Geoscience Reference
In-Depth Information
Snow
Low
Depression
Freezing rain
Rain
> 0˚ C
< 0˚ C
< 0˚ C
A
B
Snow
Snow
Rain
Freezing
rain
Schematic representation of precipitation patterns for a mid-latitude cyclonic depression in winter in the northern hemisphere.
Fig. 3.20
air is rapidly pushed into the low behind the warm air
mass. In this region, there is rapid uplift of moist, warm
air. However, condensing moisture is rapidly turned to
snow. Here, snow may fall during thunderstorms, but
the amounts are generally less than that accumulating
in the path of the warm front. In exceptionally intense
cases, another cyclonic frontal system can develop
to the equatorial side of the first, forming twin low-
pressure cells.
Figure 3.21 is typical of the big snowstorms to have
affected eastern North America in the late twentieth
century. It illustrates a cyclonic situation that brought
snow to the eastern half of the United States at the end
of February 1984, paralyzing transport. The eastern
low was associated with very warm air from the Gulf of
Mexico that brought widespread rain to the southern
part of the United States. The western low was domi-
nated by uplift of moist air along the polar front. Heavy
snow extended across 1500 km, from New York to
Chicago, and from Canada to the southern states. In
some cases, as the cyclonic depression moves down the
St Lawrence Valley and occludes, spiraling winds may
turn the snow-bearing cloud back over parts of the
continent already affected by the main storm, and
prolong the fall of snow.
interact with moisture-bearing air originating from the
Gulf of Mexico. One of the worst snowstorms was 'The
Great Snow of 1717', which resulted from four late-
winter snowstorms that dumped 1.5 m of snow. New
England's Blizzard of 1888 dropped between 1.0 and
1.5 m of snow over several days. It is most notable for
the high winds that turned it into a
blizzard
. A century
later the 12-15 March 1993 'Storm of the Century'
replicated the havoc. This latter storm covered twenty-
six states - from Texas to Maine - and the eastern
provinces of Canada. The storm equated to a category
3 hurricane on the Saffir-Simpson scale. It began in
the western Gulf of Mexico, tracked across Louisiana,
and then rapidly intensified up the eastern seaboard
before moving north-east over eastern Canada. It
generated maximum winds of 232 km hr
-1
at Mt
Washington, waves 19.8 m high off Nova Scotia, a
3.7 m storm surge in Florida and a minimum pressure
of 961 hPa over New York State. The snowfalls were
most notable, reaching 1.4 m in Tennessee. These
occurred with record-breaking cold temperatures in
conjunction with power failures that left three million
people without heating. The storm killed 270 people,
paralyzed communications and air traffic, and impaired
people's ability to get to work. Fifteen tornadoes
formed over Florida, accounting for 44 of those killed.
The storm cost $US6 billion, making it the most expen-
sive extra-tropical storm in United States history.
Three years later, on 6-8 January 1996, a similar storm
Notable events
The United States is most susceptible to large snow-
storms because of the ability of mobile polar highs to
