Heat of Earth – Temperature , Temperature Inversion Insolation and its distribution :
The Earth’s surface receives most of its energy in short wavelengths. The Energy received by the earth is known as incoming solar radiation which in short is termed as insolation.
The factors that cause these variations in insolation are :
- Rotation of earth on its axis – Earth’s axis makes an angle of 66 1/2 degree with the plane of its orbit round the sun has a greater influence on the amount of insolation received at different latitudes.
- Angle of Inclination of sun’s rays – This depends on the latitude of a place. The higher the latitude the less is the angle they make with the surface of the earth resulting in slant sun rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases.
- Length of the day
- Transparency of the atmosphere – Atmosphere is transparent to short wave solar radiation. Within the troposphere water vapour, ozone and other gases absorb much of the near infrared radiation
- Configuration of land in terms of its aspect
Last two however, have less influence.
Maximum insolation is received over the subtropical deserts, where the cloudiness is the least. Equator receives comparatively less insolation than the tropics.
Generally, at the same latitude the insolation is more over the continent than over the oceans.
Heating and Cooling of Atmosphere :
Conduction takes place when 2 bodies of unequal temperature are in contact with one another, there is a flow of energy from the warmer to cooler body. The transfer of heat continues until the both the bodies attain the same temperature or the contact is broken.
Conduction is important in heating the lower layers of the atmosphere.
The earth after being heated by insolation transmits the heat to the atmospheric layers near to the earth in long wave form. The air in contact with land gets heated slowly and since air is very poor conductor of heat, transfer of heat is only effective upto a few metres in the lower atmosphere.
- Convection :
The transfer of heat energy through the movement of a mass of substance from one place to another place is called convection.
Process- The earth’s surface gets heated after receiving insolation from the sun. Consequently , the air coming in contact with the warmer earth’s surface also gets heated and expands in volume. Thus, warmer air becomes lighter and rises upward and a vertical circulation of air is set in. Conversely, the relatively colder air aloft becomes heavier because of contraction in volume and thus descends to reach earth’s surface. The whole mechanism of ascent of warmer air and descent of colder air generates convection currents in the lower atmosphere
The Convective transfer of energy is confined only to the Troposphere.
The Transfer of heat through horizontal movement of air is called advection. Horizontal movement of air is relatively more important than vertical movement.
Dirunal (day and night ) variations in Middle latitude, ‘Loo’ – summer season local winds in northern india are the outcome of advection process.
- Terrestrial Radiation :
The insolation received by the earth is in short waves forms and heats up its surface. The earth after being heated itself becomes a radiating body and it radiates energy to the atmosphere in long wave form. This energy heats up the atmosphere from below. This process is known as terrestrial radiation.
The long wave radiation is absorbed by the atmosphere gases particularly by Carbon Dioxide and other green house gases. Thus, the atmosphere is indirectly heated by the earth’s radiation. The atmosphere in turn radiates and transmits heat to the space. Finally the amount of heat received from the sun is returned to space, thereby maintaining constant temperature at the earth’s surface and in the atmosphere.
Heat Budget / Heat Balance of the Earth : A heat budget is the perfect balance between incoming heat absorbed by earth and outgoing heat escaping it in the form of radiation. If the balance is disturbed, then earth would get progressively warmer or cooler with each passing year.
Temperature : It is referred to be amount of heat present in the atmosphere involving outgoing terrestrial radiation as its principal source where as incoming solar radiation forms the secondary source.
Factors controlling Temperature Distribution :
- Latitude – The Temperature of a place depends on the insolation received. As explained earlier that the Insolation varies according to the latitude hence the temperature also varies accordingly.
- Altitude – The temperature generally decreases with increasing height. The rate of decrease of temperature with height is termed as the Normal Lapse Rate. It is 6.5 degree Celsius per 1000 m.
- Distance from Sea – Compared to land, the sea gets heated slowly and loses heat slowly. Land heats up and cools down quickly. Therefore, the variation in temperature over the sea is less compared to land.
- Air -Mass and Ocean currents– The places that come under the influence of warm air-masses experience higher temperature and the places that come under the influence of cold air-masses. Similarly, the places located on the coast where the warm ocean current flow record high temperature than the places located on coast where cold ocean current flow.
- Local Aspects
- Under certain conditions the situation where temperature decreases with increasing height is reversed and the Normal Lapse Rate is inverted which is known as Temperature Inversion.
- The mechanism of temperature inversion is more strongly applicable during low sun season when the process of conduction throughout the long winter nights generates cold air column in contact with it, With air being poor conductor of heat , significantly warm air column present over it. For development of such inversion other fundamental requirements includes cloudless night and still air.
- Over the Polar region areas, the temperature inversion is normal throughout the year.
- Surface inversion promotes stability in the lower layers of the atmosphere.
- Smoke and dust gets collected beneath the inversion layer and spread horizontally to fill the lower strata of atmosphere. Dense Fog is common occurrence in the morning especially during winter season.
- The inversion in hills and mountains takes place due to Air drainage. Cold air produced during night flows under the influence of gravity. Being heavy and dense, cold air acts like water and moves down the slope & gets piled up deeply in pockets & valley bottoms with warm air above. This regular sequence of inversion relates to range of economic benefits as it facilitate favourable sun rays to the mountain wall which supports cultivation , human settlement, tourism in such mountainous region.
Distribution of Temperature :
- The temperature distribution is generally shown on the map with the help of isotherms. Isotherms are lines joining places having equal temperature.
- In general the effect of the latitude on temperature is well pronounced on the map, as the isotherms are generally parallel to the latitude. The deviation from this general trend is more pronounced in January than in July, especially in the northern hemisphere.
- In the northern hemisphere the land surface area is much larger than in the southern hemisphere. Hence, the effects of land mass and the ocean currents are well pronounced.
- In January the isotherms deviate to the north over the ocean and to the south over the continent
- The effect of the ocean in well pronounced in the southern hemisphere. Here the isotherms are more or less parallel to the latitudes and variations in temperature is more gradual than in the northern hemisphere.
- In July the isotherms generally run parallel to the latitude.
- The highest temperatures in july are in the northern hemisphere ; in January they are in the southern hemisphere.