You have heard that land near Equator is always hotter while round the Poles it is terribly cold and is never warm. Why is it so? Sun shines all the year round for 12 hours out of 24 hours at the Equator. It adds up to 6 months in a year. The period for which Sun shines at the Poles is also 6 months. Then why the difference? The answer lies elsewhere. It is the difference between the heating power slanting rays by morning or the evening and the more direct rays falling at the midday. Now the noontime Sun at poles is never more than 23 1/2° above horizon. So, its rays are always slanting and have little heating power despite the long summer days. On the other hand at equator, Sun is never less than 66 1/2° above the horizon. The heating power of ever directly falling rays is great. Thus, when the Sun’s ray are vertical over the Equator, they fall at an angle of 40° at 50° N and S latitudes and at 0° at the poles. As Earth has a spherical shape, the angle of Sun’s rays on surface of the Earth differs from one place to another and temperature changes at every latitude. We should also remember that the angle of Sun’s rays and the length of day and night during different seasons change all the year round due to the tilt of Earth’s axis. Had it not been so, we would have no seasons and the climate in each belt would remain the same throughout the year. Hence the temperature of a particular place at a particular time of the year depends, on angle of Sun’s rays and the length of day and night. Both in turn depend on the latitude. On the basis of differences in latitudes, the temperatures are high in the torrid or the tropical zone and seasonal contrasts are lesser. In the North and South temperate zones, Sun’s rays are always slanting and seasonal variations in temperatures are large. In the North and South polar regions i.e., in frigid zone, Sun’s rays are at a very low angle during summer and during winter these are always below the horizon. So the temperatures in this zone are very low. As the rays of the Sun fall vertically on the Equator, the temperature is always high there. Places within the tropics have the mid-day Sun almost overhead, and the temperatures remain high. Outside the tropics, the Sun’s rays reach the earth’s surface obliquely, and the temperatures are correspondingly lower. There are two similar bands of rays coming from the Sun to two different latitudes on the earth’s surface. The band of rays falling vertically over the equatorial latitudes heats up a smaller surface, and there the temperature is high. The band of rays falling obliquely over the temperate latitudes heats a larger area (this is due to the curvature of the earth), and there the temperature is low. Thus, we conclude that the temperature of places away from the Equator (on higher latitudes) goes on decreasing according to their distances from the Equator. Hence, the temperature of a place depends on its latitude.
Altitude or Height above Sea Level
Yet the temperature does not depend on latitude alone. Mt. Kenya in East Africa is on the Equator. Yet its peak is always covered under snow, because it is over 5000 metres above sea level. Bermuda islands are situated at 30° N latitude in the Atlantic Ocean, over 3200 km from the Equator. These islands never have snowfall. Bangalore and Chennai as well as Ludhiana and Shimla respectively in South and North India are situated on two different but same latitudes. Yet both Bangalore and Shimla remain cooler than Chennai and Ludhiana respectively because of their higher altitude. It is because the atmosphere is not directly heated by the Sun’s rays. First, the Sun’s rays heat the earth’s surface then, the lower atmosphere is heated by conduction from the earth. Close to the earth’s surface, the layers of air are denser than at higher altitudes. So the temperature nearer the earth’s surface is more than that higher up. Thus, as we go to higher altitudes, the temperature goes on decreasing. In general, on climbing every 166 metres, there is fall of 1°C in temperature (or a fall of 1°F for every 300’. This is known as normal lapse rate. Hence, if the temperature at a place at sea-level is 26°C, then at the top of a mountain of 1,660 metres height the temperature will be 16°C approximately). We can draw a conclusion that temperature of an object or of a place does not depend only on the amount of heat which it receives but also upon the amount of heat it loses. If an object loses more heat than the heat received from the Sun, it gets colder and its temperature falls and the vice-versa. An elevated plateau received more heat than a piece of low ground because Sun’s rays have crossed a lesser thickness of atmosphere to reach it. But loss of heat is rapid from the clearer, dust-free rarefied air of the plateau than from the low ground. In the low area the air is dense and dust particles as well as water vapours are more helping greater retention of heat. The result is that elevated areas remain colder of the two. However, in mountain regions, at times, the temperature is less in valleys than at greater heights. It occurs particularly on calm, cold winter nights, when the sky is clear and the air very dry. Because of these conditions, the heat from the earth’s surface and the lower layer of the atmosphere escapes rapidly back into space, making the upper slopes warmer. The cold is further increased by the sinking of the cool air from the mountain sides. Thus, there is a reversal in the vertical distribution of temperature. This is known as the Inversion of Temperature. This phenomenon is very common in the mid-latitude regions. Due to this, the fruit-growers have their orchards on the upper slopes of the hills.
Distance from the Sea
At seaside in summer, water is cool and the sand nearby is hot though each is receiving the same amount of heat from the Sun. Why? Land and water differ in their behaviour when heated. Sun’s rays pass through water to a great depth as water is transparent. Water is also mobile and so the warm water mixes easily with the cold water. Because of these reasons, the water is neither heated not cooled quickly. Its temperature rises very slowly. In contrast to this, the Sun’s rays heat a piece of land more rapidly because the heat obtained by the piece does not mix with other pieces or layers of land. Therefore the land gets heated or cooled more quickly than water. So during the day, the land is hotter than the sea. The heated air over the land becomes lighter and rises. This creates a low pressure area over the land. At this time, the air above the sea is cooler. When this air blows towards the low pressure area over the land, it lowers the temperature of the air on land. This is known as Sea Breeze. This sea breeze has a moderating influence on coastal regions. During the night, conditions are just the reverse. At this time, the sea remains warmer than the land. So the land breeze blows from the land to the sea, and relieves the cold of the land. The areas, close to the sea have lower daily and annual ranges of temperatures.
