Thursday, November 15, 2007

Antarctica is a Desert

This is a question I got from Children in Room 133 at Talahi Community School:

“We are studying deserts this week. We heard that Antarctica might be considered a desert! Can you tell us about this?”

Antarctica is indeed a desert, in fact it is the largest desert (about 14.2 million square kilometers in size) on planet Earth. Specifically it is a cold desert. This may surprise many people, because people usually think of deserts as being hot, like the Sahara Desert (just over 9 million square kilometers), but that does not have to be the case. Deserts are defined as regions that have less than 254 mm (10 inches) of precipitation per year. Precipitation means water that can fall as rain, sleet, hail or snow.

Another definition of a desert is ‘an area where there is a greater rate of evaporation than precipitation (rain, hail or snow)’. Because the average temperature in Antarctica is typically below zero degrees Celsius (32 degrees Fahrenheit), combined with the fact that cold air cannot hold as much moisture as warm air, it is usually too cold for precipitation to occur.

In the interior of the Antarctic Continent the average annual precipitation (snowfall is recalculated at its “water equivalent”) is only about 50 mm (about 2 inches). The amount of precipitation does increase towards the coasts, but it is still only about 200 mm (8 in). Air over Antarctica is generally too cold to hold water vapor – so there is very little evaporation. This means that when snow does fall in Antarctica, it usually stays there – or gets blown around, and it eventually accumulates over hundreds and thousands of years into thick ice sheets, like the West Antarctic Ice Sheet and the East Antarctic Ice sheet.

I have noticed the really strong winds that typically come from the south. What I have noticed is that the winds are usually blowing snow, which makes it look as though it may be snowing, but it isn’t. The ever-present winds pick up snow that has already fallen and move it around from place to place, producing ‘white-out’ conditions, which would constitute a ‘Condition 1’ in McMurdo-speak. What I have noticed is that I can tell when the snow around McMurdo has just been blown around, because it usually has a slightly brown color; this is because it has picked up lots of tiny grains of silt or mud (‘rock dust’) from the rocks that are sticking up above the ice and snow, and carried it with the blowing snow, then dropped them with the snow when the winds died down.

In fact, I was helping take some core boxes down to the helo pad one day, and we were also taking some big yellow buckets down there. Apparently the people out at the drillsite had noticed all the windblown dust with the snow, and they wanted to collect some, because they were curious about how much there was, and what the composition of the particles was. I’ll have to ask them how they set the giant buckets up to collect the wind-blown dust, and find out whether they have managed to collected dusty particles.

By the way, I went for a very nice short walk this evening part way around Observation Hill. It was one of the calmest (wind-free!) evenings we have had since I have been here.


Anonymous said...

thanks for this article it really helped me for my school project.

Anonymous said...

heeyyy..... that was extermely helpfull.. thnks heaps