North America

North America is the third largest continent, extending from the Arctic Ocean in the north to South America in the south. It covers about 24,249,000 sq km or about a’sixth 6f the world’s land area, and extends between (approxi­mately) 84° N to 7° N latitudes and 180° W to 20° W longi­tudes.
The continent has four major physical divisions-the Canadian Shield, the Appalachian Mountains or Eastern Highlands, the Central Plains, and the Western Cordilleras. The Canadian Shield, a huge area of ancient rocks, covers most of Canada.

It is a reservoir of valuable mineral resources like gold, silver, nickel, iron, copper, platinum, radium, cobalt and uranium. A large part of the shield is made up of swamps and a number of lakes such as the Great Bear, Winnipeg and the Great Lakes (Lakes Superior, Michigan, Huron, Erie and Ontario). The famous Niagara Falls is located between Lake Erie and Lake Ontario.

The Appalachian Mountains, located in the north-east, are constituted of low rounded mountains, plateaus and valleys. A number of minerals are found here.
The Central Plains cover central Canada, mid-western United States and the coastal plains in Texas. The mid and southern parts are a vast low and flat river basin of the Missouri-Mississippi.

The Western Cordilleras are a mountainous region along the north to south of the western part of the continent. There are several parallel ranges. The Rocky mountains are North America’s largest mountain range. The Alaska Range, the Coastal Range, the Sierra Nevada, the Cascade Range, etc., are the other ranges. Mt McKinley (6,194 m) in the Alaska Range is North America’s highest mountain. Fertile valleys between the mountain ranges are major agricultural centres. The ranges also enclose inter­montane plateaus-the Great Basin is the largest in termon­tane plateau ot the continent. Most of the rivers of North America rise in the Western Cordilleras. There are many acttve volcanoes in the Western Cordilleras, especially in Alaska and Mexico.

The Mississippi-Missouri-Ohio is the continent’s longest river system. The Grand Canyon of River Colorado is the largest of its kind in the world. Lake Superior is the largest freshwater lake in the world. Great Salt Lake in Utah is even saltier than the ocean. Most North American deserts lie in the southwestern United States and northern Mexico.

North America’s coastline-about 300,000 km-is longer than that of any other continent. A number of countries lie off the coasts of the continent-Greenland (part of the North American continent, but governed as a province of Denmark), Newfoundland, Vancouver, the Aleutian Is­lands, and the West Indies.
North America experiences the most varied range of climate-from extremely dry cold of the Arctic type to the hot tropic.

African Continent

Africa is the second largest continent in terms of size, covering about one-fifth of the land area of the earth. Spread over 30,330,000 sq km, Africa is located between 35° 5 and 37° N latitudes and 50° E and 17° W longitudes. The African continent is separated from Europe by the Mediterranean Sea and from Asia by the Red Sea. The Strait of Gibralter in the north-west, the Suez Canal in the north east and the Strait of Bab-el Mandeb connect Africa to Eurasia.

Africa is an enormous plateau,. most of which is covered by deserts, forests and grasslands. The continent can be divided into two major land regions-Low Africa and High Africa. Low Africa, consisting of northern, west ern and central Africa, is further subdivided into the Coastal Lowlands, the Northern Highlands, the Saharan Plateau, the Western Plateau, the Nile Basin and the Congo (Zaire) Basin. High Africa covers eastern and southern Africa and may be further subdivided into the Rift System, the Eastern Highlands, the Southern Plateau, the Coastal Lowlands and Madagascar. The Rift System, a spetial physical feature of Africa, consists of the Great Rift Valley, which is a series of parallel cracks in the earth that form deep, steep-sided valleys. Many of these valleys house lakes. There are several large lakes in High Africa. Lake Victoria is the largest lake in Africa and is the source of River Nile, the longest river in the world. Other large rift lakes include the Tanganyika, Nyasa, Albert and Turkana. The Rift System extends from Ethiopia in the east to Mozambique in the south-east. It is rich in volcanic soil and has some of Africa’s best farmlands.

