of less than 0.65. Drylands can be classified into four sub-types:
* Dry sub-humid lands
* Semi-arid lands
* Arid lands
* Hyper-arid lands

Some authorities regard hyper-arid lands as deserts (United Nations Convention to Combat Desertification) although a number of the world's deserts include both hyper-arid and arid climate zones. The UNCCD