Geoscience Reference
In-Depth Information
turned many off sailing forever, but was also astonish-
ing in its intensity. A rescue helicopter provided one of
the highest measurements of a wave ever recorded. As
it hovered 30 m above a stricken yacht buffeted by high
winds, a single wave, for a brief second, rose suddenly
towards the helicopter and touched its skids. However,
the worst failing surrounding the race was the fact that
no warning was given at the final, pre-departure
briefing - despite ominous signs that an unusual
meteorological situation was developing.
The storm began with the passage of a cold front
across the south of the continent on Christmas Day, the
day before the race began. At the same time, an east-
coast low had developed off the Queensland coast with
a central pressure of 992 hPa. This latter low moved
south-west and linked up with a low-pressure cell
preceding the cold front. In the early hours of
27 December, both cells merged in Bass Strait between
the mainland and Tasmania, to explosively produce a
complex storm cell with a central pressure of 982 hPa.
This cell then drifted east into the Tasman Sea and the
path of most of the yachts sailing down the coast. Tem-
porarily, the center of the maelstrom developed an eye
structure characteristic of tropical cyclones. As it did so,
the fleet was hammered by strong westerly winds
gusting to 167 km hr
-1
. At this time, average maximum
wave heights reached 14 m, with isolated rogue waves
theoretically reaching the 30 m height cited above.
Many of the waves were over-steepened, probably by
the East Australian current moving in the opposite
direction. Not since the 1979 Fastnet Race off Britain -
in which a freak storm killed 15 competitors - had the
best intentions of organizers of a yacht race been so
destroyed. Six people drowned in the Sydney Hobart
storm, five boats sank, and fifty-five sailors were rescued
under some of the most treacherous conditions ever
experienced by helicopter rescue crews. The subse-
quent coroner's inquiry found that the Cruising Yacht
Club of Australia, which organized the race, was blind
to reports of impending disaster and had 'abdicated its
responsibility to manage the race'.
flooding, conditions will always be worse if the precip-
itation falls in the form of snow. The fact that snow
volume exceeds rainfall by a factor of 7-10 implies that
even small amounts of precipitated water falling as
snow can totally paralyze large sections of a continent.
Even if the volume of snow in individual storms is
small, it can accumulate over many falls to present a
serious flood hazard lasting one to two weeks during
the spring melting season. If snowstorms occur too
frequently, then the effects of the previous storm may
not be cleared away and urban transport systems can
be slowly crippled. Whereas snow generally incapaci-
tates transportation, freezing rain can cause severe and
widespread damage to power transmission lines.
Most large snowstorms originate as mid-latitude
depressions following the
meandering
path of the jet
stream across the continents. In winter, the jet stream
tends to be located across North America and Europe,
so that the area affected by snowstorms is likely to
persist over periods of several weeks. There is also the
risk that abnormal or unseasonable jet stream paths
can cause snowstorms to occur in areas unprepared for
them. In North America, the preferred path for winter
snowstorms follows jet stream looping down over
the United States Midwest towards Texas and
then northward parallel to the Appalachians, Great
Lakes-St Lawrence River Valley, and eastward over
Newfoundland. This jet stream path is topographically
controlled by the Appalachian Mountains. Movement
of the jet to the eastern side of the Appalachians can
bring exceptionally heavy snowfalls to the east coast
of the United States. In Europe, the jet stream tends
to loop down over England and northern Europe.
Movement southward is partially hampered by the
Alps. Extended looping southward can bring very cold
conditions to northern Europe, and unseasonable
snowfalls to the Mediterranean region of Europe.
In winter, mid-latitude cyclonic depressions can
affect very large areas of the continental United States.
Figure 3.20 shows areas of intense snowfall and the
development of a low-pressure cell. In this diagram,
rain is falling in the warm part of the air mass. As the
warm air is pushed up over the cold air, there is a small
zone of freezing rain (to be discussed in detail later). As
the air is forced higher along the warm front, temper-
atures drop below freezing point and precipitation falls
as snow. The area in front, and to the poleward side of
the advancing warm front, is the area of heaviest
snowfall, which can cover a distance of 1000 km. Cold
SNOWSTORMS, BLIZZARDS
AND FREEZING RAIN
Sno
wstorms
(Rooney, 1973; Whittow, 1980; Eagleman, 1983; Lott, 1993)
While many mid-latitude cyclonic depressions can give
rise to exceptionally heavy rain and widespread
