Climate of extremes - Antarctica: Global Science from a Frozen Continent - page 115

Figure 4.12 Annual cycle of monthly

mean temperature at the South Pole.
The coreless winter of uniformly cold
temperatures from April to September
is clearly shown in this plot.

-20

-25

-30

-35

-40

-45

-50

-55

-60

-65

Jan Feb Mar Apr May Jun

Month

Jul Aug Sep Oct Nov Dec

standing on the high plateau of the Antarctic continent during the winter that the
temperature could increase 5 C or more from your feet to the top of your head.
Another unique feature of the cold Antarctic winter temperatures is their
duration. In mid-latitudes temperatures gradually decrease through the autumn
with the coldest temperatures of the year con ned to one or two mid-winter months.

In the Antarctic, once the sun sets in March, temperatures quickly drop in early
winter and remain nearly constant for 4 to 5months of the Antarctic winter, at
extremely cold temperatures, only to rise once the sun returns in the spring. Early
Antarctic meteorologists called this unique seasonal change in temperature with
a long period of nearly constant very cold temperature the ' coreless winter ' .

Wind: why is Antarctica so windy?

Everyone knows that warm air rises. This is true in the atmosphere and is how
thunderstorms form. Analogous to this is the concept that cold air sinks. This
rising or sinking motion has to do with the density of the air. As air cools it becomes
more dense and as it warms it becomes less dense. In a fluid refers to

both liquids and gases, less dense objects rise and denser objects sink. This can be
seen with a glass of water. An ice cube placed in the glass of water will

fluid, where

oat because
ice is less dense than liquid water. Similarly if a pebble is placed in the glass of
water it will sink since the pebble is denser than the liquid water. In the atmosphere
horizontal temperature contrasts lead to density contrasts which result in less
dense warm air rising and denser cold air sinking.
Consider a location over the sloping edge of the Antarctic ice sheet. Air in
contact with the ice sheet will cool by contact with the radiatively cooling ice surface.
As one moves horizontally away from the ice surface the air becomes warmer. This
horizontal temperature contrast creates a density contrast, with dense air adjacent to
the ice sheet and less dense air further from the ice sheet. This dense air will sink,
accelerating down the surface of the ice sheet to produce a wind known as a katabatic wind.