Dear Tom,
How is the dew point determined? Is a "thermometer" that measures dew point temperatures available to the public?
---Scott Jansen
Dear Scott,
Unfortunately, it's not that simple. The dew point temperature is the temperature at which the relative humidity reaches 100 percent as air is cooled.
For many years the dew point was calculated from the air temperature and a "wet bulb" reading, a temperature obtained by placing a wet cloth cover on a thermometer bulb and noting the temperature drop as the water evaporates.
Now, sensors calculate the dew point by starting with a warm mirror, chilling it and noting its temperature -- the dew point -- at the instant when condensation first appears on the mirror's surface, as determined by a reduction in the intensity of a focused light beam aimed at the mirror.
Dear Tom,
On July 4, 1956, Unionville, Md., got a record 1.23 inches of rain in a minute. What caused the rain to fall so heavily that day?
--Jeff Kozinski, Mokena
Dear Jeff,
It was what is known as a "rain gush," a colloquial term used to describe a sudden, intense rainfall, usually at a rate of 4 inches or more per hour. Gushes are almost always associated with thunderstorms. Incredible bursts of rain producing 2 inches in one minute are occasionally reported.
Very large raindrops fall at a top speed of about 30 feet per second (20 mph), but rising currents of warm air (updrafts) within thunderstorms easily attain much greater speeds. Consequently, thunderstorm updrafts often support immense quantities of water. Should such updrafts suddenly weaken or collapse, their burdens of water will pour out in a sudden gush.
Dear Tom,
You mentioned that last summer the city recorded only four 90s. Has there ever been a summer without any?
-- Max Friedman
Dear Max,
Only once in Chicago's weather history dating from 1871 has the city experienced a summer without a single 90-degree day, and that was in 1875. The highest readings that summer were 89 degrees on June 11 and 88 degrees on July 15. However, the city's official thermometer in 1875 was located downtown and close to Lake Michigan, and it is almost a certainty that readings reached 90 degrees at inland locations.
The city's official weather observation station was moved inland in 1942 (to Midway Airport) and then to O'Hare International Airport (1980). The fewest number of 90-degree days logged at the inland stations was two in 1979.
Recent summers have produced relatively few 90-degree days, but historic trends indicate the potential for a substantial number of 90s this summer.
Dear Tom,
If warmer air can hold more moisture and should therefore be heavier, why does it rise toward the ceiling when we have our heat and humidifier running?
---Janis Franz
Dear Janis,
A parcel of air will rise whenever it is less dense than the air around it. When air is warmed, the molecules of its component gases vibrate more rapidly, collide more frequently and become more widely spaced, so the air expands and becomes less dense. The effect of moisture is similar.
Contrary to popular belief, moist air is less dense than dry air at the same temperature. The molecular weight of a water molecule is less than the proportionally averaged molecular weight of the gases in air. Consequently, adding moisture to air causes it to become less dense. The conclusion: Air becomes less dense and rises when it is warmed and/or when moisture is added.
Dear Tom,
Averages, such as daily and monthly average temperatures, are often quoted in the media. How are those averages figured?
--Cynthia Radcliff
Dear Cynthia,
Daily and monthly average temperatures, like averages for other intervals of time, are calculated by determining the arithmetic average using daily maximum and minimum temperatures.
The calculation for a daily average is as follows: Add the day's (midnight-to-midnight, local standard time) maximum and minimum temperatures, divide by 2 and round to the nearest whole number. For a 31-day month, like May, add the 31 daily highs and 31 daily lows, divide by 62, and round to one decimal place.
Long-term averages (the so-called climatological "normal temperatures") are calculated from a 30-year base period, currently using 1971-2000, and updated every 10 years.
Dear Tom,
Does Hudson Bay have any effect on Chicago weather?
Bill Peters, Beach Park
Dear Bill,
Though Hudson Bay is more than 1,000 miles to our north, its icy waters do play a role in Chicago's weather. That region is a frequent host to large, stationary high pressure systems that keep a sustained northeast flow of air into the Midwest and Chicago. The air of arctic origin is not only cold, but also dry and can bring extended periods of cool and dry weather to Chicago during spring, summer and fall. When this type of flow occurs in winter, the result is often a prolonged period of cloudy, snowy weather. The snow is usually light due to lack of moisture except where the lake-effect from the Great Lakes enhances totals. When low pressure is parked over Hudson Bay in winter Chicago can be in line for major arctic outbreaks and sustained periods of severe cold.
Dear Tom,
You recently told us about all the 90s in May 1977. Didn't that May come after one of the city's worst Januarys?
--Donna Winslow
Dear Donna,
You are correct. In one of the most dramatic meteorological turnarounds in Chicago's history, the city's warmest May, with a record 10 days in the 90s, came after the coldest January on record that logged 17 days of zero or below and temperatures that never once reached the freezing mark. January 1977 averaged 10.1 degrees and was 14.2 degrees below normal while May's 69.3-degree average was 9.3 degrees above normal. The summer of 1977 went on to produce 23 more 90-degree days for a total of 33, well above the city's long-term average of 16. The temperature extremes in 1977 spanned 118 degrees, from 19 below zero on Jan. 16 to 99 degrees on July 15.
Dear Tom,
What would Chicago's weather and climate be like without Lake Michigan?
--Jeff Kozinski, Mokena
Dear Jeff,
Without Lake Michigan on Chicago's eastern doorstep, the city would have a far more continental climate characterized by warmer days, colder nights and a greater daily temperature range. The loss of cooling lake breezes would bring a more uniform temperature distribution across the metropolitan area in spring and summer. Overall there would be a decrease in cloudiness and lower annual precipitation total with the loss of lake-effect snow and rain, lake breeze-generated thunderstorms and the fact that rain-generating frontal boundaries would no longer stall at the south end of the lake. Late fall storms would bring more snow to the city as the northeast winds accompanying them would not carry warm air from 50-degree lake waters inland.
Dear Tom,
The recent question about Chicago's 100-degree temperatures in May prompts me to ask about 100-degree days in June. Have we ever had any?
--Patricia Sanders
Dear Patricia,
In 139 years of temperature data (1871-2009), Chicago's official thermometer has hit or exceeded 100 degrees in June in eight years. That's about one June in 17. Those eight Junes produced 13 triple-digit days, most recently in the drought summer of 1988. But the statistics are misleading because the city's weather observation station was located near Lake Michigan (in the Loop or at the University of Chicago) until 1942, and lake cooling often prevented 100-degree readings that occurred across most of the city. Midway Airport data (1928-2009) provide a better picture: 19 occurrences of 100-degree days in nine Junes out of 82, or one June in nine (versus one in 17 officially).
WGN Weather on Twitter