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What
is a thunderstorm?
Technically, according to the National Weather Service, a thunderstorm
occurs when an observer hears thunder. Radar observers use the intensity
of the radar echo to distinguish between rain showers and thunderstorms.
Lightning detection networks now routinely track cloud-to-ground flashes,
and therefore thunderstorms. Thunderstorms arise when clouds develop sufficient
upward motion and are cold enough to provide the ingredients (ice and
supercoooled water) to generate and separate electrical charges with the
cloud. The cumulonimbus cloud is the perfect lightning and therefore thunder
factory. That is why its is nicknamed the “thunderhead.”
What
is a severe thunderstorm?
By definition, the National Weather Service classifies a thunderstorm
as severe if it contains hail of three-quarter inches or larger and/or
winds gusts of 58 mph or higher. Severe thunderstorm watches, meaning
conditions are suitable for severe storm development during the next several
hours, are issued for areas several hundred miles on a side by the NWS
Storm Prediction Center in Norman, Oklahoma. A severe thunderstorm warning
is issued by the local National Weather Service Office, usually for several
counties or parts thereof for the next hour or so based upon spotter reports
of conditions exceeding severe levels and/or by radar indications of the
same. If there is a distinct threat of a tornado, a tornado warning is
issued.
What
creates a thunderstorm?
Warm, moist air rising in sufficiently large volume with a high enough
velocity results in a thunderstorm. The fuel for these storms is warm,
moist air present near the surface of the earth. If the atmosphere around
the cloud is unstable, that is the temperature of the air falls faster
than that of the rising parcel air within the storm, then the updraft
becomes ever more warmer than the air outside, and therefore more buoyant.
The release of latent heat when water vapor turns to liquid and then the
liquid to ice further warms the rising parcel, stoking the “fires”
of the updraft. A trigger is often necessary to get the warm bubble of
air rising in the first place. Sometimes it can be a warm air thermal
rising from a large, heated field or a sunlit mountain top, or the upward
motion produced by fronts pushing air together so it has no place to go
but up. GO
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What
type of cloud produces thunderstorms?
The cumulonimbus (Latin, for “rain heaps”) is the massive cloud
factory which spawns lightning and thunder. This cloud can span the depth
of the troposphere, with its base near the group, to its icy characteristic
“anvil” top, sometimes more than 12 miles high. In extreme storms,
updrafts can reach 100 mph. Downdrafts can often attain even greater speeds.
Aircraft avoid the cumulonimbus due to the extreme turbulence often found
inside.
What
are the ”four horsemen” of thunderstorms?
Floods, hail, lightning and wind/tornadoes. All are spawned by thunderstorm
clouds, sometimes all at the same time. The cumulonimbus cloud is an amazingly
efficient weather factory.
How
much energy does a thunderstorm release?
The energy in even a modest thundercloud can be impressive. The first
atomic bomb was detonated in the desert near Alamagordo, NM on 16 July
1945. Though the energy released was awesome, it was several times less
than that generated by the almost daily thunderstorms which dot the New
Mexico mountains on a typical summer day.
How
high do thunderstorm go?
Almost all thunderstorm clouds grow to heights above 20,000 feet. With
35,000 feet being typical. The more intense ones continue upwards until
they hit the top of the troposphere, called the tropopause. Since penetrating
into the stratosphere takes a lot of energy, many cumulonimbus clouds
flatten out on the tropopause into the classic anvil shape with the tip
streaming off downwind. If the storm is unusually intense, the updraft
may punch into the stratosphere in cauliflower-like turrets. These “trop
busters” are usually severe storms, with internal updrafts perhaps
exceeding 100 mph. At any given place and time the height of the tallest
storms is thus controlled by the height of the troposphere. Over the U.S.
the tops of the stronger storms range from 40,000 to 65,000 feet from
spring through summer and from north to south, respectively. There are
some radar reports of echoes exceeding 70,000 feet, but if these reports
are correct, this would be a very rare event. In any case, most thunderstorms
are high enough that commercial jet traffic does not fly over most storms
but rather circumnavigates since there can be “surprises” inside
thunderstorm tops including extreme turbulence, hail, lightning, and wind
shears. GO
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Are there different types of thunderstorms?