Deserts cover about two-fifths of Africa. The Sahara, the world’s largest desert, stretches across northern Africa from the Atlantic Sea to the Red Sea. Other deserts include the Namib desert and the Kalahari desert in southern Africa. Grasslands cover more than two-fifths of Africa. Forests cover less than a fifth of the continent.
Other major African rivers include the Congo (Zaire) which carries the greatest volume of water among all the rivers of Africa-and the Niger, which discharge into the Atlantic, and the Limpopo and the Zambezi which drain into the Indian Ocean. The Nile flows northward from east central Africa to the Mediterranean Sea.
Most of Africa’s highest mountains have been created by volcanic activity. Mt Kilimanjaro (5895 m) in Tanzania and Mt Kenye (5199) in Kenya are extinct volcanoes. The Atlas mountain range, extending from Morocco to Tunisia, forms Africa’s longest mountain chain and is of non volcanic nature.
About 90 per cent of the African continent lies within the tropics-Africa has the largest tropical area in the world.

AIR MASSES AND FRONTS

AIR MASSES An air mass is a mass of air with similar properties of temperature and moisture covering a large area of the earth’s surface and bounded by fronts. Regions where homogenous air masses tend to be created are known as source regions. Some of the well known source regions are sub-tropical and tropical oceans, low-latitude deserts, and the continental interiors. An air mass is said to be cold when it is colder than the surface over which it rests or is moving. An air mass is said to be warm when it is warmer than the surface over which it rests or is moving.

On the basis of source region, air masses are of three types: tropical air mass, polar air mass, and Arctic or Antarctic air mass. Tropical air mass is an air mass that has its source region within the sub-tropical high pressure areas between 20° and 40° north and south of the equator. Polar air mass is an air mass having its source region between latitudes 40° and 60° north and south of the equator. Arctic or Antarctic air mass is an air mass with its source area between latitude 60° and the poles. On the basis of humidity, tropical and polar air masses are further divided into tropical maritime, tropical continental, polar maritime and polar continental. Maritime air masses originate over an ocean, while continental air masses originate over a con­tinent or landmass.

FRONTS The boundary zones between air masses that have originated in different source areas have differing temperature and humidity characteristics. Where the bound­ary zone intersects the earth’s surface, it forms a line of separation, called a front. It is represented by a line in weather charts. A front is usually associated with a trough of low pressure and is very important as a considerable amount of weather is generated along it. Fronts may be nearly vertical as in the case of air masses having little motion relative to one another. They may -be almost horizontal as in cases where one air mass slides over
,another.

Cold Front is a front separating a retreating warm air mass and an advancing cold air mass; the cold air, being heavier, remains near the surface, forcing the warm air to rise over it. The cold front is steeper than the warm front. Another is associated with strong atmospheric disturbances. At a cold front, pressure rises, temperature falls, the wind, veers and there are heavy showers often accompanied by thunder.
Wann Front is a front separating a retreating cold air mass and an advancing warm air mass. Warm air is forced to rise here also. Warm fronts have lower slopes and are commonly attended by stable atmospheric conditions, lack­ing the turbulent air motion of the cold front. If the wal1I\ air is unstable, it results in heavy showers and thunder­storm. The weather associated with warm fronts is seen long before the front reaches the observer. The first indication of its approach is high cirrus cloud, which gradually lowers and thickens into low cirrostratus, fol­lowed by altostratus and then rain-producing nimbostratus at the front itself, frequently a distance of 1000 km over
World Geography 25
all. As the front passes, temperature and humidity rise, pressure stops falling and winds veer.
Occluded Front forms when, during the later stages of the evolution of a cyclonic storm, the more rapidly moving cold front catches up with the warm front and lifts the warm sector above the surface. This is fairly common over Western Europe. The weather associated with an occlusion is similar to that occurring at the original fronts, but does not last as long and eventually dies out as the air in the warm sector riSes and cools.
Stationary Front is formed when neither the warm nor the cold front are able to replace each other. At the stationary fronts rain occurs, sometimes lasting for several days.