Air Masses: Types, Characteristics, and Their Impact on Weather"
Learn about air masses, their types, characteristics, and how they influence weather patterns. Discover their role in climate and meteorology.
What is an Air mass?
An air mass is a body of air that extends over a large area and has nearly uniform temperature and humidity in any horizontal direction. Places where air masses form are called source regions, and they are generally flat with light winds which enable them acquire uniform conditions. Ideal source regions are those dominated by large high-pressure areas, such as the arctic plains in winter and the subtropical oceans and desert regions in summer, where air settles for a long period of time. Air masses are classified into two according to their source region;
Map of Eastern and Southern Africa Showing Air Masses.
The latitudinal position of a place on the globe which in turn determines the temperature of a place. Air masses categorised on this basis are polar and tropical air masses.
Polar air masses originate over cold regions.
Tropical air masses originate over the warm tropics.
The second classification is the underlying surface over which the air mass originates. Continental air masses originate over land, and maritime air masses originate over water. The surface whether continent or ocean, further determines the moisture content of the air mass. By combining types based on latitudinal position with those based on underlying surface, a list of four important types of air masses result.
Types of air masses.
1. Polar air masses
2. Tropical air masses
Polar air masses
Polar air masses originate over cold regions. There are two types of Polar air masses.(Polar Maritime air mass and Polar continental air mass)
Polar maritime air mass.
Originally, this air is cold and relatively dry. It originates from the Arctic Ocean. As it moves southwards, it picks-up moisture and becomes heated from below. Ultimately, the air becomes unstable. The weather associated with this air mass gives broken cumulus-nimbus clouds, local showers, and heavy at times with bright periods of good visibility. During the winter, Alaska of USA and the Yukon and the Northwest Territories of Canada are dominated by cold, dry air masses which results in little precipitation.
Polar continental air mass.
This cold, dry air mass forms over land in continental Polar Regions. This air mass thus originates from the continental interiors. The northern Tundra lands of North America and Asia, and the Greenland ice caps. The air too is cold and dry. As the air mass spreads outward towards the coasts and over the North Atlantic, it is warmed from below and its water content increases. In summer, the weather is warm and cumulus is the typical cloud type. In winter, the air is very cold and stable. Stratus clouds develop as aresult of moisture picked up over the North Sea.
Tropical air masses.
Tropical maritime air mass.
This is a warm, moist air mass that forms over ocean water in the tropics. This air usually originates in the Atlantic Ocean between 35° to 40°N. It is warm moist air which cools as it moves north over the sea. There is a high relative humidity, but, with the consequent surface cooling of the lower layers, the air becomes stable. The weather associated with this type of air mass in sea fog over coastal areas, heavy rainfall and high humidity.
Map of Africa Showing Air Movements(ITCZ)
Tropical continental air mass.
The air mass originates in latitudes 35° to 40° north of the equator particularly over sub-tropical deserts of continents. It travels northwards over North Africa and the Mediterranean it commences as warm, dry air but it cools over the Mediterranean and picks some moisture, which it drops as rain over the continent. This type of air produces cloudless skies, poor visibility due to fog, however sometimes isolated showers and thunder storms.
Characteristics of air masses
· Air masses can be warm or cold depending on the source of origin. Air masses originating from the tropics tend to be warm while those originating from the poles tend to be cold.
· Air masses can be maritime or continental depending on the path taken. When the underlying surface is ocean, (water) air masses are maritime and when the underlying surface is land (continent), the air masses are continental.
· Air masses move from one region to another following the pattern of barometric pressure. Thus, they move from regions of high barometric pressure to regions of low barometric pressure.
· Air masses have a homogeneous or uniform temperature over a considerable distance.
· Air masses have uniform humidity content throughout the air body.
· Air masses have a definite direction of movement from a definite source region.
· Polar air masses tend to be stable while continental air masses tend to be unstable.
· Formation of air masses.
· Air masses from due to several reasons:-
Differences in temperature.
Temperature differences at the sources of air masses affect atmospheric pressure. Areas with high temperatures have low pressure while areas with low temperature have higher pressure. Differences in temperature will result into air moving from regions of higher pressure to regions of lower pressure. For example, the inter-tropical convergence zone is a belt of low pressure where wind system from southern hemisphere converges with those from the northern hemisphere.
Coriolis force.
The rotation of the earth results into the formation of a drag - force known as the coriolis force. Any free body on the surface of the earth is deflected to its air right in the northern hemisphere and to its left in the south hemisphere. This is termed as Ferrel's law. The force is proportional to the speed of the moving object, and it varies with latitude, being lowest at the equator and at a maximum at the poles.
Nature of earth's surface.
Air masses develop over areas, which are extensive and uniform e.g. desert surface such as the Sahara or ocean surfaces like the Arctic Ocean. These result into a large mass of air having same characteristics in terms of temperature and humidity.