Basically any cloud that generates lightning, and therefore thunder, is
a thunderstorm. However, there are a wide range of sizes and shapes of
such storms, indicative of the atmospheric processes which give them birth.
There are dozens of informal classifications of storm types. The air mass
thunderstorm is perhaps the most common. It is relatively small and short
lived and forms in semi-random patterns within large, moist high pressure
systems. Sea breeze thunderstorms are indicative of their triggering mechanism,
as are cold frontal storms. Squall lines are long thin chains of storms,
which on occasion have been known to extend for more than 1000 miles along
or ahead of cold fronts. Larger, non-frontal thunderstorms are often called
mesoscale convective systems or, the biggest of them all, the mesoscale
convective complex (MCC). These monsters can be the size of several eastern
states and live for twelve hours or more. Supercells, which often rotate
as a whole, are often relatively small in size but long lived, often producing
tornadoes and major hail storms. Sometimes these cells split into two
pairs, one moving to the right and the other to the left. Trying to decipher
which “mode” of convection that the atmosphere will take on
a given day is one of the challenges of contemporary forecasting.
What
is a downburst and a microburst?
Inside a thunderstorm there are powerful updrafts and, as a the storm
matures, downdrafts (what goes up does come down). The updrafts can reach
many tens of miles per hour. Turns out storm downdrafts can be equally
as intense. The downdrafts are caused by factors such as the drag from
heavy masses of rain and hail, and especially the fact that falling precipitation
evaporates and cools the air, making it heavier than its environment.
Most thunderstorms generate downdrafts, the cooling, outward rushing air
that often breaks the heat of any oppressively hot summer afternoon. The
leading edge of the downdraft is called the gust front. It is sometimes
marked by spectacular cloud features called shelf or roll clouds. But
on some occasions, downdrafts can become locally very intense, slamming
into the surface with wind gusts well in excess of hurricane force. That
is a downburst. The smallest of these are called microbursts, some of
which may be only several hundred yards across. Recent research has shown
that much storm damage once ascribed to tornadoes is actually the result
of microbursts. Their winds can equal that of small tornadoes and, to
the untrained eye, the damage looks as if a tornado went through. They
can also be accompanied by very loud roaring noises. The second greatest
cause of aircraft accidents, after pilot error, are the extreme wind shears
associated with thunderstorm downburst winds. Since 1964, at least 29
major airline accidents have been caused by downbursts.
What
is a derecho?
A derecho (der-ray-cho) is one more type of severe storm. It is often
very long lasting, covering many hundreds of miles, while generating a
continuous series of very strong downburst winds. In July 1995 an extremely
severe derecho storm swept through upstate New York. Wind gusts up to
106 mph were recorded. Striking mostly in rural areas, the storm devastated
over one million acres of trees, felling tens of millions, piling trees
10 to 20 feet high in places. Worse, five people, campers, were killed
by the falling timbers. GO
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How
strong can thunderstorm winds be without a tornado?
Wind speeds above 120 mph in downbursts are not that uncommon. Downburst
winds were clocked at 136 mph at Grissom Air Force Base, Indiana in 1995,
with some estimates as high as 140 mph in Miami County, IN. The highest
recorded thunderstorm microburst wind ever clocked was a 149.5 mph gust
which struck Andrews Air Force Base near Washington, DC, causing considerable
damage... and occurred just five minutes after President Reagan landed
in Air Force One.
How
much does a rainstorm weigh?
One inch of rainfall over an acre of land weighs 226,000 pounds. If that
same rainstorm occurs uniformly over a ten by ten mile area, a relatively
modest size for a shower, we are dealing with 7.2 million tons of rain
water.
How
many thunderstorms occur each year?
Nearly 2000 thunderstorm cells are estimated to be present over the planet
at any given time. It is estimated that globally there are 16 million
thunderstorms each year. In the United States, central Florida has almost
100 thunderstorm days annually. Other areas with large numbers of thunderstorms
include much of the Gulf Coast region and the Rocky Mountains and adjacent
High Plains. Kampala, Uganda may hold the world record for thunderstorms,
averaging 242 rumbly days each year, though portions of Indonesia may
have more. Between 1916 and 1919, the city of Bogur averaged 322 thunderstorms
per year. There the rainy season could also be called the noisy season.
