Dear Tom,
What are the largest and smallest tropical cyclones (by area) on record?
Tom Polivka Algonquin
Dear Tom,
Based on a National Hurricane Center study by Dr. Chris Landsea and updated by Neal Dorst, it appears that the largest tropical cyclone on record was Typhoon Tip which roamed the Northwest Pacific in October 1979. The storm had gale force winds (39 m.p.h. or higher) extending 675 miles out from the eye. The massive storm also holds the record for this planet's lowest sea level air pressure of 25.69 inches of mercury measured 520 miles northwest of Guam on October 12. The smallest tropical cyclone is thought to be Tropical Storm Marco that packed gale force winds that extended only 12 miles from the storm's center of circulation as it moved across the far south Gulf of Mexico on Oct. 6-7, 2008.
Dear Tom,
Wasn't the period from Aug. 17-24, 1947, a very hot one? Didn't the highs reach at least 95 every day?
Brian Beecher, Villa Park
Dear Brian,
August 1947 stands out as the city's hottest August on record, and the Aug. 17-24 period certainly helped it gain its lofty status. The high temperatures did not quite reach 95 every day (Aug. 21-22 topped out at 94) but the other six days recorded highs of at least 97 degrees. The final day, Aug. 24, was the hottest of all with the mercury peaking at 100. Not only were the days hot, the nights were warm and muggy. With the exception of a morning low of 68 degrees on the morning of Aug. 17, overnight lows never dropped below 75 degrees. Four record highs still remain from that period: 98 degrees on the 18th and 20th, 97 on the 23rd and 100 on the 24th.
Dear Tom,
In a book about the St. Louis Cardinals' Gashouse Gang, the author mentioned that St. Louis experienced 30 consecutive 100-degree days during the 1934 season. Is that true, and was it also hot in Chicago?
---Alex Miller
Dear Alex,
The summer of 1934 was during the "Dust Bowl" and it was beastly hot in St. Louis; but not quite as hot as the author implied. St. Louis officially experienced 23 days of 100 degrees or higher that summer plus many more days in the 90s. There were eight consecutive 100s from July 18 to 25, including the season's highest reading of 110 degrees on July 24. It was also very hot in Chicago and most of the Midwest. Temperatures at Midway Airport topped the century mark 12 times that summer, including an all-time high reading of 109 on July 23 and six straight 100s from July 20 to 25.
Dear Tom,
What does a half-inch of rain mean?
Pat Broz
Dear Pat,
When we say that one-half inch of rain has fallen, we mean a blanket of water exactly 0.50 inches in depth would cover all horizontal surfaces if none of the rainwater had soaked in, run off, evaporated or been blocked by nearby or overhead obstructions such as vegetation, wires or buildings.
Rainfall is measured and reported in hundredths (0.01) of an inch, an increment too tiny to measure directly. Rain gauges therefore employ the principle of multiplication. In a standard gauge, rainwater falls into a circular collection area (a few inches in diameter) whose area is exactly ten times that of an accumulation tube into which it drains. Each one-hundredth inch of rainfall therefore stands one-tenth inch deep in the accumulation tube, and that water is easily measured with a ruler graduated to tenths of an inch.
Dear Tom,
My husband says that Chicago and Oak Park have never been hit by tornadoes. Is he right?
Kelly McCreary
Dear Kelly,
No portion of the Chicago metro area is immune from tornadoes and it should be noted that four twisters just occurred in the area during the recent severe weather outbreak on Aug. 19, 2009. At least a dozen tornadoes have been documented within the city of Chicago, including one in the downtown area in May 1876. The tail end of the deadly Oak lawn tornado traveled through the South Side on April 21, 1967 and another twister tore up portions of the South Side on March 4, 1961. Your husband is correct that Oak Park has not recorded a significant tornado strike, but twisters have touched down in nearby areas like Maywood, Melrose Park, Bellwood and North Riverside.
Dear Tom,
How did the word "depression" get associated with hurricanes?
David Ligare
Dear David,
When an area of disturbed weather in the tropics begins to acquire a circulation but has top wind speeds of less than 39 m.p.h. it is classified as a tropical depression. The term depression is related to the low barometer readings at the center of the circulation which is depressed when compared to the surrounding air pressure. According to the American Meteorological Society's Glossary of Meteorology, the word depression can be applied to any area of low pressure or trough not just to tropical systems. If the tropical depression starts to strengthen, it becomes a named tropical storm when sustained winds reach 39 m.p.h. and a hurricane when speeds reach 74 m.p.h.
