Precipitation rates vary globally and over time. Rain doesn’t fall in the same quantity in any place around the world. There are a range of reasons for differing rainfall patterns:
- Desert areas (most lie between 15° and 35° north and south of the equator) have limited rainfall as they receive sinking, dry air from high pressure systems.
- Large continental interiors tend to be dry because of their distance from moisture sources and many clouds lose moisture before they can reach the centre of large continents. Central North America is an example of this.
- Polar areas are dry because cold air cannot hold as much moisture as warm air so precipitation can’t occur so often.
- Areas near the equator receive high rainfall amounts because constant solar heating produces intense heating, large-scale evaporation, moist rising air that cools with altitude and forms convectional rainfall. In addition, air masses converge here which results in heavy rainfall.
- Mid-latitudes may experience convectional rainfall and polar and sub-tropical air masses meet here too which causes frontal (or depressional) precipitation. This is when a warm, moist air mass cools as it is forced to slide up over a colder, denser mass of air.
- Mountain ranges near water sources can receive high levels of precipitation because of uplift. Precipitation occurs when clouds rise to go over mountains and the air cools which encourages the water molecules to join and then precipitation occurs. Known as relief, or orographic precipitation, this accounts for many uplands receiving higher precipitation than similarly-located lowlands. This can also result in a sharp reduction in rainfall in regions behind (downwind of) the mountains. This phenomenon is commonly known as the rainshadow effect.
climate change could affect global precipitation patterns considerably. Some
areas may see an increase in precipitation from increased evaporation while
areas which have currently have regular rainfall could see a reduction.
For example, Monsoon rainfall, which provides
many countries in Asia with essential rainfall is reducing in duration.