Big question
Why is Antarctica a desert?
Antarctica is a desert because it receives very little precipitation, less than 200 millimetres of snow a year across most of the continent, which is drier than many sandy deserts. A place counts as a desert based on how little rain or snow falls there, not on how hot or cold it is.
Deserts Are Defined by Dryness
When most people picture a desert, they imagine hot sand dunes, but scientists actually define a desert as any place that receives very little precipitation, meaning rain or snow, usually under about 250 millimetres a year. By that measurement, Antarctica easily qualifies: the interior of the continent gets less than 50 millimetres of snow a year in some places, making it drier overall than the Sahara Desert. It just happens to be a cold desert, covered in thick ice built up over hundreds of thousands of years, rather than a hot desert covered in bare sand and rock.
Cold Air Cannot Hold Much Moisture
The main reason Antarctica gets so little snowfall is that cold air simply cannot carry much water vapour, the invisible moisture that eventually condenses into clouds and falls as precipitation. Warm air near the equator can hold a lot of moisture, which is why tropical regions often get heavy rain, but the freezing air over Antarctica holds only a tiny fraction of that amount. Antarctica is also surrounded by cold ocean water rather than warm tropical seas, so even the moisture that does evaporate nearby carries less water vapour into the atmosphere above the continent, and much of what little falls stays locked as ice rather than melting away.
Ice That Has Built Up for Ages
Even though very little new snow falls on Antarctica each year, almost none of it ever melts, because temperatures there rarely rise above freezing, even in summer. Over hundreds of thousands of years, this small yearly amount of snow has piled up, layer upon layer, compressing under its own weight into an ice sheet that in places is more than 4 kilometres thick. That is why Antarctica, despite being one of the driest places on Earth in terms of yearly precipitation, holds roughly 70 percent of the planet's fresh water, all of it locked away as ancient, slowly accumulated ice rather than flowing water.
Try it yourself
Look up how much rain your own town gets in an average year, then compare it to Antarctica's roughly 150 to 200 millimetres. You might be surprised to find your home city gets more precipitation than the driest continent on Earth.
Questions children ask
Is Antarctica the driest place on Earth?
Antarctica is one of the driest places on Earth, with parts of its interior getting less than 50 millimetres of snow a year, making it drier than the Sahara. Some ice-free valleys there have received almost no precipitation for millions of years.
Can a cold place really be a desert?
Yes, a desert is defined by how little precipitation it receives, not by temperature, so cold places with very little snowfall count as deserts too. Antarctica and the Arctic both include cold desert regions.
Why does Antarctica have so much ice if it barely snows there?
Antarctica has huge amounts of ice because the small amount of snow that does fall almost never melts, since temperatures rarely rise above freezing even in summer. It has slowly built up over hundreds of thousands of years.