Ask Tom Why

Dear Tom,

My wife and I just returned from our winter home in Gilbert, Arizona. One day the humidity was 1 percent. What’s Chicago’s lowest humidity? And the world’s?

--Bob Hansen, Lindenhurst

Dear Bob,

Chicago climatologist Frank Wachowski reports that the city’s all-time low relative humidity is 13 percent, recorded on three occasions. It first occurred during a dust storm on May 10, 1934, and was tied on April 11, 1956, and April 8, 1971. Chicago’s low humidity episodes generally occur on warm, windy afternoons when dry air aloft can easily mix down to the surface. For the record, one of the world’s lowest humidities was recorded Oct. 24, 2006, in Coober Pedy, Australia, when a temperature of 93 and a dew point of minus 23 produced a relative humidity of less than 1 percent.

Dear Tom,

 
What is the distinction between humidity and dew point?  Thank you.

 
Thanks,

Arlene Steinhoff

Dear Arlene,

 
The relative humidity is determined by a relationship between the air temperature and the dew point. Only a certain maximum amount of water vapor can be present in the air at a given temperature and any surplus will condense out as dew, frost, clouds or fog. The dew point is that saturation or condensation temperature. Relative humidity is the ratio, expressed as a percentage, of the water vapor present to that maximum possible amount. When the air temperature equals the dew point, the relative humidity is 100 percent. The warmer the air, the greater its capacity for water vapor. Thus, the relative humidity increases as the temperature falls toward the dew point and decreases as it rises. That's why humidity usually peaks around sunrise and reaches its lowest value in the late afternoon.

Dear Tom,

I live in the country, and it’s always very windy here, except at night. The wind usually quits at dusk, and by night it’s completely still. Why does that happen?
— Ashley Leutloff, Chebanse, Ill.

Dear Ashley,

Sun-induced heating during the day disappears at dusk, and, if the weather is not stormy, winds fade to calm or near calm around sunset. Nighttime cooling sets up a temperature inversion: Cooler, denser air collects near the ground, and milder temperatures prevail above. Such inversions move large-scale organized winds (that reach the ground during the day) away from the surface. Those winds continue to blow through the night several hundred feet aloft. When sunlight returns in the morning, the inversion dissipates after a few hours of heating. Large-scale organized winds return to the surface.

Dear Tom,

I recall a weather event in either the late 1960s or early '70s that had some people thinking it was the end of the world. It got as dark outside as if it were midnight. Can you help?

— Richard Murphy, Chicago

 
Dear Richard,

While it's hard to pinpoint the exact event you remember, Chicago climatologist Frank Wachowski used your exact words when he described the eerily dark conditions before the deadly F4 Oak Lawn tornado struck about 5:20 p.m. April 21, 1967, producing extensive damage and killing 33 people. The cumulonimbus clouds that produce severe weather often tower 11 or 12 miles into the atmosphere and are very effective at blocking sunlight. Virtually no light can penetrate such a deep cloud layer, giving approaching severe storms their uniquely dark and ominous appearance.

Dear Tom,

Do you have any information about the ice tsunami that just occurred in Minnesota?

-- Paul Crimo, Highwood

Dear Paul,

“Ice tsunami” is a term coined by news media to describe an ice event, more correctly called an “ice heave,” that occurred about May 10 on Mille Lacs, a large, shallow lake in north-central Minnesota. The winter’s ice cover on Mille Lacs was breaking up when strong, persistent northwest winds pushed floating ice onto the southeast shore.
Ice jammed onto the shore and, as northwest winds continued, loose ice pushed across the lake’s shore and a few hundred feet landward. The leading edge of that ice field consisted of a slow-moving “ice wall” 1 to 3 feet in height, advancing a foot or two per minute, until it jammed against lakefront houses, doing extensive damage.

Dear Tom,
Chicago’s temperature was in the 80s on May 1 and in the 40s the next day. Was that temperature drop a record? '

— Al Jehl

Dear Al,

Chicago’s temperature plunged 42 degrees in the situation that you referred to, from an afternoon high of 84 degrees on May 1 to a nighttime low of 42 degrees on the 2nd. While noteworthy, a drop of that magnitude is not without precedent here. A computer scan of the city’s official temperature data, 1871-2012, reveals 298 occurrences of declines of at least 42 degrees from the high temperature on one day to the low on the next. That’s an average of two such events per year.

Chicago’s all-time greatest decline, a numbing drop of 61 degrees, took place November 11-12, 1911, when arctic air blasted into the city and sent temperatures crashing from 74 degrees to 13.

Dear Tom,

I remember some very heavy rainfall Aug. 13, 1987, when we were moving from Joliet to Naperville, with lots of flooding in DuPage County. Can you provide details?
— Nate
Dear Nate,
You are recalling one of the Chicago area’s benchmark rainfall events, the Aug. 13-14, 1987, deluge that officially brought 9.35 inches of rainfall to Chicago in just under 16 hours.
The O’Hare area was hardest hit, with floodwaters cutting off access to the airport for nearly 24 hours.
The core of heaviest rain extended west from the O’Hare area just north of the I-88 corridor into north central Illinois. The storm claimed three lives and flood damage totaled nearly a quarter billion dollars.
August 1987 — with 17.10 inches of rain — claims the title of the city’s all-time wettest month.

Dear Tom,

Is there any place in the United States (excluding Hawaii) that has the best weather? Certainly not Chicago!

Jake Sievers

Dear Jake,

You pose a subjective question. Some people like snow measured in feet; for others, it can't get too hot. However, for the weather attributes that seem most enjoyable to most people, it's hard to beat coastal southern California: moderate temperatures year around; plenty of sunshine (at least inland a few miles, away from coastal fog); no snow and little rain. By those criteria, San Diego experiences an ideal climate. Moderated by mild air from the Pacific Ocean, its normal daily high temperatures run from 66 degrees in January to 76 degrees in July -- only a 10-degree annual range. Compare Chicago's annual range of 54 degrees (from 31 degrees in January to 85 in July).

Dear Tom,
The weather on a smartphone app that I use reported “Fair” conditions today. What does that mean exactly?
--- John Mannos, Andersonville

Dear John,

Meteorologists are always searching for a variety of terms to use in forecasts to best convey their idea of the expected weather to the user while avoiding repetition of the same descriptors.

“Fair” is used to describe benign and generally pleasant weather conditions. It implies no precipitation, limited low-level cloud cover, good visibilities and light winds.

Many forecasters rely on a forecast of “fair” for the hours of darkness when thin, high-level cirrus clouds are present; the stars and planets are still visible, but the presence of these high clouds precludes the use of “clear.”

Dear Tom,

Why does the sun make me sneeze?

Julianna Mariotti (age 4), South Elgin

Dear Julianna,

Perhaps as many as two or three people out of ten experience the same thing that affects you, and it's called "sun sneezing." The technical name is "photic sneeze reflex," defined loosely as a burst of several uncontrolled sneezes, usually no more than 10, that occur when a person is suddenly exposed to bright light like sunshine. It's harmless, but it can be annoying and embarrassing. Unfortunately, the cause of sun sneezing is unknown, though it is possibly genetically inherited. Shielding your eyes by wearing sunglasses and a hat might help. Sun sneezing has not been carefully studied even though it has been described for many centuries. Even Greek philosopher Aristotle (384 BC-322 BC) discussed it more than 23 centuries ago.