Dear Tom,
What exactly does it mean when the forecast says there is a 50 percent chance it will rain for a given area? Does it mean there is a 50 chance it will rain in that entire area, or that 50 percent of that area will definitely get rain
James Shaw, Northrop Grumman Corp.
Dear James,
Neither of your statements is precisely correct. In weather forecasts, the probability of rain is a so-called "point probability," the chance of measurable precipitation (at least 0.01 inch) at a specific location, such as at your residence. Regarding a 50-percent forecast, the forecaster may believe the entire area will experience rain if it rains, but his confidence that rain will actually arrive is only 50 percent. Or he may believe scattered showers will definitely occur, but affect only 50 percent of the area. Regardless of the weather situation, the meaning for you is always the same: a 50 percent chance of rain on your head
Dear Tom,
I was curious how many days in the 139 years of Chicago weather records have had the same high and low temperatures on a given day.
Cole Turgeon, Brookfield
Dear Cole,
A computer sweep of Chicago's entire official temperature data base, from Nov.1, 1870, through Sept. 30, 2009, reveals only three days on which temperatures remained absolutely steady through the entire 24-hour day: March 13, 1878, when the city's official thermometer registered a constant 44 degrees; March 24, 1891, with 32 degrees; and Feb. 6, 1942, with 35 degrees.
On an additional 24 days, the range between the city's maximum and minimum temperatures was only one degree. The most recent occurrence of that event was about six and one-half years ago -- April 8, 2003, when the high and low temperatures were 32 and 31 degrees, respectively.
Dear Tom,
What are the effects of El Nino on Chicago's winters?
Mario Ortiz
Dear Mario,
El Nino, a widespread warming of the surface waters of the equatorial Pacific Ocean, occurs every few years. An El Nino that began in July is likely to persist well into 2010. When it is in progress, El Nino causes changes in atmospheric wind patterns that have worldwide weather
ramifications.
Past El Ninos have had little effect on Chicago summers, but they have dramatically altered our winters. Chicago's "El Nino winters" tend to be milder, drier and "quieter" than normal: fewer occurrences of bitterly cold arctic air and fewer big storms. Snowfall in 17 El Nino winters since 1950 has averaged a few inches below our seasonal normal of 40 inches, but snow totals in any given winter have ranged from well below to far above normal.
Dear Tom,
Kendall, my six-year-old son, asks if there is any place on Earth where blizzards, tornadoes and hurricanes never occur. He wants to live there!
Lisa Gerhold-Dirks and Kendall Dirks, Elmhurst
Dear Lisa and Kendall,
Such places do exist, but "never" is a powerful word. The reality of the world's weather is such that locations blessed with a benign climate will on very rare occasions nonetheless experience one or more of the three phenomena that you wish to avoid.
Blizzards and occasionally harsh winter weather threaten most places outside tropical and subtropical areas, hurricanes threaten the subtropics (except for locations far inland) and tornadoes and severe thunderstorms constitute a worldwide menace outside polar regions. Taking it all into consideration, only the world's harshest desert regions, such as Death Valley or the Sahara Desert, meet your criteria, but other aspects of the climate in those places are certainly undesirable.
Dear Tom,
What happens when cold and warm fronts collide?
John Padna, Chicago
Dear John,
"Collision" implies a violence inappropriate to the situation. The collision of solid, fast-moving objects like automobiles is violent, but in the atmosphere we're dealing with air, a medium so fluid that it readily gives way. Air density always determines the outcome when fronts collide. In the cold frontal situation, advancing cool air, heavier and more dense than warm air, pushes under and lifts warm air; advancing warm air (the warm frontal situation), lighter and less dense, slides over cooler air. A typical frontal collision situation involves a cold front overtaking a warm front. When this occurs, the cold front always "wins" in the sense that the cool air slides under and bodily lifts the warm frontal zone. The surface warm front disappears (it's no longer a ground-level feature) and the advancing cold front is now called an occluded front.
Dear Tom,
In 1960 when I was in the Navy, our ship was in Chesapeake Bay and was caught in Hurricane Donna. I think it was a Category 5 storm? Can you provide details?
--Bob Phillip
Dear Bob,
Hurricane Donna was a blockbuster storm that wreaked havoc on the entire East Coast and remains the only hurricane to produce hurricane force winds (74 mph or higher) from Florida to New England. At its peak intensity, Donna was indeed a Category 5 storm with top winds of 160 mph, but that was early on as it approached the Virgin Islands. When you encountered the storm on Sept. 12, 1960, it was a Category 2 storm with peak winds around 100 mph. However, much higher gusts were recorded offshore, at 138 mph on the Chesapeake Lightship located 17 miles off of Virginia Beach.
