Antarctica is the largest desert in the world.

A desert is identified by aridity rather than by sand dunes or high temperatures. Antarctica is a polar desert: much of it receives very small amounts of new rain or snow. A landscape can therefore be both ice-covered and desert-like, provided that its continuing precipitation is low enough to create an extremely dry climate.

Snowfall needs careful interpretation in this comparison. Scientists often report precipitation as water equivalent, meaning the depth of liquid water that the snow would produce if melted. A deep-looking layer of fluffy snow does not represent the same amount of water as an equal depth of rain. Water-equivalent measurements allow more meaningful comparisons between cold and warm environments.

The Antarctic interior is especially dry. Educational material associated with the British Antarctic Survey reports less than 50 millimeters of annual water-equivalent precipitation across much of it. Coastal regions can receive much more, so the desert label should not be read as a claim that every part of the continent has identical weather.

The continent's roughly 14 million square kilometers makes Antarctica larger than the Sahara, which remains the largest hot desert. Those are compatible records because hot desert is a narrower category. Removing the temperature restriction brings polar deserts into the comparison and changes which landscape comes first in the familiar size ranking.

Antarctica's enormous ice sheet is not evidence of abundant present-day precipitation. It is a stored accumulation built over long periods. In the cold interior, relatively little of that store is removed by surface melting. Small additions can accumulate when losses are also small, so the amount already present and the annual rate of supply answer different questions.

Cold conditions help explain the scarcity of moisture. Very cold air can contain little water vapor, and the high interior is remote from many oceanic moisture sources. Antarctic climate also varies with elevation, latitude, winds and distance from the coast. These factors make a single continent-wide precipitation figure less informative than a description that acknowledges the regional differences.

The ice sheet must also be distinguished from surrounding sea ice. Antarctica is a rocky continent covered by land ice, while sea ice forms from ocean water around it and expands or contracts seasonally. Adding winter sea ice to the continent's area would mix two different geographical measurements. The desert-size comparison concerns the continental landscape, not the maximum seasonal extent of frozen ocean.

Images of dunes describe only one kind of desert surface. Other deserts are rocky, gravel-covered, salty or icy. The Antarctic example is useful because it separates the physical appearance of a place from the climatic process used to classify it. A white landscape can be extremely dry, just as an occasional storm can occur in a hot desert without ending its long-term aridity.

Antarctica separates the idea of a desert from the familiar image of hot sand. Its ice is stored precipitation accumulated over time, not proof of a rainy present climate. The apparent contradiction disappears once a water inventory is distinguished from a yearly water input. A landscape can contain an immense frozen reservoir while receiving very little new moisture.