Dear Tom,
A friend who lives near Glasgow, Montana, tells a wild, unbelievable weather story. He claims, once upon a time, the temperature in Glasgow shot up into the 90s in the middle of the night. 90s at night? Maybe in Las Vegas, but in Montana?
Ron Smithson
Dear Ron,
Believe him. The event in question took place on Sept. 9, 1994, in Glasgow. It was an odd atmospheric event known as a "heat burst" and it occurs near thunderstorms.
Thunderstorms usually produce cooling gusts of rain-chilled wind, but on rare occasions a parcel of dry air is pushed from an altitude of 20 or 30 thousand feet down to the surface, warming by compression all the way down. On that Sept. 9, Glasgow's temperature at 5:02 a.m. was 67 degrees. A thunderstorm heat burst shot the reading to 93 degrees at 5:17 a.m., but by 5:40 a.m. it was back 68 degrees.
Dear Tom,
How big is Lake Michigan and how does it affect the weather?
Janet
Dear Janet,
Lake Michigan is the third largest Great Lake (after Superior and Huron) and the world's sixth largest freshwater lake. It boasts more than 1,600 miles of shoreline and covers more than 22,000 square miles. It is more than 300 miles long and nearly 120 miles wide. The average depth is about 280 feet and its deepest part (925 feet) is mid-lake east of Manitowoc and Two Rivers, Wis. The lake has a great deal of influence on the surrounding area's weather. Among the most important; it keeps areas near the lake cooler in summer and warmer in winter, extending the growing season by several weeks as compared to inland areas. It causes more cloudiness and generates both lake effect rain and snow in the cold season but often suppresses thunderstorm development in the warm season.
Dear Tom,
What was the most destructive hurricane that ever occurred?
Luke Budil, Grayslake
Dear Luke,
Dealing solely with U.S. hurricanes, Katrina is the hands-down winner in terms of dollar damage, estimated at more than $84 billion when it devastated New Orleans in August 2005. The Galveston Hurricane that roared ashore in September 1900, leads in loss of life, with fatalities estimated from 8,000 to 12,000.
Expanding the question to tropical cyclones around the world, the Bhola Cyclone that struck Bangladesh (then east Pakistan) in November 1970, ranks as one of the world's all-time deadliest storms. Making landfall on Nov. 12, that killer storm packed 125 m.p.h. winds and sent inland a devastating 15-20 foot storm surge that inundated the area's low-lying landscape. The incredible death toll: at least 500,000 but possibly as high as one million.
Dear Tom,
I was born on Labor Day, Sept. 7, 1953, and my mother likes to remind me of how hot it was (no air conditioning) just before I was born. What caused the heat wave and when did it finally break?
Beth Swanick
Dear Beth,
The days leading up to your birth set new standards for late-season heat in Chicago. With the jet stream far to the north in Canada and high pressure anchored over the eastern United States, Chicago was locked in a record-breaking heat wave. The city recorded 11 consecutive days of 90 degrees or higher from Aug. 24-Sept. 3 that included back-to-back highs of 101 on Sept. 1-2. The hot weather established six straight record highs from Aug. 29-Sept. 3, and all remain in effect today. The heat wave finally broke on Sept. 4 when a cold front passed, dropping the high temperature to pleasant 76 degrees.
Dear Tom,
I've heard that fireflies are decreasing in numbers, but in early August the fields south of Davenport were aglow with them. Why do they flash? Does the weather affect their flashing?
Steve Pond, Chicago
Dear Steve,
Firefly numbers worldwide have declined precipitously in recent years, but the cause is unknown. All 2,000+ species of fireflies have one thing in common: bioluminescence, the ability to produce visible light, but does the weather have anything to do with it?
Dr. James Case of the Department of Ecology, Evolution and Marine Biology at the Univ. California, Santa Barbara, says, "Behaviorally, luminescence in fireflies functions in mating, with each species tending to have a distinct communication flash code." The timing and sequence of flashes enable females and males to communicate with each other in preparation for mating, and weather has nothing to do with it.
Dear Tom,
A few years ago you stated we were in a 25-year period of numerous hurricanes and each year would be well above average, yet only the 2005 season was above average. Are you still predicting above-average activity?
