't convey an accurate picture. There has been 60-degree day as early as Jan. 1 (65 degrees in 1876) and as late as April 23 (63 degrees in 1881).

Dear Tom,
What is the warmest Groundhog Day in Chicago history?
--Brandon Borbath, Gurnee

Dear Brandon,
With the help of Chicago climatologist Frank Wachowski, we combed the city's high temperatures for Groundhog Day and found the furry prognosticator has never emerged from his den on a truly warm day. Chicago's record high for Feb. 2 is just 51 degrees, recorded in 1992, the lowest record high for any day of the year. The city's next lowest record high is 55 degrees, established Dec. 26, 1971 and again on Jan. 30, 1988. However, the groundhog has encountered some frigid weather, most notably the day's record low of minus 16 in 1996. Last year, Groundhog Day marked the end of the city's crippling blizzard that brought 21.2 inches of snow, and the day featured falling temperatures, strong winds, and 6.6 inches of snow.

Dear Tom,
January is Chicago's coldest month, but is it always the coldest month every winter?
---James Proust

Dear James,
On average, January is Chicago's coldest month, but it's not necessarily the coldest month in any given winter. Long-term temperature averages at Midway Airport (1929-2011) rank January as the city's coldest month with 23.9 degrees followed by February at 27.9 degrees and December at 28.9. However, it's not often that the city's wildly variable weather adheres closely to those averages. More often than not, Chicago's month-to-month temperatures consist of large swings above and below the averages. In that vein, the data suggest it's premature to assume winter's coldest temperatures are behind us. February has been Chicago's coldest month in 18 of the past 83 winters, and March was the season's coldest in the winter of 1931-32.

Dear Tom,
How common is it to go an entire winter without a below zero temperature?
--Larry Voves, Orland Park

Dear Larry,
It's quite unusual. In the 141 winters dating to 1870-71, Chicago has logged only a dozen winters that failed to produce a subzero day, about 1 every 12 years. It's been nearly 30 years since the city last had a winter without below-zero temperatures (in 1982-83), but subzero cold definitely has been an infrequent visitor in recent winters. The winter of 2004-05 produced just one such day; there were only two in 2005-06, one in 2009-10 and four last winter. In contrast, Chicago's winter with the most subzero days was in 1884-85 when the mercury dropped into negative territory 25 times. Despite this winter's ongoing mild character, early February is the city's most frequent period for subzero readings, so we may still drop below zero this winter.

Dear Tom,
Since 1959, how many days below zero have been recorded in Chicago?
---Jim Nichols

Dear Jim,
Subzero temperatures are recorded in most Chicago winters, and since 1959 the mercury has dropped into negative territory 420 times, an average of about eight  per winter. During that period only the winters of 1959-60 and 1982-83 failed to produce at least one subzero day. Since 1959 the city has endured several bitter-cold months with the number of below-zero days reaching double digits. January 1963 leads the list with 15 subzero days, followed by January 1982 with 13, and January of 1977, 1979 and 1994 with 12 each. The city's last month that logged at least 10 subzero days was December 2000, with 10. This winter has failed to produce below-zero weather with the city's last subzero reading on Feb. 10, 2011, when the mercury plunged to minus 9.

Dear Tom,
The blizzard of 1967 was nasty. With all the advances in meteorology since then, would Chicago still be crippled the same?
--Jeffrey Kozinski, Mokena

Dear Jeffrey,
The 23.0-inch snowfall in Chicago on Jan. 26-27, 1967, shut the city down for nearly a week. Just an hour before the snow began, forecasts called only for 4 or more inches of snow. Last year's 21.6-inch blizzard on Jan. 31-Feb. 2 brought almost as much snow as the 1967 storm and its 70 mph winds were stronger than the 53 mph gusts recorded in 1967. Yet, the city was able to rebound quickly after some short-term problems, especially on Lake Shore Drive. The reason; with a slew of modern tools, forecasters were able to alert the city of the upcoming storm a week ahead of time, warning of as much as 2 feet of snow.

Dear Tom,

Chicago's normal daytime high has been stuck at 31 degrees for some time now. I thought it went down to 28 for the normal high at this time of year?

--Robert Calhoun

Dear Robert,

You are correct to remark that Chicago's normal daily high temperature has been at 31 degrees for some time -- since Jan. 2, to be exact, and it will remain there through Jan. 29. After Sunday, the normal daily high goes to 32 degrees and begins a gradual ascent to a peak of 85 degrees during the second week of July. Those normals are derived from a smoothed average of temperatures as recorded at O'Hare International Airport during the 30-year period of 1981-2010. The previous normal daily highs, from 1971-2000, ranged from 29 degrees to 84. Reflecting an ongoing warming trend, Chicago's new daily normals are generally 1 or 2 degrees higher than the previous normals.