Insolation and Temperature: Heat Budget, Global Warming, Temperature and Its Distribution

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Heat Budget

  • Insolation is the amount of solar radiation that reaches the earth’s surface through shortwaves. The earth also radiates heat energy like all other hot objects. This is known as terrestrial radiation. The annual mean temperature on the surface of the earth is always constant. It has been possible because of the balance between insolation and terrestrial radiation. This balance is known as the heat budget of the earth.

  • Let us suppose that the total incoming solar radiation received at the top of the atmosphere is 100 units. Roughly 35 units of it are reflected back into space even before reaching the surface of the earth. Out of these 35 units, 6 units are reflected back to space from the top of the atmosphere, 27 units reflected by clouds and 2 units from the snow and ice-covered surfaces.

  • Out of the remaining 65 units (100-35), only 51 units reach the surface of the earth and 14 units are absorbed by the various gases, dust particles, and water vapour of the atmosphere.

Image of Image heat budget of earth

Image of Image Heat Budget of Earth

Image of Image heat budget of earth

  • The earth in turn radiates back 51 units in the form of terrestrial radiation. Out of these 51 units of terrestrial radiation, 34 units are absorbed by the atmosphere, and the remaining 17 units directly go to space. The atmosphere also radiates 48 units (14 units of incoming radiation and 34 units of outgoing radiation absorbed by it) back to space. Thus, 65 units of solar radiation entering the atmosphere are reflected back into the space. This account of incoming and outgoing radiation always maintains the balance of heat on the surface of the earth.

  • Although the earth as a whole, maintains balance between incoming solar radiation and outgoing terrestrial radiation, but in the tropical region the amount of insolation is higher than the amount of terrestrial radiation. Hence it is a region of surplus heat. In the Polar Regions the heat gain is less than the heat loss. Hence it is a region of deficit heat. Thus, the insolation creates an imbalance of heat at different latitudes. This is being nullified to some extent by winds and ocean currents, which transfer heat from surplus heat regions to deficit heat regions. This is commonly termed latitudinal heat balance.

Global Warming

  • As we know that the upper portion of the stratosphere contains a layer of ozone gas. Ozone is capable of absorbing a large amount of sun’s ultra-violet radiation, thus preventing it from reaching the earth’s surface. Scientist have realised that the thickness of the ozone layer is reducing. This is disturbing the balance of gases in the atmosphere and increasing the amount of ultra-violet radiation reaching the earth. Ultra-violet radiation is responsible for increasing the global temperature of the earth’s surface besides it can severely burn human being’s skin, increase the incidence of skin cancer, destroy certain microscopic forms of life, and damage plants.

  • There is a gradual increase in the carbon dioxide content of the atmosphere. It is estimated that the carbon dioxide content of the atmosphere has increased 25 % in the last hundred years. Increase of carbon dioxide in the atmosphere has the effect of raising the atmospheric temperature. It is estimated that the temperature of atmosphere has increased by about 0.5oC in the last 1000 years. Large scale deforestation, fossil fuel burning, burning of garbage, combustion processes in factories, and volcanic eruptions are some of the factors responsible for the increase of carbon dioxide in the atmosphere.

  • If the depletion of ozone layer and the increase in the carbon dioxide content continue, the time would come when the temperature of the atmosphere will rise to the extent that it would melt the polar ice caps, increasing the sea level, and causing submergence of coastal regions and islands. The phenomenon of worldwide increase of atmospheric temperature due to depletion of ozone layer and the increase of carbon dioxide content is known as global warming.

Temperature and Its Distribution

Temperature indicates the relative degree of heat of a substance. Heat is the energy which make things or objects hot, while temperature measures the intensity of heat. Although quite distinct from each other yet heat and temperature are closely related because gain or loss of heat is necessary to raise or lower the temperature. Difference in temperature determines the direction of flow of heat. Distribution of temperature varies both horizontally and vertically.

Horizontal Distribution of Temperature

Distribution of temperature across the latitudes over the surface of the earth is called its horizontal distribution. On maps, the horizontal distribution of temperature is generally shown by Isotherms, the lines connecting points that have equal temperatures. The factors responsible for the uneven distribution of temperature are:

  • Latitude: We have already studied that higher the angle of incidence, higher is the temperature. Lower angle of incidence leads to the lowering of temperature. It is because of this that higher temperatures are found in tropical regions and they generally decrease at a considerable rate towards the poles.

  • Land and Sea Contrast: Land gets heated more rapidly and to a greater degree than water during sunshine. It also cools down more rapidly than water during night. Hence, temperature is relatively higher on land during daytime and it is higher in water during night. During summer the air above land has higher temperature than the oceans. But the air above oceans gets higher temperature than landmasses in winter. There are differences in the rate of heating of different land surfaces. A snow-covered land as in polar areas warms very slowly because of the large amount of reflection of solar energy. A vegetation covered land does not get excessively heated because a great amount of insolation is used in evaporating water from the plants.

  • Relief and Altitude: Mountains act as barriers against the movement of winds. The Himalayan ranges prevent cold winds of Central Asia from entering India, during winter. Because of this Kolkata is not as cold as Guangzhou (Canton) in winter though both are situated almost on the same latitude. As we move upwards from sea level, we experience gradual decrease in temperature. Temperature decreases at an average rate of 6oC per 1000 m altitude. It is termed normal lapse rate. As a result, mountains are cooler than the plains even during summers

  • Ocean Currents: Ocean currents are of two types i.e., warm and cold. Warm currents make the coasts warmer, along which they flow, while cold currents reduce the temperature of the coasts along which they flow. The north-western European coasts do not freeze in winter due to the effect of warm North Atlantic Drift, while the Quebec on the coast of Canada is frozen due to the cold Labrador Current flowing along it, though the Quebec is situated in lower latitudes than the north-west European Coast

  • Winds: Winds also affect temperature because they transport heat from one region to the other, about which we have already studied under advection.

  • Vegetation Cover: Vegetation cover absorbs much of sun’s heat and then prevents quick radiation from the earth whereas the soil devoid of vegetation cover radiates it more rapidly. For example, annual range of temperature in equatorial regions is about 5°C while in hot deserts, it is as high as 38°C.

  • Nature of the Soil: Black, yellow and clayey soils absorb more heat than sandy soils. Heat radiates more rapidly from sandy soils than from black, yellow and clayey soils. Hence temperature contrasts are relatively less in black soil areas than those of sandy soils.

  • Slope and Aspect: The angle of incidence of sun’s rays is greater along a gentler slope and smaller along a steeper slope. Greater concentration of solar energy per unit area along gentler slope raises the temperature while its lesser concentration along steeper slopes lowers the temperature. For this reason, the southern slopes of the Himalaya are warmer than the northern ones.

    • The slopes, in terms of aspect, exposed to the sun receive more insolation and are warmer than those which are away from the direct rays of the sun. The northern slopes of the Himalaya not facing the sun are exposed to cold northernly winds are colder. The southern slopes of the Himalaya on the other hand, are sun-facing and are also shelter from the northernly cold winds is warmer. The horizontal distribution of temperature over the globe can be studied easily from the maps of January and July months, since the seasonal extremes of high and low temperature are most obvious in both northern and southern hemispheres during these months.

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