Ocean currents also affect the temperature of a place. These ocean currents may be warm or cold. When the winds blow over them, they get their warmth or cold and affect the temperatures of coastal areas. The warm North Atlantic Drift raises the winter temperature of North-West Europe, especially those of the British Isles and Norway. Because of the influence of the Warm Drift, the port of Bergen (about 60°N) in Norway remains open during the winter season, whereas ports on the north-east coast of Canada, located in the same latitudes, remain frozen for several months because of the influence of the cold Labrador Current. But it should be noted that cold currents have less effect upon temperature, because they usually lie under off-shore winds. However, there are exceptions like the on-shore wind blowing over the cold Labrador Current, and the cold Benguela Current. But the cool Canaries has little cooling effect in summer on the coast of Iberian Peninsula (Portugal and Spain), because at that time that region comes under the influence of off-shore north-east trade winds. In those areas, where the warm and cold current meet, the temperature falls below the dew-point, giving rise to foggy conditions. The mixing of warm moist air over Labrador Current causes dense fogs in the neighbourhood at the Grand Bank of New Foundland and at the mouth of the St. Lawrence. The fogs extend several kilometres in land, reduce visibility and hinder communication.
- Prevailing Winds : Winds indirectly change the temperature of a place in the neighbourhood of sea or according to the areas from which they blow. The prevailing winds, in temperate latitudes, coming from the land area lower the temperature in winter and raise it in summer. In contrast, the prevailing winds coming from the sea areas raise the winter temperatures, but lower the summer temperatures. In winter, central and northern parts of China get winds from cold Central Asia, and, therefore, their temperatures are sufficiently lower than those of other places lying in the same latitudes elsewhere. The high Himalaya does not allow the cold central Asian winds to cross towards the northern plains of India. Calcutta and Guangzhow (South China) are both on the same latitude. But the January temperature of Calcutta to the south of Himalaya is about 20°C while that of Guangzhow to its north is 14°C.
- Local Winds : Several local winds bring a marked change in the temperature. During the day the land is hotter than the sea. The heated air over the land becomes lighter and rises. This creates a low pressure area over the land. At this time, the air above the sea is cooler. When this air blows towards the low pressure area over the land, it lowers the temperature of the air on land. This is known as Sea Breeze. This sea breeze has a moderating influence on coastal regions. During the night, conditions are just the reverse. At this time, the sea remains warmer than the land. So the land breeze blows from the land to the sea, and relieves the cold of the land. The areas, close to the sea have lower daily and annual ranges of temperatures.
In the plains of northern India, the hot wind, called Loo, blowing from Rajasthan makes western U.P. very hot. Every year, in the summer season, it is the cause of sun-stroke to thousands of people. The warm dry Chinook wind, blowing across the Rocky Mountains from the west, causes a considerable rise in temperatures in the Central Plains of the U.S.A. This wind gets warmed up, when it descends along the eastern or leeward slopes of the Rocky Mountains. Like the Chinook, the Foehn also is a warm dry descending wind. It blows across the Alps down the northern or leeward slopes in winter. When it descends down the mountain slopes, it warms up. The Harmatten, a dry wind that blows over North-West Africa from north-east areas, brings relief to those areas from the damp heat of the tropics. Although full of dust, it is welcomed by the people. It is known as the ‘Doctor’. The Sirocco, hot dry wind blowing from the Sahara to the Mediterranean, is most unpleasant. When this wind crosses the Mediterranean Sea, it picks up sufficient moisture. This makes it an oppressively hot moist wind. Its scorching nature brings a great loss to vegetation and crops. The Mistral and the Bora are the cold winds which blow from the north of Europe. The Mistral is taken as the violent wind in the lower Rhone Valley and across the Mediterranean coast up to Genoa. It often causes the winter temperature to fall below freezing point. The Bora occurs in winter, when the atmospheric pressure over continental Europe is more than that over the Mediterranean Sea. This is a dry and very cold wind. It is more violent than the Mistral.
Clouds in the sky also affect the temperature of a place. The presence of clouds in the sky prevents the amount of solar radiation coming to the earth’s surface. It also prevents the ground radiation, leaving the earth’s surface. Because of this, the places, where clouds are less, experience a high day temperature (more than 38°C) and a much lower night temperature (less than 21°C). Such is the condition in deserts. In the equatorial areas, because of the abundance of cloud cover, day temperatures are not so high and night temperatures not so low.
Slope of the Land
The effect of slope of the land is not so much noticed in the tropical areas, because in those latitudes the mid-day Sun is always high in the sky, and its rays fall more or less vertically over the whole land. But in temperate latitudes its effect is noticed significantly. In these latitudes, the Sun facing slope of mountains that are more or less parallel to the equator gets warmer than the land sloping away from the Sun or towards the Poles. This is because the slope facing the Sun will get more or less vertical rays, whereas the slope away from the Sun will get slanting rays. In the Northern Hemisphere, the land, having its slope towards the south, is warmer than the land having its slope towards the north. In the Southern Hemisphere the position is just the reverse. In mountain regions, as for example, of the Himalaya, as soon as the Sun rises, it starts warming the south facing slopes. It favours the creation of human settlements and orchards or croplands along these slopes.