ASK TOM: Has Chicago ever experienced a day with the same high and low temperature?

Dear Tom,

Has Chicago ever experienced a day with the same high and low temperature?

— Isaac VanDeBerg, Barrington
Dear Isaac,
A Chicago day with the same high and low is extremely rare, with just one occurrence since Chicago weather records began in 1870. The lone day was Feb. 6, 1942, with a high and low of 35 degrees. It was a raw, damp and windy day, with brisk east-northeast winds gusting up to 40 mph. Heavy rain fell much of the day, eventually mixing with snow in the afternoon and night, with the day’s precipitation totaling 1.98 inches. The string of steady temperatures actually extended for 30continuous hours, from 9 p.m. Feb. 5 to 3 a.m. Feb. 7.

In sharp contrast, the day with the largest range between the high and low was Feb. 8, 1900, with a 52-degree span, from a high of 62 to a low of 10.

ASK TOM: What is the best temperature for snow?

Dear Tom,

What is the best temperature for snow?

— Jon Rosecrans


Dear Jon,

Snow can occur only when water vapor is present in air at subfreezing temperatures, a mechanism exists to chill that air below its saturation temperature, and there are conditions that allow tiny ice crystals to grow into snowflakes that can fall to the ground.

Airborne ice crystals spring into existence when water vapor condenses (technically, sublimes) at subfreezing temperatures.

But because the amount of water vapor that can be present in air increases dramatically with increasing temperature, the greatest amount of snow is possible in any given situation when the temperature (at the level at which snow is forming) is as high as it can be for snow to occur: 32 degrees.

ASK TOM: The difference between “Normal” and “Average” temps

Dear Tom,
What is the difference between “average” and “normal” temperatures?
— James Byrne, Lansing

Dear James,
By international convention, daily high and low temperatures are defined as the maximum and minimum temperatures that occur in the 24-hour period, midnight to midnight, local standard time. An average daily temperature is the sum of those high and low readings, divided by two.
The average temperature over a longer period of time, say March 1-31, is the sum of the 31 daily highs and the 31 daily lows, divided by the number of summed values (62 in this case). A “normal” temperature at a location is also an average temperature, but it’s a special average — calculated over a precise 30-year period, currently Jan. 1, 1981 to Dec. 31, 2010. New normals will be calculated when data become available for 1991-2020.

ASK TOM WHY: When was the last time Chicago had measurable snow in May?

Dear Tom,

When was the last time Chicago had measurable snow in May?

Mary, Geneva

Dear Mary,

Measurable snowfall in May is very rare in Chicago having occurred in only eight years dating back to 1885. Chicago climatologist Frank Wachowski reported that the most recent occurrences were 0.5 inches on May 6, 1989 and 0.1 inches on May 2, 1976. Most May snowfalls are minor, amounting to just a few tenths of an inch, but there have been a few exceptions. In 1907, 1.3 inches of snow fell on May 3, and in 1940 the Chicago area was whitened by a 2.2 inch snowfall on May 1-2 at the official downtown location while Midway Airport received a slushy 3.7 inches and some suburban areas measured as much as 5 inches. The snow quickly melted as temperatures rebounded into the 70s by May 5.

ASK TOM: Tornado almost hit President Gerald Ford’s motorcade in Chicago?

Dear Tom,

My parents told me about a tornado that almost hit President Gerald Ford’s motorcade in Chicago. Did that really happen?

— Melanie O., Island Lake

Dear Melanie,

It sure did. During the early afternoon hours of March 12, 1976, an F-2 tornado cut a 17-mile path of destruction across the Chicago area from Northlake to Wilmette. The twister, which killed two and injured 66, heavily damaged Chicago’s Northwest Side, including areas around O’Hare Airport. The storm made national headlines when it passed within one-quarter of a mile of the motorcade of President Gerald Ford, who was in town campaigning for the upcoming presidential election. Ford was back in town a few days later, this time assessing the damage and declaring the region a federal disaster area.

