SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE

The earth receives all of its energy from the sun and in turn, radiates back into space.  As a result, the earth neither warms up nor cools down. The amount of energy received by the earth varies from place to place due to pressure differentiations. This leads to the transformation of heat from one place to another by winds.

1. Solar radiation

• The earth's surface receives most of its energy in short wavelengths known as incoming solar radiation or insolation. On average earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.
• The earth is farthest from the sun [152 million km] on 4^th July. This position of the earth is called aphelion.
• On 3^rd January, the earth is nearest to the sun [147 million km]. This position is called perihelion. Therefore the annual insolation received by the earth on 3^rd January is slightly more than 4^th July.
• The solar output is masked by other factors like the distribution of land and sea and atmospheric circulation.

Variability of insolation at the surface of the earth

The amount and the intensity of insolation vary during the day, in a season and a year. The following are the factors which cause these variations in insolation,

1. The rotation of the earth on its axis: The earth’s axis makes an angle of 66 with a plane of its orbit around the sun. It has a greater influence on the amount of insolation received at different latitudes.
2. The angle of inclination of the sun’s rays: It depends on the latitude of a place. The higher the latitude the less the angle they make with the surface of the earth resulting in slant sun rays. The area covered by vertical rays is always less than the slant rays. If more area is covered, the energy received per unit area decreases. The slant rays are required to pass through a greater depth of the atmosphere resulting in more absorption scattering and diffusion.
3. The length of the day
4. The transparency of the atmosphere
5.  The configuration of land in terms of its aspect

The atmosphere is largely transparent to short-wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth’s surface. Infrared radiation is absorbed by troposphere water vapour, ozone and other gases. Very small suspended particles in the troposphere scatter visible spectrum both to space and towards the earth’s surface. This process adds colour to the sky. The red colour of the rising and setting sun and the blue colour of the sky are the results of the scattering of light within the atmosphere. The insolation received at the surface varies from about 320 watts/ M2 in the tropics is about 70 watts/ m2 in the poles. Maximum insolation is received over the sub-tropical deserts. Equator receives comparatively less insolation than the tropics. In winter, the middle and higher latitudes receive less radiation than in summer.

2. Heating and cooling of the atmosphere

The earth after being heated by insolation transmits the heat to the atmospheric layers near to the earth in the long waveform. The air in contact with the land gets heated slowly and the upper layers in contact with the lower layers also get heated. This process is called conduction. Conduction takes place when two bodies of unequal temperature are in contact with one another till they get equal temperatures. Conduction is important in heating the lower layers of the atmosphere. The process of vertical heating of the atmosphere is called convection. Its transfer of energy is confined only to the troposphere. The transfer of heat through the horizontal movement of air is called advection. It is more important than vertical movement. Advection alone causes diurnal [day and night] variation in daily weather in middle latitudes. Summer season local winds are called ‘loo’ in northern India.

Terrestrial radiation: The earth's surface after being heated up by the insolation starts to radiate energy to the atmosphere in the form of long waves. Carbon dioxide and other greenhouse gases absorbed the longwave radiation, Resulting in the atmosphere being heated by the earth’s radiation.

Heat budget of the planet Earth: The amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation.

• Consider that the insolation received at the top of the atmosphere is 100 per cent. While passing through the atmosphere some amount of energy is reflected, scattered and absorbed. Only the remaining part reaches the earth's surface.
• Roughly 35 units are reflected in space even before reaching the earth’s surface. Of these, 27 units are reflected from the top of the clouds and 2 units from the snow and ice-covered areas of the earth.
• The reflected amount of radiation is called the albedo of the earth.
• The remaining 65 units are absorbed, 14 units within the atmosphere and 51 units by the earth’s surface. The earth radiates back 51 units in the form of terrestrial radiation. Of these, 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere (6 units absorbed directly by the atmosphere, 9 units through convection and turbulence and 19 units through latent heat of condensation). 48 units absorbed by the atmosphere (14 units from insolation +34 units from terrestrial radiation) are also radiated back into space.
• Thus, the total radiation returning from the earth and the atmosphere respectively is 17+48=65 units which balances the total of 65 units received from the sun. This is termed the heat budget or heat balance of the earth.

3. Temperature

The heat of the earth’s surface is measured in terms of temperature. The temperature is measured in degrees of hot [or cold] a thing [or a place].

Factors controlling temperature distribution

The temperature of the air at any place is influenced by

1. The latitude of the place:
2. The altitude of the place;
3. distance from the sea, the air mass circulation;
4. The presence of warm and cold ocean currents;
5. Local aspects.

The latitude: The temperature of a place depends on the insolation received.

The altitude: The atmosphere is indirectly heated by terrestrial radiation from below.

The places near the sea level record higher temperatures than the places situated at higher elevations. The rate of decrease of temperature with height is termed as the normal lapse rate of 6.5®C per 1000 meters.

• Distance from the sea: The sea gets heated slowly and cools slowly compared to land. Lands heat up and cool quickly. The place situated near the sea comes under the moderating influence of sea and land breezes which moderates the temperature.
• Air mass and ocean currents: Warm-air masses experience higher temperatures and cold air- masses experience low temperatures. The coast has warm ocean currents flow record higher temperatures than the coast where the cold currents flow.
• Distribution of temperature: The global distribution of temperature was well understood by studying in January and July. The temperature distribution is generally shown on the map with the help of isotherms. The isotherms are lines joining places having an equal temperature. Isotherms are parallel to the latitude.

January the isotherms

• In January the isotherms deviate to the north over the ocean and to the south over the continent. This can be seen in the North Atlantic Ocean.
• The presence of warm ocean currents, Gulf Stream and North Atlantic drift, make the Northern Atlantic Ocean warmer and the isotherms bend towards the north.
• Over the land, the temperature decreases sharply and the isotherms bend towards the south in Europe.
• Southern hemisphere the isotherms are more or less parallel to the latitudes and the temperature variation is slower than in the northern hemisphere. In July the isotherms generally run parallel to the latitude.

3.1. Inversion of temperature

• Temperature decreases with an increase in elevation is called the normal lapse rate. Inversion of temperature means when the situation is reversed and the normal lapse rate is inverted. Inversion is usually of short duration but quite common nonetheless. Winter nights with clear skies and still air is an ideal situation for inversion. Over polar areas, a temperature inversion is normal throughout the year.
• The cold air acts like water due to being heavy and dense and moves down the slope to pile up deeply in pockets and valley bottoms with warm air above is called air drainage.
• Plank’s law states that the hotter a body, the more energy it will radiate and the shorter the wavelength of that radiation.
• Specific heat is the energy needed to raise the temperature of one gram of a substance by one Celsius.