Should
you stand under trees during thunderstorms?
Most people seem to know (and ignore the fact) that they shouldn’t
stand near trees in a thunderstorm due to the lightning hazard. But there
is another reason. Trees blow down. In a recent 15 year period in Ohio
alone, over 40 persons were injured and at least 9 were killed by trees
toppling during thunderstorms. Many of the victims were in vehicles. If
you pull your car over during a downpour, be sure you are not in the potential
path of a tree which could fall as the next gust of wind strikes. In June
1993, a severe thunderstorm with several microbursts downed more than
13,000 trees in the northern part of Cincinnati and its suburbs. Over
60,000 utility customers lost power. A woman was killed when a tree crushed
her car, which she had parked because the rainfall was too heavy for driving.
GO
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How
can thunderstorms cause record low temperatures?
Record low temperatures do occur in Miami during the summer. But how?
Certainly not from Arctic cold fronts. The fact that the record lows usually
occur in late afternoon gives away the answer - thunderstorms. The rain-cooled
air masses in the thunderstorm’s outflow are often the source of
the coolest summertime readings in tropical areas. Temperatures only have
to be cooled into the low 70s or upper 60s in order for a new daily minimum
to be set.
Where
can you go to avoid thunderstorms?
Many people really enjoy thunderstorms, and some don’t. For the latter,
may we suggest St. Paul Island in the Bering Sea off Alaska. On 8 November
1992, they reported a thunderstorm - the first one in 40 years! The coastal
deserts of Chile and Antarctica are also pretty free of thunderstorms.
What
is hail?
Hail are more or less spherical balls of ice which fall from spring and
summer thunderstorms. Hail forms as the result of small frozen raindrops
or graupel being continuously recycled through multiple up- and downdrafts.
They continuously accumulate new layers of ice until they become so heavy
that they can no longer be supported. Hail is not to be confused with
sleet, frozen raindrops, which falls during winter storms.
What
are the impacts of hail upon crops?
Worldwide, annual crop losses from hail represent about 1% of the total
annual agricultural production. Many U.S. farmers insure their crops against
hail losses.
Can
hailstones speak German?
Yes, and they were crying for help. Five German glider pilots made the
mistake of flying into a thunderstorm over the Rhone Mountains in 1930.
Fearing their fragile craft might break up, they bailed out. They all
essentially became human hailstones encased in ice. All but one froze
to death.
Are
hailstorms a threat to animals?
The natural world can be a cruel place. On 15 July 1978 a hailstorm spewing
stones up to baseball size killed more than 200 sheep in Montana. A hailstorm
in Alberta on 14 July 1953 was found to have killed some 36,000 ducks
and thousands of other birds. As if that weren’t enough, another
storm several days later killed another 30,000 ducks in the same area.
GO
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Are
humans at risk from hailstorms?
It is rare in the U.S. for people to be killed by hail. But on 14 April
1986, 92 persons perished in a hailstorm around Gopolganj, Bangladesh.
Some of the individual stones were reported to weigh in at 2.25 pounds!The
deadliest hailstorm on record? Perhaps it was on 30 April 1888, in the
Moradad and Bareilly districts of India where 246 people perished. There
are reports of 100 persons killed and 9000 injured in Sichuan Province,
China on 22 March 1986. By contrast there have been only several known
hail fatalities in the U.S. this century. One occurred on 30 July, 1979
in Fort Collins, CO, where a baby was struck by grapefruit sized hail,
to the despair of its mother who was rushing to carry the child to safety.
In addition, 25 others were injured, including an 84 year old man whose
arm was broken. Considering a baseball-sized hailstone hurtles out of
the sky at the speed of a major league fastball, one might wonder, since
most of us don’t wear batting helmets, that there haven’t been
more hail related tragedies. Several were killed in a Dallas-Ft. Wort,
TX hailstorm in 1995.
What
part of the U.S. has the most hail?
About 4800 hailstorms strike the nation every year. Of these perhaps 500
to 700 produce hailstones large enough to cause damage or injury. Hailstones
form within thunderstorms. Yet the region with the greatest number of
thunderstorms, Florida, has one of the lowest hail rates in the nation
(less than one event per year at any one place). Cheyenne, WY is the U.S.
city with the most hailstorms, averaging about ten hail days per year.