Dear Tom,
Has it snowed in all 50 states?
Maggie Drummond
Dear Maggie,
All 50 states, including Hawaii and Florida, have recorded snow. Most states get snowfall every winter as part of their normal cold-season weather, especially in the higher elevations of the warmer states. In Florida, some snow occurs at least every couple of years across the far north, but even southern portions of mainland Florida have experienced flurries on rare occasions, though snow has never been reported in the Keys. During an exceptionally strong arctic outbreak in January 1977, snow fell at Homestead, Fla., south of Miami. In Hawaii snowfall is usually confined to the highest elevations of Mauna Loa and Mauna Kea on the Big Island and Haleakala on Maui, but it has fallen on some of the other islands as low as the 4,000 foot level during cold snaps.
Dear Tom,
In a January article you wrote that on perihelion day the Earth is 93.1 million miles from the sun and "the Earth actually passes closer to the sun during winter and swings further from it during summer." Is this correct? Text material at my son's school says the opposite: "The summer sun is closer and the winter sun is further."
Tilly Dushman
Dear Tilly,
The school text material that you have quoted is incorrect. Triton College astronomer Dan Joyce tells us perihelion, the point of the Earth's closest passage to the sun, occurs on or about Jan. 3 and aphelion, the Earth's farthest passage, occurs on or about July 4. Those are winter and summer dates in the Northern Hemisphere.
The average Earth-sun distance, 93.0 million miles, diminishes to 91.3 million miles at perihelion and increases to 94.4 million miles at aphelion.
Dear Tom,
Vesuvius is considered to be one of the world's most dangerous volcanoes. Are that mountain's eruptions in any way weather-related, and should I be concerned for the safety of friends who live in Naples?
John Liska
Dear John
Indeed, the storied and sometimes catastrophic eruptions of Mount Vesuvius rank it among the world's most dangerous volcanoes, but its eruptions (and the eruptions of volcanoes worldwide) are determined by forces deep within the Earth and are in no way weather-related.
Italian authorities are gravely concerned that, in the event of a sudden eruption of Vesuvius, it might be impossible to evacuate the three million people who would be threatened. Vesuvius is quiet now, but Giuseppe Mastrolorenzo of the Vesuvius Observatory in Naples says, "It could remain in this condition for centuries, or even erupt next year."
Dear Tom,
Has Chicago or any other place ever established a record high and a record
low on the same day?
Douglas H. Hanbury East Peoria Ill.
Dear Douglas,
Setting a record low and a record high on the same day is an extremely rare event; one that has never occurred in Chicago. Accomplishing this feat normally means a large range between the day's low and high, a condition that almost always requires a dry atmosphere. Other contributing factors would be unlimited sunshine to send daytime temperatures soaring and clear
skies and light winds at night to allow the mercury to plummet. Such conditions were present in August 2002 when a rash of same-day record highs and lows were set in western United States. Park City, Utah broke or tied its record high/low three times in one week (Aug. 11,13,15) and Alamosa, Colo. did it on consecutive days (Aug. 25-26).
Dear Tom,
Many East Coast hurricanes seem to develop off of the west coast of Africa. Where do the hurricanes that hit Baja California form?
Ann Marie Perkins Naperville
Dear Ann Marie,
The hurricanes that roam the eastern Pacific tend to develop in the warm tropical waters off the west coast of southern Mexico, with some of them forming even further south off Guatemala. In general, the storms tend to track east and northeast and eventually dissipate as they encounter the colder waters in the central Pacific. However, when steered by favorable winds, these tropical cyclones can move north affecting not only Baja California, but also areas along the west coast of northern Mexico. On a few occasions, a storm has moved north through the Gulf of California bringing heavy rainfall to portions of the southwestern U.S.
Dear Tom,
I enjoy how you indicate the cities with the highest and lowest temperatures of the day. Has Chicago ever been host to either extreme?
Laurence K. Marks
Dear Laurence,
The National Weather Service releases the daily list of the nation's highest and lowest temperatures, but Chicago has never appeared there. Many would say that's fortunate because, more often than not, those temperatures are, at the very least, uncomfortably hot or cold.
On the very rare occasions when Chicago's temperatures are either so hot or so cold that they might qualify for the national extremes on any given day, some other Midwestern city invariably registers a more extreme value.