ASK TOM WHY: What is the lowest the humidity has ever been in Chicago and the U.S.?

Dear Tom,

I have noticed Chicago’s relative humidity rarely drops below 25 percent. What is the lowest it has ever been in Chicago? In the United States?

Tim Coates, Chicago

Dear Tim,

Low relative humidity values are one aspect of weather records that are not well documented. On the high side, humidity readings of 100 percent are common here and elsewhere, often occurring when it is foggy. In contrast, low values of relative humidity are rare, confined almost exclusively to the hottest afternoons in the world’s deserts. The lowest relative humidity value in the United States seems to be two percent, and has been recorded in Las Vegas, Nev., Phoenix and Yuma, Ariz.. Chicago’s lowest relative humidity: 13 percent, measured on three occasions: 5/10/1934, 4/11/1956 and 4/8/1971.

Dear Tom, 

How often has Mother Nature decided to play an April Fools’ Day joke with snow in the Chicago area? When did it last happen?

— Mark Orloff and Stephen Verhaeren, Palos Park

Dear Mark and Stephen,

It hasn’t happened often, but Chicagoans have dealt with 10 measurable April 1 snowfalls since 1885. Most recently, 0.9 inches fell in 2002, and a more significant 3.7 inches fell in 1993. Nature’s biggest April Fools’ prank occurred in 1970, when the city was hammered by a 10.7-inch snowstorm on April 1-2. Accompanied by thunder and lightning, the heavy, wet snow was piled into large drifts by northeast winds gusting to 52 mph. The weight of the snow caused much damage to trees, shrubs and power lines. The storm also forced the relocation of the then-annual preseason Cubs-White Sox City Series to Tulsa, Okla.

ASK TOM: What are the official rules for measuring snowfall?

Dear Tom,

What are the official rules for measuring snowfall?

— C. Michael Becker

Dear C. Michael,

Frank Wachowski, who has been measuring Chicago’s snowfall for nearly 60 years, provides this information:

Two measurements are taken: snowfall and snow depth.

Snowfall is measured on a snow board, a 2-foot square piece of plywood painted white to reflect heat, with a ruler to the nearest 10th of an inch. When falling snow does not accumulate, it’s reported as “trace — melted as it fell,” and if no measurable snow (0.1inches or more) occurs during the calendar day, a trace of snow is entered in the record books.

Snow depth is measured to the nearest inch. If drifting is a problem, many measurements from high spots to low spots must be averaged to obtain a representative reading

ASK TOM: What is black ice?

Dear Tom,

What is black ice?

— Jack Anderson, Glen Ellyn

Dear Jack, Black ice can be a major problem on cold mornings during snowmelt, as the moisture left from the melting snow refreezes. The ice forms a nearly invisible, thin layer on roads or sidewalks because it takes on the color of the underlying pavement, which on asphalt surfaces appears black. Black ice can also form from freezing drizzle, wind-blown snow or frozen condensation. It is dangerous because unsuspecting motorists or pedestrians traveling on dry pavement fail to realize they are on an icy surface and can fall or lose control of their vehicles. The term originally was used to describe a thin layer of ice on a pond or lake, but in recent years its use to describe the road hazard has become much more common.

ASK TOM: Recording seasonal snowfall in Chicago

Dear Tom,

I say that recording snow as part of the seasonal snowfall ends when we stop having snow, but my husband says it ends on the first day of spring. Who’s correct?

— Chris Schulz, Naperville

Dear Chris,

You are the winner on this one. Seasonal snowfall totals are the sum of all measurable snowfalls (0.1 inches or more) from the first snow in the autumn to the last snow in the spring. Over the 129-year span of the city’s snowfall records, the earliest measurable snow occurred on Oct 12, 2006, (0.3 inches) and the latest on May 11, 1966, (0.2 inches). This season the first measurable snow was logged Nov. 11, when 0.4 inches fell. Typically, Chicago sees its last measurable snow around April 3, but given our persistent chill this spring, it may be a while before we are declare the end of the 2013-14 snow season.