ASK TOM WHY - WGN Weather Center Blog

The Myanmar cyclone disaster

Dear Tom,

The Myanmar cyclone disaster is horrific. What could account for such a catastrophe?
Steve Bartik

Dear Steve,
The disaster in Myanmar, formerly known as Burma, is staggering. The official death
toll, now at more the 20,000, could climb to 100,000 as a worst-case possibility, said
Shari Villarosa, the U.S. chargé d’affaires in Yangon.

On the evening of May 2, Cyclone Nargis moved from west to east along Myanmar's
southern coast. It brought sustained winds of 125 m.p.h. and a 12-foot storm surge
that swept across the flat, low-lying and densely populated delta of the Irrawaddy
River.

The river branches into several large channels that drain south into the Indian Ocean.
Unfortunately, the north/south orientation of those channels allowed the cyclone's
onshore winds to push storm surge many miles inland.

Posted by wgnweather at 9:56 PM | Permalink | TrackBacks (0)

When is Chicago's "growing season"?

Dear Tom,
Please define the "growing season." When does it start and end?
Gregg Dubin, Huntley, Ill.

Dear Gregg,

No single definition of "growing season" is satisfactory for all situations.

Generally speaking, the growing season is the period of the year during which air
temperatures remain continuously high enough for the growth of cultivated plants.

A commonly accepted standard is the number of days between the average dates of
the last spring and first fall occurrences of a temperature of 32 degrees. By that
measure, the growing season varies from 191 days (April 17 to Oct. 24) along
Chicago's lakefront to 162 days (May 1 to Oct. 9) in outlying suburban locations.

Ideally, the growing season should be defined biologically rather than
meteorologically because plant species vary greatly in their tolerance to cold
temperatures.

Posted by wgnweather at 9:16 PM | Permalink | TrackBacks (0)

Does large hail always come before a tornado?

Dear Tom,
Weather-knowledgeable friends say large hail always comes before a tornado, but I
wonder about that. Is it true?
Peter Micham, southwest suburbs of Chicago
Dear Peter,
It is not true. Only the wind fields within and in the area several miles around a severe
thunderstorm provide reliable clues as to the storm's tornado-producing capabilities.
Unfortunately, those telltale signs are rarely apparent to an observer. Only Doppler
radars can detect them.

Severe storms forecaster Roger Edwards of the National Weather Service's Storm
Prediction Center in Norman, Okla., says: "Does hail always come before a tornado?
Rain? Lightning? Utter silence? High winds? Not necessarily for any of those. Hail can
indicate the presence of an unusually dangerous thunderstorm, but it is not a reliable
predictor of tornado threat."

Posted by wgnweather at 9:53 PM | Permalink | TrackBacks (0)

Sound travel at night vs. day

ATW GRAPHIC HEADER LINK:

Dear Tom,

I've noticed that sounds carry better at night than during the day. Could you provide
an explanation?

Louise, Park Forest

Dear Louise,

Sounds do carry better and farther at night. Because it tends to be quieter then,
individual sounds are easier to distinguish. By far the most important factor, however,
is a difference in the temperature structure of the lower few hundred feet of the
atmosphere between day and night.

Sound travels through cold, dense air more slowly than through warmer, less dense
air. When air temperatures change on the path along which sound waves are
traveling, the waves always bend toward the colder air.

By day, it's warmer near the ground and colder above; sound bends up and away from
the ground (and you). At night, it's colder near the ground and warmer above; sound
waves bend down.

Posted by wgnweather at 9:33 PM | Permalink | TrackBacks (0)

Hail in the Great Plains

Dear Tom,
On a visit to the Black Hills of South Dakota, my family and I noticed outdoor neon signs
had screens over them for protection from hail. Is hail that frequent there?
--James Walker, Madison, Wis.
Dear James,
It is. Large hail is massively damaging. It flattens crops, shatters windows, dents cars and
kills birds and small mammals. Chicago experiences an average of two days per year with
hail. Most of it is small and causes no damage. But the frequency of hail, and especially
the frequency of large, damaging hail, increases steadily westward from Chicago. Hail is a
costly problem in the Great Plains, especially across Wyoming and Colorado. Cheyenne,
Wyo., with nine hail days annually, is the "hail capital" of the nation. The Black Hills, 200
miles northeast of Cheyenne, logs six hail days per year.

Posted by wgnweather at 10:36 PM | Permalink

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May 8, 2008

The Myanmar cyclone disaster

May 7, 2008

When is Chicago's "growing season"?

May 6, 2008

Does large hail always come before a tornado?

May 5, 2008

Sound travel at night vs. day

May 4, 2008

Hail in the Great Plains

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