Hail is most frequently found in “hail alley” which covers portions
of eastern Colorado, Nebraska and Wyoming. It is also rather common throughout
the High Plains, the Midwest and sometimes the Ohio Valley. The lowest
frequency of hail is along the immediate Pacific shoreline. What hail
falls on the West Coast is usually small, accompanying the winter thunderstorms
that blow inland during winter storms.
What
are other hail prone parts of the world?
Northern India appears to be the region where the greatest frequency of
extremely large hail events occur. India also appears to have the dubious
distinction of having the most human fatalities from hail. The world’s
“hail belts” are generally found at mid-latitudes, often downwind
of large mountain ranges. Among the “hailiest” places are the
High Plains of the U.S. and Canada, central Europe eastward to Ukraine,
the Himalayan region, southern China, and portions of Argentina, South
Africa and southeastern Australia. Keriche, Kenya averages 132 days per
year with hail. This may be by far the highest frequency anywhere in the
world. One theory holds that large amounts of pulverized tea leaf litter
from the local tea plantations get stirred into the atmosphere and serve
as excellent "ice nuclei" once in the rain clouds overhead.
Iced tea, anyone? GO
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Are
all hailstones round?
More often than not, hailstones are basically spherical. But not always.
They sometimes have strange protruding spikes (punk hail?). Sometimes
they are star shaped or highly oblong. And in 1979, in Norwich, England,
a nearby thunderstorm generated flakes of ice about 2 by 4 inches in size
which fluttered out of the sky like falling leaves
What
do the rings in hailstones mean?
Alessandro Volta, in the year 1806, is believed to be one of the first
scientists to study hail stones. Upon cutting some specimens open he noted
the alternating bands of clear and cloudy ice, indicative of multiple
trips through up- and down drafts before the stone became too heavy to
be supported by the storm’s updrafts. He also correctly speculated
that at the center of each stone is an “embryo” often composed
of a snow flake or a frozen raindrop. As many as 25 concentric rings have
been counted in larger specimens.
The
most damaging hailstorm ever?
It would appear that Munich, Germany holds that dubious distinction. In
1984 a massive hailstorm caused at least $1 billion in damages. A hailstorm
pounded the Colorado Front Range, including the City of Denver, on 11
July 1990. The resulting property losses were at least $625 million. Forty
seven people were injured at an amusement park, some seriously, when a
power failure trapped them on a Ferris wheel and they were battered by
softball-sized hail. They were not amused. A 1995 hailstorm in the Dallas-Ft.
Worth area is reputed to have been even costlier. GO
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How
are hailstone sizes categorized?
There was a lot of hail in the central U.S. during the hot month of July
1991. Marble-sized hail reports were legion. Golf-ball-sized hail was
reported in Illinois, baseball sized hail struck in Iowa, and softball-sized
hail fell in Colorado. But one unconfirmed report of basketball-sized
hail near Manhattan, IL leaves one suspecting a serious case of hoop dreams.
Weather observers report hail stone sizes either directly (three quarter
inch) or in the following equivalents:
0.25 inches pea
0.50 marble
0.75 penny/large marble/dime
0.88 mothball/nickel
1.00 quarter
1.25 half dollar
1.50 walnut
1.75 golf ball
2.00 hen egg
2.50 tennis ball
2.75 baseball
3.00 tea cup
4.00 grapefruit
4.50 softball
These classifications are used in the U.S. Other countries probably use
different terms (hail the size of wontons in China?). GO
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How
large can hailstones become?
No one knows for sure the absolute upper limit of hailstone size. In the
spring of 1995, reports circulated on the Internet from the Guandong Province,
China of falling hailstones, some of which were reported to be the size
of basketballs! The Chinese story does seem not very credible.
Keeping in a sports
vein, there are credible reports of baseball-sized hail failing in Texas
in 1995. More specifically, it fell on the Ballpark at Arlington. To be
precise, it fell DURING a Texas Rangers/ Cleveland Indians baseball game.
And it should be noted that baseball size hail falls at about the same
speed as Nolan Ryan's fastball in his prime.
