I would say that the precipitation amount per year if it is only 6 1/2 inches per year is the main reason Antarctica would qualify for a dry climate. Presumably though, at one time it must have had much higher precipitation in the form of copious amounts of snow to be able to accumulate the ice that formed its extensive glaciers. A comparison to show the opposite of a dry climate is the town of Ocean Falls in the rainforest of British Columbia, on the westcoast of Canada which I believe has an average rainfall of about 400 inches per year.