Benjamin Hoekstra
Dear Benjamin,
National Weather Service researchers have learned that hurricane activity in the North Atlantic Basin fluctuates alternately through periods of diminished and heightened hurricane numbers, each approximately 20 to 40 years long. A period of increased activity that began in the middle 1990s is likely to persist into the 2020s. That prediction addresses hurricane activity averaged over the entire active period, not specific years. Tropical cyclone activity since 1995 has averaged 15 named storms per season and almost every season (not just 2005) has exceeded the long-term annual average of 10 named storms.
Dear Tom,
Summers in the 1930s were very hot. What was the tally of "century days" in
those years?
Paul Sarewich, Chicago
Dear Paul,
When it comes to 100-degree occurrences in Chicago, the decade of the 1930s yielded a bumper crop of 100-degree days that is unrivaled by any other decade in the city's 139 years of documented weather history. From 1930 through 1939, 37 "century days" were registered at Midway Airport. And in the 1930s two blistering summers stand out: 1934 and 1936. Each of them produced 11 days of 100-degree heat that were without precedent in two dangerous aspects, duration and intensity---six consecutive 100-degree days, July 20 to July 25, 1934;
and eight consecutive days, July 7 through 14, 1936. The daily highs during the 1934 period were 103, 108, 104, 109, 107, 105 and in 1936 the highs were 102, 106, 100, 106, 107, 100, 102, 104.
Dear Tom,
What is the latest date that the first named Atlantic storm has formed?
Steve Hanan
Dear Steve,
Atlantic Basin storms have been named since 1950, and since then the latest starting season was 1967 when Arlene was named on Aug. 30. Arlene never made landfall and moved north through the open Atlantic mainly as a tropical storm, but it briefly became a Category 1 hurricane with top winds of 85 m.p.h. Runner-up for a late-season start was 1977 when Anita was named on Aug. 29. Anita quickly grew into a monster storm, slamming into the Mexican coast between Brownsville and Tampico on Sept. 2 as a Category 5 hurricane with top winds of nearly 175 m.p.h. The current season, which started with Tropical Storm Ana on Aug. 15, marked the latest start to a season since 1992 when the infamous Hurricane Andrew was named on Aug. 17.
Dear Tom,
As we head toward fall with shortening days, I contend that the sun sets later as you go north. Am I correct?
Ginger Bolte, Arlington Heights
Dear Ginger, At this time of year, you are correct. On Sunday, the sun will set at 7:49 p.m. in Chicago, while to the north, our neighbors in Green Bay will not experience sunset until 7:55 p.m. Heading up to Marquette, Mich. the sun sets two minutes later than at Green Bay. As we get into October and the hours of daylight continue to decrease, sunset occurs earlier as you travel north of Chicago. By Dec. 1, sunset will be at 4:21 p.m. in Chicago, seven minutes after setting in Green Bay at 4:14 p.m. and 17 minutes after setting in Marquette.
Dear Tom,
While on a trip to Tennessee last May we drove through southern Illinois. In the area from Mt. Vernon to Fulton, Ky., we noticed many trees uprooted, broken in two, twisted and scattered along the road. What kind of storm produced such widespread damage?
Roy Martin
Dear Roy,
You witnessed the aftermath of one of the worst squall lines to pass through southern Illinois in more than a decade. On May 8, 2009, the long-lived, fast-moving thunderstorm complex known as a derecho left a widespread trail of destruction from southern Missouri and southern Illinois all the way to the western Carolinas. Winds peaked in excess of 100 m.p.h. and the damage was likened to that of a hurricane because of the vast extent of the damage path. Several tornadoes were reported in addition to the very strong winds. Four people perished in the storms and at least a dozen were injured.
Dear Tom,
Now that the weather has gotten hot why has Lake Michigan turned so chilly?
Mehul Jethva, Joyce and Ted Husak, Brett Konken
Dear Readers,
It seems counter-intuitive, but when the city turns hot, Lake Michigan water temperatures usually drops. A process called upwelling is to blame. As the lake warms during the summer the sun's rays heat the water closest to the surface while the deeper water remains cold. Hot weather arrives in the city on brisk south and southwest winds that push the warm surface water out into the lake toward the east shore, allowing the colder bottom water to upwell and replace it. As a result between August 7-10 Chicago's shore water temperature dropped from 71 to 63 degrees while the water at Michigan City jumped from 55 degrees to 67.
Dear Tom,
I'm not complaining because I love this cool summer, but how many hours of temperatures in the 90s have we had in July and August?
