What is a desert? For a place to be classified as a desert, they need at least less than 40cm of rain water per year, NOT a sandy dry place with cacti. Think about it is Antarctica a sandy dry place with cacti? No Antarctica is a desert. Did you know?... * That 20% of the Earth is covered in desert. * The largest coldest desert in the world, is Antarctica. * The largest hottest desert in the world is the Sahara. * Hot deserts are usually cold at night, and hot during the day.