Geoscience Reference
In-Depth Information
d i s c o v e r…
FRIGID TEMPERATURES ON MOUNT WASHINGTON
Where do you think you would find the worst weather on
Earth, the North or South Pole? You might be surprised to
learn that many meteorologists think the harshest condi-
tions are found in New Hampshire on Mount Washington,
which rises 1917 m (6,288 ft) in the White Mountains. Here
the polar jet stream routinely slams directly into the highest
point of land along the eastern United States, with results
that rival the brutal conditions in Antarctica.
The highest wind speed ever recorded anywhere
on Earth was measured at 372 km/h (231 mi/h) at the
weather station on Mount Washington's summit in 1934.
Annual average temperature at the summit is only -3
o
C
(26.5
o
F), with annual average snowfall of about 6.5 m
(21 ft). One year the snowfall on Mount Washington was
over 14 m (47 ft)! Visitors often marvel at the pictures
showing the summit buildings covered in rime ice, which
forms when fog droplets freeze as they come into con-
tact with structures. The rime ice grows out into the wind,
which is the direction from which the particles are com-
ing, giving it a feathery appearance. Since the summit is
covered with dense fog about 60% of the time, the ice
builds up into thick layers.
(a)
(b)
(a) Tuckerman's Ravine is famous for spring skiing. (b) Rime ice on the observatory on Mount Washington. Note the view of clouds
below the summit.
With the onset of meridional flow, warm air on the east side
of the wave pushes poleward because the winds are southerly.
At the same time that this northward push of warm air occurs,
cold air from the north plunges southward on the west side of
the system, where the winds are northerly. Such an influx of
cold air into a region is called an
Arctic outbreak
and can result
in extremely cold air reaching latitudes far south of its origin.
If you have spent any winters in the northern part of the United
States, you probably know that an Arctic outbreak can bring
frigid temperatures into the region for a long time. An extreme
Arctic outbreak can even bring freezing temperatures as far
south as Florida, which can threaten its agriculture. As time
progresses, the cold pool of air can literally be pinched off from
the main body of arctic air, resulting in the reestablishment of
zonal flow conditions. Such a pool of cold air can persist in
a region for several weeks, gradually warming because of the
higher Sun angles at these somewhat lower latitudes.
Polar Circulation
In contrast to the complex patterns associated with tropical and
midlatitude circulation, atmospheric circulation in the polar
regions is associated with a simple circulatory loop known
as a
polar cell
. Air that flows northward at the polar front
cools considerably and subsequently descends at very high
latitudes, producing a weak high-pressure system (see Fig-
ure 6.16 again). This system is called the
Polar High
and
Zone of high atmospheric pressure at high
Polar High
latitudes.
