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near the base of the mountains. This increase in pressure results in air starting
to
flow away from the barrier, opposite to its original direction, from higher pressure
towards lower pressure. Under the in
'
s rotation the westerly
wind turns to the left, eventually turning parallel to the topography, and
becoming a southerly wind. These terrain parallel winds are referred to as
barrier winds. Southerly barrier winds are common on the eastern side of the
Antarctic Peninsula and on the east side of the Transantarctic Mountains, which
form the western boundary of the Ross Ice Shelf.
uence of the planet
Precipitation: why is Antarctica so dry?
On average Antarctica is the driest continent, with average annual precipitation
of 13 cm, and can be considered a desert. Why is Antarctica so dry? As discussed
earlier, the Antarctic continent is located in an area of sinking motion, associated
with the polar cell. Sinking motion in the atmosphere tends to cause clouds to
dissipate, and suppresses the development of precipitation. This, in large part, is
why Antarctica gets so little precipitation. In addition the atmosphere tends to
contain less water vapour as temperature decreases and as the coldest continent
the Antarctic atmosphere also contains very little water vapour.
The fact that Antarctica can be considered a desert may seem surprising given
that 99% of the continent is permanently covered by ice. How could thousands of
metres of ice come to exist on the Antarctic continent when just over 1 cm of
precipitation falls per year in the interior of the ice sheet? The answer to this
riddle is that what little snow does fall over the interior of the continent does not
melt, and thus can accumulate year after year, century after century, and millennia
after millennia. Over these long periods the small annual snowfall can accumulate to
great depths, resulting in an ice sheet that is more than 4 km thick at some locations.
In fact, ice retrieved from ice cores at Concordia Station on the East Antarctic
plateau has been found to be over 720 000 years old.
If Antarctica is the driest continent, in terms of annual precipitation amount,
is the continent dry or humid when one considers the amount of water vapour in the
Antarctic atmosphere? The answer to this question is that it is both very dry and also
quite humid, at the same time and place. How can this be? When meteorologists
discuss humidity they can refer to either absolute or relative measures of humidity.
An absolute measure of humidity is an indication of the actual amount of water
vapour present in the atmosphere. From this perspective Antarctica is quite dry.
As air temperature decreases the air is able to contain less water vapour. At the
very cold temperatures found in the Antarctic atmosphere during the winter the air
can contain less than 1% of the water vapour that could be contained in air on a
warm summer day in the mid-latitudes.
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