One relevant fact: The daily tabulation of the nation's highest and lowest temperatures is limited to cities and locations in the 48 contiguous states; Alaskan and Hawaiian cities are excluded.
Dear Tom,
We once noticed the full moon low on the eastern horizon glowing orange. It was beautiful, and as it rose it became yellower until finally it was white. What caused this?
Joanne Prusik, Chicago
Dear Joanne,
The spectacle of the moon's color change as it gradually rose from the horizon was caused by the effect of the Earth's atmosphere on moonlight. Moonlight (actually, reflected sunlight which is white when pure) is massively scattered when the moon is on the horizon, and its light must pass through a great distance of dense air. Only the longer wavelength components of light, shades of red and orange, arrive at our eyes.
As the moon climbs, its light makes a shorter passage through dense air, and less scattering occurs. Shorter wavelength colors, like yellow, appear and finally the full white spectrum.
Dear Tom,
In a recent column you mentioned that is has been as cold as 15 degrees at Death Valley, Calif. Has it ever snowed there?
Neil Walker
Dear Neil,
Once again we invoked the services of our Death Valley expert, Paul Kubecka, and his answer was a resounding "yes." Snow has been recorded on two occasions at the official weather station there, located at 168 to 194 feet below sea level. Traces of snow fell on Jan. 9 and 11, 1949, and again on Jan. 4 and 5, 1974. In addition, ice pellets (sleet) fell there on Jan. 2, 1949. A more significant snowfall of 4 inches was observed at the National Park Service headquarters at an unofficial weather station located at Cow Creek, at an elevation of 151 feet below sea level on Jan. 12, 1949.
Kubecka also found reference to a mention of 0.5 inch snowfall on Jan. 29, 1922, but could not verify the validity of that report.
Dear Tom,
It seems that September and October usually receive a great deal of sunshine. Have either of these months ever received 100 percent of possible sun?
William J. Ooms Jr., Alsip
Dear William,
September with an average of 62 percent and October with 58 percent of possible sunshine, are usually the city's last sunny months before the dark, dreary days of late autumn and early winter set in. We checked with Chicago climatologist Frank Wachowski and found that dating back to 1893 the two sunniest Septembers (percentage of possible sunshine) were in 2004 (90 percent) and 1895 (86 percent.) The city's sunniest October was in 1924 (84 percent) followed by 1938 (79 percent.) Wachowski noted that Chicago never has and probably never will log a totally sunny month. The sunniest month on record here was July 1916 with 95 percent of the possible sun.
Dear Tom,
You've told us that Chicago has twice recorded a trace of snow in September. What kind of snow season followed those early flakes?
Mark Keenan
Dear Mark,
You're correct that Chicago has twice recorded a trace of snow in September since snowfall records began in 1884. Both instances occurred on Sept. 25, first in 1928 and again in 1942 following the passage of strong cold fronts. The winter of 1928 turned out to be a subpar snow season with just 30.1 inches compared to around 40 inches in a typical Chicago winter, while the 1942-43 season was snowier with 45.2 inches. The typical date for the arrival of the season's first snow flurries is around Halloween, though they have occurred as late as Dec. 5 in 1999. Last year the city's first snowflakes fell on Oct. 26.
Dear Tom,
How rare is it for Chicago to experience a 90-degree day in October?
Megan Samuels
Dear Megan,
October 90s are extremely rare in Chicago with only six on the books since 1870. The combination of shortening daylight and lowering sun angle make 90s unlikely so late in the year.
The last October 90s recorded in the city took place in 1971 when back-to-back 90s were logged on Oct. 1 (92) and 2 (91). Both of these readings established new record highs for the day.
The latest occurrence for a 90-degree day here is Oct. 6, 1963 when the mercury peaked at 94 degrees---not only breaking the record high for the day, but set the all-time high for the month.
Since 2000, the highest October temperature recorded here has been 87 degrees; peaking at that value on Oct. 4, 2005 and for three straight days during a mini-late-season heat wave from Oct. 6-8, in 2007, which affected the Chicago Marathon.
Dear Mr. Skilling,
Does lightning come up from the ground or down from the sky?
Sarah
Dear Sarah,
In answer to your question, lightning researcher Dr. Martin Uman of the University of Florida says, "In a sense it does both. The usual lightning flash between cloud and ground begins with a visually-undetected downward-moving traveling spark called the stepped leader. On the other hand, when the stepped leader reaches ground (or a spark jumping up from the ground), the leader channel first becomes highly luminous at the ground and then at higher and higher altitudes. This is the return stroke."
