Geoscience Reference
In-Depth Information
blizzard. A military operation had to be mounted, with
2000 troops digging out the city. Not only was Buffalo
cut off from the rest of the world, it was also completely
immobilized within. Over one million people were
stranded in streets, offices, their own homes, or on
isolated farms. It took two weeks to clear the streets. To
minimize the local spring flooding risk, snow had to be
removed to other parts of the country using trains. The
final clearance bill totalled over $US200 million, more
than five times the budgeted amount for snow removal.
In contrast, the city of Hamilton was relatively
unaffected by this blizzard, and ended up loaning snow
removal equipment to Buffalo as part of an inter-
national disaster response.
The Buffalo blizzards of 1976-1977 illustrate the
additional hazard caused by abnormal amounts of
snow: flooding in the subsequent spring melting
period. While snow accumulation can be easily
measured, the rate of spring melting is to some degree
unpredictable. If rapid spring warming is accompanied
by rainfall, then most of the winter's accumulation of
snow will melt within a few days, flooding major rivers.
The historic flooding of the Mississippi River in the
spring of 1973 occurred as the result of such con-
ditions, after a heavy snowfall season within its
drainage basin. Snowmelt flooding is a major problem
in the drainage basins of rivers flowing from moun-
tains, and in the interior of continents with significant
winter snow accumulation. This aspect is particularly
severe in the Po Basin of northern Italy, along the
lower Rhine, and downstream from the Sierra Nevada
Mountains of California. On a continental scale, the
flooding of the Mississippi River and its tributaries
is well known. However, for river systems draining
north to the Arctic Ocean, snowmelt flooding can be a
catastrophic hazard. Because the headwaters of these
rivers lie in southern latitudes, melt runoff and ice
thawing within the river channel occur first upstream
and then downstream at higher latitudes. The ice-
dammed rivers, swollen by meltwater, easily flood
adjacent floodplains and low-lying topography. In
Canada, spring flooding of the Red River passing
through Winnipeg, and the Mackenzie leading to
the Arctic Ocean, is an annual hazard. Nowhere is the
problem more severe than on the Ob River system
in Russia. All northward flowing rivers in Russia
experience an annual spring flooding hazard; but on
the Ob River, the problem is exacerbated by the
swampy nature of the low-lying countryside.
Blizz
ards
(Orville, 1993; Environment Canada, 2002; Henson, 2002)
There is one special case of snowfall which does not
depend upon the amount of snow falling but upon the
strength of the wind. The term 'blizzard' is given to any
event with winds exceeding 60 km hr
-1
, visibility below
0.4 km for more than three hours because of blowing
snow, and temperatures below -6°C. The word
derives from the description of a rifle burst and was
first used to describe a fierce snowstorm on 14 March
1870 in Minnesota. The term 'purga' is used in Siberia.
A severe blizzard occurs if winds exceed 75 km hr
-1
with temperatures below -12°C. Strong mid-latitude
depressions with central pressures below 960 hPa (less
than some tropical cyclones), and accompanied by
snow, will produce blizzard conditions. On the prairies
of North America strong, dry north-westerly winds
without snow are a common feature of winter out-
breaks of cold Arctic air. These winds can persist for
several days. Snow is picked up from the ground and
blown at high velocity, in a similar manner to dust in
dust storms. Surprisingly, the death toll annually from
blizzards in North America is the same as that caused
by tornadoes. The 28 January 1977 blizzard at Buffalo,
described above, resulted in over 100 deaths across the
eastern United States, and produced winds in excess of
134 km hr
-1
. In March 1888, the late New England
winter snowstorm mentioned above turned into a
blizzard as winds gusted 128 km hr
-1
. Temperatures in
New York City dropped to -14.5ºC - the coldest on
record for March.
The main hazard of blizzards is the strong wind,
which can drop the
wind-chill factor
, the index
measuring equivalent still-air temperature due to the
combined effects of wind and temperature. Wind chill
quantifies terms such as 'brisk', 'bracing' and 'bone-
chilling'. Since 1973, wind chill has been measured
using the Siple-Passel Index based upon how long it
takes to freeze a water-filled plastic cylinder positioned
10 m above the ground. It is assumed that, at temper-
atures of less than -35°C, lightly clothed skin or bare
flesh will freeze in 60 seconds. If body temperature
drops by more than 5°C, hypothermia and death will
result. However, water in a plastic cylinder bears no
relationship to reality. In addition, body heat forms a
layer of warm air adjacent to exposed flesh. This layer
is dispersed faster by increasing wind speeds. Finally,
there is biofeedback such that flesh exposed to wind