Peggy Pannke-Smith Park Ridge
Dear Peggy,
Hours of 90 degree temperatures have been few and far between this summer. Frank Wachowski supplied the data from Midway Airport and reported that there were no hours of 90s in July and so far in August just seven, recorded last weekend, Aug. 8-9. Wachowski noted that we did have 20 hours spread over four days in June so the total number of hours for the entire summer is just 27. Compare that to some of the city's hottest summers where we have logged as many as 48 days in the 90s and you realize just how little heat there has been this year. Of course, summer is not over yet and more 90s are expected, possibly as early as next weekend.
Dear Tom,
You recently mentioned a hurricane near Hawaii. Isn't that rare and why isn't it being called a typhoon?
Leon Panofsky
Dear Leon,
To be classified a typhoon, a tropical cyclone (in the northwest Pacific) must develop or move west of the 180th meridian (International Date Line), so the tropical cyclones affecting Hawaii are called hurricanes. The threat of a landfall in Hawaii is considerably lower than in the Caribbean or along the U.S. Gulf and Atlantic coasts. The storms in that part of the Pacific seldom make a direct hit on the islands, usually just skirting them with fringe effects. Since 1950 only two storms have made a direct hit, both on Kauai: Hurricane Dot in 1959 and Hurricane Iniki in 1992. Recently in 2003 Hurricane Jimena passed within 120 miles of the Big Island's South Point bringing 50 m.p.h. winds, 15 foot waves and considerable flooding.
Dear Tom,
I'm a dispatcher at United Airlines and I see all kinds of weather across
the country. In Phoenix they have dust storms called haboobs. I recently
learned a new term, derecho. What other interesting terms can you share with
us?
Matt Duxbury
Dear Matt,
How about sastrugi, sharp ridges on snow drifts, and its singular form,
sastruga? And then there's aftersummer, a period of unusually warm, quiet
weather in the autumn, and foreshore, the part of a lake shore that lies
between the low and high water marks. A psithurism is the sound of wind in
trees and frazil is slushy ice crystals that form in supercooled water that
is too turbulent, as in gushing streams, to permit the formation of an ice
sheet. In Australia, there's Fremantle Doctor, a strong sea breeze (like our
lake breeze, but stronger) and Cock-eyed Bob, a typhoon. In Britain, they
call squally and tempestuous weather a blunk.
Dear Mr. Skilling,
What are the lowest and highest temperatures on Aug. 13 (my birthday) and also in all of August?
Maggie Westover, age 7
Dear Maggie,
You'll be 8 on Aug. 13 (happy birthday!) and you were born in 2001. It was a pleasantly cool day in Chicago on Aug. 13, 2001. Afternoon temperatures across the area remained in the middle 70s (the normal high on the 13th is 82 degrees).
We cracked open Chicago's record books and found that the city's temperatures on Aug. 13 have been as high as 101 degrees (in 1936, at Midway Airport) and as low as 51 degrees (1982 and 2004, at O'Hare). Chicago's extremes in all of August range from 104 degrees (Aug. 6, 1934, Midway Airport) to 42 (Aug. 28, 1986, at O'Hare).
August is arguably Chicago's most summery month. July is slightly warmer, but August weather tends to be more tranquil: more sunshine, longer rain-free periods and lots of warm days.
Dear Tom,
My birthday is on Aug. 9 and since 1987 it seems that we've had at least some rain on that day almost every year. Is that true or am I imagining it?
Samantha Moore
Dear Samantha,
Chicago weather historian Frank Wachowski confirms that Aug. 9 has been rainier than usual in recent years, but he added that you have also celebrated many dry birthdays. The average probability for measurable rain (0.01 inch or more) on any August day is 29 percent, but since 1987 the probability of rain on Aug. 9 has been 41 percent (measurable rain on 9 of the 22 days). It rained on your birthday last year (but just a trace) and in 2007 and 2005. It was dry in 2006. For the most part, the rains have been light. Since 1987, the heaviest amount that fell was 0.47 inches in 2001. In official records since 1871, the heaviest Aug. 9 rain was 1.60 inches in 1952.
Dear Tom,
What is the difference between a hurricane, typhoon and a cyclone? Do we ever hear about typhoons and cyclones here?
