Deserts cut cross our planet along two fringes parallel to the equator, at 25-35° latitude in both the northern and southern hemispheres. The Desert Biome can be defined climatologically as the sum of all the arid and hyper-arid areas of globe; biologically, as the ecoregions that contain plants and animals adapted for survival in arid environments, and, physically, as large contiguous areas with ample extensions of bare soil and low
vegetation cover. A map produced by overlaying areas under these three criteria shows a composite definition of the world's deserts, occupying almost one-quarter of the earth’s land surface, some 33.7 million square kilometres.