What is Arid?

Arid= Areas (14.6% of world's land mass) characterised by high temperatures and low, unreliable precipitation (100-300mm/yr).

Semi-arid= Account for 12.2% of the world's land mass and experiencing precipitation of between 300-600mm/yr.

Hyper-arid= Account for 4.2% of the world's land mass and experiencing precipitation of <100mm/yr.

It can be measured using the UN aridity index:

Which is amount of precipitation (P) over potential evapotranspiration (PET), i.e. water that would evaporate if it could.

Aridity is when an area has a low level of precipitation and very high potential evapotranspiration.

Climate characteristics:

1) Low, rainfall levels and high rates of evapotranspiration 

2) Altitude and Temperature (Temperature decreases when altitude increases)

3) Most have high diurnal (24 hour) temperature ranges (up to 38°C range) due to lack of cloud cover, heat gained by land is lost rapidly.

4) High wind speeds (Most) typically due to lack of vegetation cover or shelter, therefore sediment blown by the wind can be used to shape the land.