Weather

View from the Amundsen-Scott Station observation deck – MAR 2012

When most people think of the South Pole, often their first thought is of its defining feature: weather…extreme weather!  I would describe the weather here as “seasonal.”  In the summer it is cold and sunny; in the winter it is colder and dark.  There might be some transient changes in cloud cover, barometric pressure, or the occasional wind storm, but for the most part, you know the forecast without having to ask.

 So why is Antarctica so cold?  Temperatures have reached minus 117 degrees Fahrenheit below zero.  One reason is the permanent ice cover that reflects 80% of the Sun’s rays back into space.  Another is the shallow angle of the sun even in summer.  The Sun is never higher than 23.5 degrees (the Earth’s tilt on its axis) above the horizon, unlike locations near the equator which receive the highest concentration of the Sun’s rays.  Antarctica is even colder than the Arctic since the Arctic is ocean surrounded by land, and therefore warmer as a result of the moderating effect of the relatively “warm” ocean water.  The Antarctic is land surrounded by water so the interior portions do not benefit from the moderating effects of the ocean.  During the winter, Antarctica doubles in size as the seawater at the edges of the continent freezes.  This effectively blocks heat from the warmer surrounding seawater.

Sea Ice in Winter & Summer – http://www.grida.no/publications/geo-ice-snow
 Unlike anywhere else in the world, a forecast of high pressure at the South Pole means bad weather and low pressure indicates good weather.  The reason for this is the fairly permanent temperature inversion.  In most places on the planet, air gets colder with increasing altitude; here it gets warmer.  The air at the pole is much colder at ground level than it is at higher altitude.  The temperature inversion layer is usually less than 1000 feet thick, but the temperature difference can be as much as 54 degrees Fahrenheit!  High pressure weather systems result in a downward vertical air motion bringing warmer, more humid air causing increased clouds and worsening weather.  Low pressure weather systems produce upward vertical air movements, which brings in colder, drier surface air to replace the air that is being forced upward.

Temperature Inversion Keeps Generator Plume Near Surface

 Except on the coldest days, we usually have steady, low velocity winds.  These “katabatic” winds are produced when cold, dense air flows down the polar plateau or down the slopes of the inland mountains. 

How Katabatic Wind Is Created

While usually not much of a problem at the pole, along the coastal areas they can produce hurricane force winds.  If the winds become turbulent, they can sweep up loose snow and cause a localized blizzard, where the skies are clear and no snow actually falls to the ground, but horizontal visibility is markedly reduced. 

Calm wind conditions at the Ceremonial South Pole – FEB 2012

What is the difference between a blizzard and a whiteout?  Blizzards are defined as winds greater than 35 miles per hour with visibility less than ¼ mile due to snow or blowing snow for at least 5 hours.  A whiteout is a condition where a uniformly gray or white sky over a snow covered surface causes a loss of depth perception and surface definition.  Winds can be calm, but you can still have a whiteout.

Whiteout Conditions South Pole FEB 2012

 As mentioned in a previous blog, the air is extremely dry at the South Pole.  This is a result of the extremely cold temperatures where moisture is frozen out of the air.  Although the relative humidity, the amount of moisture in air at a given temperature, is about 60 to 80%, when the air is heated to room temperature, the relative humidity drops to 4% making it about twice as dry as the Sahara Desert.  As a result of the extremely low humidity, my nose and throat feel dry most of the time, despite running a humidifier in my room at night.  This is the only time in my life I wake up in the middle of the night to drink water.  Theoretically, dry upper airway mucosa (membranes) can also result in an increased susceptibility to upper respiratory infections, although I haven’t seen that reflected in my clinic population thus far. 

 Ice crystals commonly form in the polar atmosphere and produce a phenomenon called “parhelion” or “sun dogs”.  As rays of sunlight are bent within the ice crystals, they produce a false image of the sun.  Sun dogs are usually produced in pair, one of each side of the Sun and are often part of a ring or halo around the Sun. 

"Sun Dogs" at the South Pole
“Sun Dogs” at the South Pole

Since there is little moisture in the air, precipitation is rare.  Actual snowflakes (branched ice crystals) are only seen during the warmest periods of summer.  Most of the precipitation at the South Pole is in the form of ice crystals, falling out of a clear sky when air becomes saturated with moisture.  Although the precipitation is very light, only about 9 inches per year, it never melts.  Over time this adds up and is the reason Antarctica contains 90% of the Earth’s ice and 70% of the planet’s fresh water.

 Meteorology is an important part of the science research conducted at the South Pole, since conditions here influence weather patterns in the rest of the world.  In addition to ground-based instruments, we launch high-altitude balloons on a regular basis.  But more about that in my next blog…

Advertisements