John Flanagan, St. Charles
Dear John,
In the context of your question, hurricanes, typhoons and cyclones are regional specific names for tropical cyclones. They are called hurricanes in the Atlantic, eastern and southwest Pacific and Caribbean, typhoons in the northwest Pacific, and cyclones in the Indian Ocean. While cyclones and typhoons do not affect this country, they become big news here when they cause problems in other parts of the world. Cyclone Nargis was a huge story when it devastated Myanmar on May 3, 2008, as was Typhoon Thelma when it killed 6,000 in the Philippines in 1991. In a non-tropical sense the term cyclone is also loosely applied to tornadoes, waterspouts, dust storms and sometimes just to any strong wind.
Dear Tom,
How does the cool summer of 2009 compare to other recent cool summers we've endured, such as 2004 and 1992?
Tim Lucole, Deerfield
Dear Tim,
With an average temperature of 70.0 degrees as of July 31, it's certainly accurate to characterize this summer as "cool." In fact, it ranks 8th coolest out of 82 years of temperature data (1928-2009) at Midway Airport.
However, this might be a premature characterization because meteorological summer runs through August 31 and one-third of the season is yet to occur. It's difficult to keep temperatures down in August. Indeed, a pattern change suggests much higher readings in a few days.
From June 1 through July 31, this summer's daily high temperatures have averaged 78.1 degrees (versus a normal of 82.7 in that period) and it ranked third coolest; 2004 (79.6 degrees) was 8th coolest and 1992 (78.8) was 4th coolest.
Dear Tom,
Last summer severe storms swept the Chicago area and Wrigley Field was evacuated. Were any tornadoes reported that evening?
Monica Salgado Chicago
Dear Monica,
A year ago on the evening of August 4 a violent squall line raced across the Chicago area producing wind gusts as high as 94 m.p.h., intense lightning and thunder and widespread flooding from torrential rainfall. The storm complex known as a DERECHO left a long legacy of downed trees and power lines all the way from northwest Illinois to Lake Michigan. In the city, tornado sirens blared as the stands at Wrigley Field were evacuated. Most of the storm damage was due to high straight-line winds but there were three confirmed tornadoes; an EF2 (winds to 135 m.p.h.) in Griffith, Indiana and EF1 twisters (winds to 110 m.p.h.) in Bloomingdale and Bolingbrook.
Dear Tom,
What causes hail to vary in size?
Leon J. Hoffman, Chicago
Thunderstorms produce hail of varying sizes for several reasons, but a key element is the strength of the storm's updraft, the powerful column of ascending air that feeds heat and moisture into the storm. "Thunderstorms that produce larger hail have stronger updrafts." That's the word from severe weather researcher and native Chicagoan Brian Smith.
"Hail starts as small ice particles supported thousands of feet aloft in the updraft, growing larger layer by layer by collecting droplets of supercooled water that adhere to them and freeze. Like the motion of an elevator, hailstones repeatedly ascend in updrafts, descend in downdrafts, then ascend again. Giant hailstones, baseball-sized or larger, usually occur in so-called supercell thunderstorms whose tremendous updrafts are rotating, circulating the hail until it finally grows too heavy and falls out."
Dear Tom,
Why haven't there been any hurricanes yet this year?
Amanda Lonis
Dear Amanda,
The tropical Atlantic has been very quiet so far this season. Not only haven't there been any named storms, but forecasters have had only a smattering of disturbances to investigate. The developing El Niño may be part of the reason. During an El Niño episode, hurricane frequency tends to decrease in the Atlantic Basin. The reason is an increase in the winds aloft which creates more wind shear, a condition that tears storms apart and disrupts their formation. With no named storms through Aug. 3, this season becomes the latest starting Atlantic season since 1992. However, that season more than made up for its late start. The first named storm which finally formed on Aug. 16 developed into Category 5 Andrew, an historic storm that devastated far south Florida.
Dear Tom,
Sometimes dew forms all night. Can it be measured? If so, how?
---Norm Victorson
Dear Norm,
The National Weather Service's weather observation program does not include measurements of dew. However, the amount of water contributed by dew can be determined, and a variety of techniques exist to do the job, though with varying degrees of accuracy. Dew can be measured most easily and accurately by weighing it. A piece of sod (or other material representative of the soil surface in the area for which dew measurements are desired) is placed on a scale that records the weight increase as dew accumulates. Unfortunately, accurate measurements of dew are not possible in densely vegetated areas. For example, little or no dew forms under a dense forest canopy, but it does accumulate on leaves at the top of the canopy.