Path taken by the air
The path taken by the air may be either a water surface or a land surface. Near the ground, however, friction tends to reduce the power of deflection due to rotation. Over water, this effect is greatly minimised. These therefore determine the general flow and speed of air mass over the surface.
Influence of air masses on the climate of East Africa.
1. The air masses that affect the climate of east Africa include the Tropical maritime air mass or South East Trade winds, the Tropical Continental air mass or North East Trade winds, and the Westerlies.
2. The South East Trade winds originate from the Indian Ocean. They blow from a high-pressure belt near 30° latitude in the Southern hemisphere, called a sub-tropical high, toward a low-pressure belt near the equator, called the equatorial low. They blow on shore along the East African coast and have the following effects on the climate:
Avideo trying to explain what are Air Masses
3. When the S.E Trade winds move over the warm tropical waters of the Indian Ocean, they pick up moisture and bring heavy rainfall to the coastal areas such as Dar es Salaam, Zanzibar, Pemba and Morogoro.
4. As the winds traverse the mainland of Tanzania, they loose their moist conditions resulting in less rainfall, clear skies and generally dry conditions.
5. Once the S.E. Trade winds cross Lake Victoria, they are recharged with moisture. They are then deflected on crossing the equator causing cloudy conditions and heavy rainfall in areas north of the lake such as Mpigi, Mukono, Wakiso, Entebbe and Jinja.
6. On the other hand, equatorial regions lack a well-defined dry season because they continue to benefit from moist, maritime air throughout the year. Their position near the equator als ensures that temperatures remain uniformly high year-round.
7. However, because of the deflection of the winds as they cross the equator, they cause arid conditions in the Ankole-Masaka corridor. These conditions are characterized by low cloudcover, low and unreliable rainfall and low humidity.
8. The Tropical Continental air mass or N.E. Trade wind originates from the Arabian landmass. As they blow, the loose moisture in the Ethiopian Highlands. The air mass then blows down the slopes as a warm dry wind. This air mass has affected the climate of East Africa in the following ways:-
9. Arid conditions in Turnana, Marsabit and Lodwar of northern Kenya as well as Karamoja in north eastern Uganda characterized by low and unreliable rainfall, low humidity, low cloud cover and very high temperatures.
The westerlies, which are also continental in origin, blow from the Congo Basin.
These winds are warm and moist and as the blow eastwards, they cause heavy relief rainfall in areas such as the windward slopes of Mount Rwenzori.
They however, descend the leeward slopes as dry winds causing low cloud cover, low rainfall and low humidity in areas of Kasese, Buliisa and Masindi.
The Tropical Continental and Tropical Maritime air masses meet at the Inter Tropical Convergence
As the two air masses meet, the warm moist tropical maritime air mass (S. E. Trades) is forced to rise leading to the formation of cumulo-nimbus clouds, and heavy convectional rainfall accompanied with lightning and thunder.
Table of Key Facts: Air Masses
| Fact | Description |
|---|---|
| Definition | A large body of air with uniform temperature and humidity covering a vast area. |
| Source Regions | Areas where air masses form, typically flat regions with stable high-pressure conditions. |
| Main Classifications | Based on temperature (Polar & Tropical) and surface type (Continental & Maritime). |
| Types of Air Masses | - Polar Maritime (Pm) – Cold & moist |
- Polar Continental (Pc) – Cold & dry
- Tropical Maritime (Tm) – Warm & moist
- Tropical Continental (Tc) – Warm & dry | | Factors Affecting Air Mass Formation | - Temperature differences
- Coriolis force (Earth’s rotation)
- Nature of the Earth’s surface (land or water)
- Path taken by the air mass | | Movement of Air Masses | Driven by atmospheric pressure differences, wind patterns, and the Coriolis effect. | | Effects on Weather | Air masses determine temperature, humidity, cloud formation, and precipitation in different regions. | | Air Masses Affecting East Africa | - Southeast Trade Winds (Tropical Maritime) – Bring rainfall to coastal regions.
- Northeast Trade Winds (Tropical Continental) – Bring dry conditions to inland regions.
- Westerlies – Cause heavy rainfall in western parts of East Africa. | | Intertropical Convergence Zone (ITCZ) | A low-pressure zone where tropical air masses meet, leading to heavy rainfall and thunderstorms. | | Role in Extreme Weather | When air masses with different characteristics collide, they form weather fronts that can cause storms, cyclones, and intense rainfall.
Conclusion
Air masses play a crucial role in shaping global weather patterns and climate conditions. Their origin, movement, and interaction with different geographical features significantly impact temperature, humidity, and precipitation in various regions. In East Africa, air masses such as the Southeast Trade Winds, Northeast Trade Winds, and Westerlies influence rainfall distribution, dry seasons, and overall climate variability. Understanding air masses helps meteorologists predict weather changes and enables better planning for agriculture, disaster preparedness, and daily activities.