The heaviest documented
hailstone ever found thudded to earth on 3 September 1970, in Coffeyville,
Kansas. It was 5.7 inches (14.5 cm) across and 17.5 inches in circumference (44.5 cm) and weighed 1.67 pounds. A slightly bigger stone fell in Aurora, NE on 22 June 2003 that was 7 inches (17.8 cm) across and 18.75 inches (47.6 cm) in circumference, though the weight has been reported as just under one pound.The low weight is likely due to the fact that part of the ice was missing due to the stone bouncing off a gutter first. On 6 July 1928, Potter, NE logged a stone weighing 1.5 pounds (680 grams) that was 7 inches in diameter (17.8 cm). The record
Canadian hailstone fell in Cedoux, Saskatchewan, and weighed in at 10.23
ounces. The largest hailstone that has ever fallen? Reports of hailstones
in India “the size of elephants” have been dismissed. But a
1925 report from Germany of a single stone weighing 2.04 kg (some 4 pounds!)
is believed to be at least possible. It could have been several large
hailstones frozen together. Elephants? Lord, we hope not.......
And then on 30 April
1985, a 13 year old boy in Hartford, CT was “startled,” to put
it mildly, when a 1500 pound block of ice fell from the sky into his back
yard near where he was playing with a friend. The source of the 6 foot
long and 8 inch thick slab which probably had an impact speed of 200 mph
was never identified but it was not a natural hailstone. Leakage from
the plumbing of a passenger plain is the most likely explanation. GO
TO TOP
What
are some record heavy rains over short time periods?
It rained 1.23 inches on 4 July 1956 in Unionville, MD. Nothing unusual
about that you say. Well, it fell in exactly 60 seconds-still the nation’s
one minute rainfall record. Barst, Guadaloupe, West Indies was reputed
to have receive 1.50 inches in one minute on 26 November 1970.
Many parts of the
western United States routinely receive less than 12 inches of precipitation
in an entire year. But that amount of rain fell in 42 minutes on Holt,
MO on 22 June 1947. And 15.78 inches deluged Muduocaidang, Inner Mongolia,
in just 60 minutes on 1 August 1977. Alvin, Texas has the distinction
of the heaviest 24-hour rainfall total in the U.S., a whopping 43 inches.
The Canadian 24-hour record is 19 inches, on Vancouver Island in British
Columbia.
Of course, record
rainfalls are all relative. Near-record Setting Rains Lash Los Angeles....
so the headlines could have screamed on 8 July 1991. A "deluge"
of 0.13 inches fell, the most in a July "storm" since the quarter
inch "inundation" of 1886. But while the amount was puny, the
event was noteworthy since it has only rained 13 times in July in downtown
LA since 1877. GO
TO TOP
From
the truly strange weather log book:
In December, 1933, huge hailstones were reported to have fallen around
Worcester, MA-containing fresh, frozen ducks inside. Daffy weather, indeed
A thunderstorm that
occurred in Germany on 9 August 1892 resulted in a heavy fall of rain...and
hundreds of fresh water mussels .
A thunderstorm near
Vicksburg, MS in 1930 resulted in some rather interesting hailstones.
One had a solid piece of alabaster 3/4 inch across. Another was a live
8" long gopher turtle, entirely encased in ice.
Bucharest, Romania
reported a rainstorm on 25 July 1872. But along with the rain fell myriads
of small black worms that littered the streets.
On 17 July 1841, a
shower of heavy rain and hail in Derby, England was accompanied by hundreds
of small fish and frogs, many of them very much alive.
Scientific American,
in 1877, reported that a rain of snakes, some up to 18 inches long, fell
over the southern part of the city of Memphis, Tennessee.
A new form of cloud
seeding? In 1687, hail fell in England containing the seeds of ivy berries.
Toads.Toads.Toads.
In 1953 the town of Leicester, MA was deluged by a fall of toads. Children
were able to gather them up by the bucketful.
The term hailstone
is a bit of a misnomer since no “rocks” are involved. However,
in Sweden in 1925 a large chunk of limestone did fall from the sky, smashing
into myriad pieces upon impact.
A peach of a storm.
In 1961, a portion of Shreveport, LA was pelted by small, unripe peaches
during a passing thunderstorm.
In Spain, in 1902,
a rain shower was observed in which the rain drops, upon touching the
ground, gave off a crackling noise and emitted electrical sparks. The
event lasted for less than ten seconds.
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