It is therefore fair to say that visible lightning moves from ground to cloud but the bolt's initial development occurs from cloud to ground.
Dear Tom,
Severe storms swept the Chicago area on Aug. 23, 2007. Was that storm a
derecho or a microburst?
Steve Plainfield
Dear Steve,
The storm you are referring to was a large derecho, a fast-moving squall line that swept the entire Chicago area during the afternoon of Aug. 23, 2007. It raced across the Chicago area at speeds approaching 60 m.p.h. The thunderstorms, some of which towered to nearly 65,000 feet, packed top winds of 80 m.p.h. that were measured at south suburban Manhattan; but almost all of the metropolitan region reported damaging winds of 60-70 m.p.h. That being said, there were likely small pockets of intense damage superimposed in the overall swath of wind-damage path that was caused by microbursts. The thunderstorm complex leveled countless trees and knocked out power to many areas.
Dear Tom,
We only hear about the blistering highs at Death Valley, Calif. What is their highest overnight low? On the cold side, what is the site's record low and lowest daytime maximum?
--Dale Kalina, Naperville
Dear Dale,
We asked retired Chicago weatherman Paul Kubecka, an expert on Death Valley climatology. The site's record high of 134 on July 10, 1913, is well-known, but Kubecka filled us in on some lesser publicized records. Death Valley is one of the few places on this planet where low temperatures have remained above 100. Kubecka cited lows of 110 on July 5, 1918, and 105 on July 22, 1917, as two of the highest. Despite the scorching summers, it does get cold there in winter. Death Valley's all-time record low of 15 was set Jan. 18, 1913, and the lowest maximum was achilly 38 recorded on Christmas Eve1990.
Dear Tom,
Was that beautiful full moon that I saw rising on Sept. 4 the Harvest Moon?
Brandy DeWitt
Dear Brandy,
The Sept. 4 full moon was indeed beautiful, but it was not the Harvest Moon. It carried a variety of other names, among them the Fruit, Barley and Corn full moon. According to folklore, the Harvest Moon is defined as the full moon closest to the autumnal equinox, the astronomical beginning of fall. However, this year the equinox occurs on Sept. 22, 18 days after the September full moon but only 12 days before the one on Oct. 4 which is designated the Harvest Moon. Having the Harvest Moon fall in October is a bit unusual and won't happen again until 2017. The full moon following the Harvest Moon is traditionally called the Hunters' Moon and will occur on Nov. 2.
Dear Tom,
Our recent seven day dry spell seemed long. Was that anything close to a
record?
Barry Bishop
Dear Barry,
Not by a long shot. Though seven straight dry days was a nice break from
precipitation after the heavy rainfall of late August, it was far from a
record.
Back in 1979 after a brief shower brought 0.01 inches of rain to Chicago on
Sept. 1, not a drop fell in the city for 22 days, the city's longest period
of absolutely dry weather. The streak was broken by some sprinkles on Sept.
23 but the month closed with no more measurable rain and remains the city's
all-time driest month. Chicago's longest span between measurable
precipitation (0.01 inches or more) was 30 days established back in 1919
from Jan. 7-Feb. 5, followed by two 29 day periods, one from Dec. 7,
1943-Jan. 4, 1944 and the other in the above mentioned month from Sept.
2-30, 1979.
Dear Tom,
Ocean currents rotate clockwise in the Northern Hemisphere, but there are also counterclockwise winds. Is the difference caused by the Coriolis force?
Sam Olinger
Dear Sam,
The terms "clockwise" and "counterclockwise" refer to the sense of rotation around a center, when viewed from above. Do not confuse those terms with the Coriolis deflection which, in the Northern Hemisphere, is always directed to the right of the motion -- for water, air and for anything that moves.
When air blows inward toward a central point (toward a low pressure center, for example), the rightward Coriolis deflection causes a counterclockwise inward spiral. When air blows out from a central point (from a high pressure center), the rightward Coriolis deflection causes a clockwise spiral outward.
Ocean currents, driven by wind and density differences, experience the same deflections.
Dear Tom,
Why is it that we get storms when a cold front arrives but not when a warm front arrives?
Steven Bloome
Dear Steven,
It's true that the weather in advance of a cold front is more turbulent and more likely to erupt into thunderstorms than warm frontal weather, but warm fronts do sometimes generate precipitation.
