The precipitation maps shown here describe rainfall plus snowfall that reaches the land surface in an average year. Rain is the liquid form of water falling to the land surface where as Snow is the solid form of water. Both rainfall and snowfall are important sources of freshwater in the earth’s land-surface system.
It is the condensation of water vapour into clouds which become heavily laden with water that leads to rainfall events. A cooling of the air and also increased water vapour in the atmosphere generally act together in formation of rain clouds.
Precipitation is key to several other factors in the water cycle, namely runoff, soil moisture and evaporation. Season and climate zone are responsible for precipitation being rainfall or snowmelt, on a global scale.
Globally over a 12 month period there are clear patterns in the amount of water falling on the land surface. In high latitudes during winter months there is little precipitation from October to May. Whereas equatorial regions have a pattern with wet and dry seasons. Africa’s central and western regions show the movement of the rain zone from Eastern areas in January to central & Western regions by June/ July. The inter-tropical convergence zone (ITCZ) is the reason for this changing pattern. The ITCZ is a band of clouds circulating the earth near the equator, with solar radiation driving its movement.
Average Total Rainfall (mm/yr)
