A desert is a barren area of land where little precipitation occurs and consequently living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation. About one third of the land surface of the world is arid or semi-arid. This includes much of the polar region where little precipitation occurs and which are sometimes called "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location. Dasht-e Lut known as the Lut Desert is a large salt desert in southeastern Kerman, Iran and is the world's 25th largest desert. The surface of the sand there has been measured at temperatures as high as 70.7 °C (159 °F) and it is one of the world's driest and hottest places.