A front is the boundary between air masses of differing characteristics, most notably temperature. We call it a cold front when cooler air is advancing, and similarly for a warm front.
Stormy weather develops when air is lifted, but cold fronts are far better lifting mechanisms than warm fronts. Advancing cold air, heavier and more dense, turbulently forces its way under warmer air, thrusting it up and thereby triggering stormy conditions. Advancing warm air, lighter and less dense, glides gently over cool air, usually generating little more than clouds and steady rain.
Dear Tom,
My son, age 7, asked a seemingly simple question that has me stumped: How fast do raindrops fall?
--Ellen Sadowski
Dear Ellen,
It might seem that raindrops tumble from the sky at great speeds as they zip past our eyes, but photographic measurements show that raindrops actually fall at speeds ranging from about 5 m.p.h. to 20 m.p.h. Larger raindrops fall faster than smaller ones. German physicist Dr. Philipp Lenard made accurate measurements of raindrop sizes, shapes and fall speeds more than 100 years ago (between 1898 and 1904). He found that raindrops can attain sizes no greater than one-fifth of an inch in diameter (they become unstable and shatter at larger diameters) and the largest drops fall at about 20 m.p.h. Tiny, misty droplets of drizzle (0.02 inch in diameter or less) descend at 1 to 4 m.p.h.
Dear Tom,
Could the deep solar minimum be responsible for our cool, cloudy summer?
Michael Leach
Dear Michael,
The answer to your question varies in scientific circles. Storms on the face of the sun -- so-called sunspots -- have all but disappeared in recent years. On Sept. 3, NASA reported: "The sun is in the pits of the deepest solar minimum in nearly a century. Weeks and sometimes whole months go by without even a single tiny sunspot. The quiet has dragged out for more than two years." Astronomer Dan Joyce reports the last Earth-sized sunspot -- something which occurs with some frequency in typical years -- occurred in December 2006.
Some researchers believe lengthy solar minima triggers global cooling. Others dismiss solar cycles as having little more than a modest effect on the planet's climate. Global temperatures continue to run above normal, but there's no question subnormal summer temperatures have occurred over much of the U.S., though the West and Southwest U.S. have baked in record warmth.
Dear Tom,
Hurricane Bill made a landfall in Newfoundland last month. How rare is that?
Brian Rohde, Tinley Park
Dear Brian,
A hurricane affecting Newfoundland is not as rare as you might think. Its North Atlantic location puts it directly in the path of northward moving tropical cyclones that skirt the East Coast of the U.S.
According to Environment Canada, Newfoundland leads all Canada with a tropical cyclone impact every 1.4 years followed by Nova Scotia with an encounter every 2.4 years. Most of the storms hitting Newfoundland are in a decaying mode, a result of the surrounding cold water. However, one of the deadliest Atlantic hurricanes on record hit eastern Newfoundland on Sept. 9, 1775. More than 4,000 sailors perished in the storm that brought 20-30 foot seas to the area.
Dear Tom,
Did this summer crack the top ten of coolest summers in Chicago?
Tim Guimond
Dear Tim,
This summer was cool, with just four official days in the 90s and about triple the typical number of sub-70 days; but not only did it not crack the top 10, it didn't even make the top 20. The summer of 2009 with its average temperature of 69.2 degrees goes into the record books as the city's 21st coolest on record and the coolest since 2004 (68.6 degrees). Chicago recorded its all-time coolest summer way back in 1875 (66.6 degrees) at a time when the official thermometer was located downtown near Lake Michigan. In fact, 16 of the 20 cooler summers were recorded before the official thermometer was moved to a warmer inland location in 1942. Since that time, only the summers of 1982,1985, 1992 and 2004 have been chillier than this past summer.
Dear Tom,
I have heard that an El Nino is now occurring. What are the likely weather consequences?
David Liu
Dear David,
El Nino, a warming of tropical Pacific Ocean waters, causes changes in planetary wind patterns that have major effects on global weather. When strong El Nino events occur, as in 1997-98, temporary climatic disruptions are widespread and, in some locations, the consequences are devastating.
Climatologists estimate the 1997-98 El Nino brought abnormal weather to 80 percent of the Earth. At Chicago, the "El Nino winter" of 1997-98 was exceptionally mild and relatively storm-free, but elsewhere there were weather disasters. A few examples: Floods swamped California, Peru and central Africa; droughts ravaged southeast Asia, Australia and Brazil. Worldwide, the 1997-98 El Nino was responsible for 2,100 deaths and $33 billion in property damage.
