World Winds and Pressure Systems. Image by Wikipedia
An air mass is a large body of air, whose properties – temperature, humidity (air moisture) and lapse rate – are largely uniform (the same) over an area several hundred kilometers across.
The regions where air masses form are referred to as air mass source regions.
An air mass acquires its distinctive characteristics in a source region where there is a large and fairly uniform surface, either water or land, over which air remains fairly stagnant for a period of at least a few days.
If air remains over a source region long enough, it will acquire the properties of the surface below it.
Examples of ideal source regions for air masses include Siberia, Oceans like the Indian Ocean and large deserts like Sahara.
The air stagnates to form a high pressure system (region).
Air masses are classified according to their temperature and moisture characteristics.
The properties of an air mass that it acquires from the source region depend on a number of factors for example the time of the year (summer, winter, autumn), the nature of the underlying surface (ocean, land or desert), and the length of time the air mass remains over its source region.
Air masses are grouped into four categories based on their source region.
Air masses that originate in the cold, polar regions are designated with a capital “P” for polar.
Air masses that originate in the warm, tropical regions are designated with a capital “T” for tropical.
Air masses that originate over land will be dry and are designated with a lowercase “c” for continental.
Air masses that originate over water will be moist and are designated with a lowercase “m” for maritime
These letters are combined to indicate the type of air mass for example Tropical Maritime, Tropical Continental etc.
In moving away from their source regions, air masses will be modified by the surface over which they pass and thus their later properties will depend not only on their source region but also on the nature of the surface they pass over and their age since being formed.
The southern pole is completely surrounded by the Antarctic and therefore the two (pole and Antarctic) are treated as one origin.
Two major air masses reach Zimbabwe i.e polar maritime and tropical maritime.
The tropical continental air mass is resident to Zimbabwe.
Types of air masses
Africa is affected almost exclusively by tropical and equatorial air masses,with tropical continental(cT) air masses dominant in the northern third of the continent and in other smaller patches, tropical maritime(mT) and equatorial (E) air masses most influential in coastal and equatorial Africa
1) Tropical continental
Originate over North Africa and the Sahara desert.
Characteristically hot and very dry.
Very unstable, yet clear conditions predominate due to a lack of water vapor.
2. Tropical Maritime
Form over low latitude oceans and as such are very warm, humid, and unstable.
The tropical maritime air mass that reaches Zimbabwe is the NE trades originating from the northern hemisphere subtropical anticyclone usually centered over central Indian ocean (over the equator).
3. Polar maritime
The SE trades originating from the southern hemisphere subtropical anticyclonic cells and whose fetch begins from Antarctic, constitute the polar maritime air mass.
The South Easterly trade winds are a winter feature in Zimbabwe.
They are cooler and moister than the resident tropical continental air during winter.
When they prevail over the country during the summer season, they still remain cooler but are drier than the local air.
Air masses affecting Zimbabwe
1. South East Trade Winds
are cool moist prevailing winds which blow through out the year.
They are usually associated with continuous light showers and drizzle especially on windward slopes.
They often give rise to guti conditions immediately after the rainy season.
When they blow strongly from the south east they often give clear weather in summer and cloudy weather in winter.
2. Zaire Air ( North West Monsoons)
These only blow in summer.
They are actually the re-curved South East trades which upon approaching Angola get drawn into Zimbabwe via DRC by the intense low pressure of the ITCZ.
They bring a lot rain to Zimbabwe and Central Africa as they collect moisture from bot the Atlantic Ocean and the Congo rain forest.
3. North East Monsoons
Only blow in the summer and cause rain to fall in the Northern parts of Zimbabwe especially in late December to early January.