🌄SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE

We live at the bottom of a huge pile of air. We inhale and exhale but we feel the air when it is in motion. It means air in motion is wind. Envelop of air is atmosphere which is formed of multitude gases. These gases support life over the earth’s surface.

Solar Radiation

i) 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 .

ii) As the earth is a geoid resembling a sphere, the sun’s rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun's energy.

iii) Aphelion: During its revolution around the sun, the earth is farthest from the sun (152 million km) on 4th July. This position of the earth is called aphelion.

iv) Perihelion: On 3rd January, the earth is the nearest to the sun (147 million km). This position is called perihelion.

v) The annual insolation received by the earth on 3rd January is slightly more than the amount received on 4th July. However, the effect of this variation in the solar output is masked by other factors like the distribution of land, sea and the atmospheric circulation. Hence, this variation in the solar output does not have great effect on daily weather changes on the surface of the earth.

Variability of Insolation at the Surface of the Earth

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

i) The rotation of earth on its axis: The earth’s axis makes an angle of 66° with the plane of its orbit round the sun has greater influence on the amount of insolation received at different latitudes.

ii) The angle of inclination of the sun’s rays: Determines the amount of insolation received is the angle of inclination of the 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. The area covered by vertical rays is always less than that covered by the slant rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases. Moreover, the slant rays are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and diffusion.

iii) The length of the day: The longer the duration of the day, the greater is the amount of insolation received.

iv) The transparency of the atmosphere: The transparency of the atmosphere depends upon the cloud cover and its thickness, dust particles, water vapour, etc. They reflect, absorb or transmit insolation. Thick cloud hinders the solar radiation to reach the earth’s surface. Similarly, water vapour absorbs solar radiation resulting in less amount of insolation reaching the surface. 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 the setting sun and the blue colour of the sky are the results of scattering of the light within the atmosphere.

v) The configuration of land in terms of its aspect : The insolation received at the surface varies from about 320 Watt/m in the tropics to about 70 Watt/min the poles. 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. In winter, the middle and higher latitudes receive less radiation than in summer.

Heating and Cooling Of Atmosphere

There are different ways of heating and cooling of the atmosphere. They are:

i) Convection

(a) The air in contact with the earth rises vertically on heating in the form of currents and further transmits the heat of the atmosphere. This process of vertical heating of the atmosphere is known as convection.

(b) The convective transfer of energy is confined only to the troposphere.

ii) Terrestrial Radiation

(a) when the earth’s surface after being heated up by the insolation (in the form of short waves), it becomes a radiating body. The earth’s surface starts to radiate energy to the atmosphere in the form of long waves. This process is called terrestrial radiation. This energy heats up the atmosphere from bottom to top.

(b) Atmosphere is transparent to short waves and opaque to long waves.

(c) The long-wave radiation is absorbed by the atmospheric gases particularly by carbon dioxide and other greenhouse gases. Thus, the atmosphere is indirectly heated by the terrestrial radiation.

(d) The atmosphere, in turn, radiates and transmits heat to space. Finally, the amount of heat received from the sun is returned to space, thereby maintaining a constant temperature at the earth’s surface and in the atmosphere.

iii) Advection

(a) The transfer of heat through horizontal movement of air (wind) is called advection.

(b) Winds carry the temperature of one place to another. The temperature of a place will rise if it lies in the path of winds coming from warmer regions. The temperature will fall if the place lies in the path of the winds blowing from cold regions.

(c) Horizontal movement of the air is relatively more important than the vertical movement. In the middle latitudes, most of diurnal (day and night) variations in daily weather are caused by advection alone.

(d) In tropical regions particularly in northern India during the summer season, local winds called ‘Loo’ is the outcome of the advection process.

iv) Conduction

(a) 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.

(b) Conduction takes place when two bodies of unequal temperature are in contact with one another, there is a flow of energy from the warm to cooler body.

Heat Budget Of The Planet Earth

i) The earth as a whole does not accumulate or lose heat. It maintains its temperature. This can happen only if the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation.

ii) This balance between the insolation and the terrestrial radiation is termed as the heat budget or heat balance of the earth.

iii) This is why the earth neither warms up nor cools down despite the huge transfer of heat that takes place.

(a) Consider that the insolation received at the top of the atmosphere is 100 percent.

(b) While passing through the atmosphere some amount of energy is reflected, scattered and absorbed.

(d) Roughly 35 units are reflected back to space even before reaching the earth’s surface.

(e) Of these, 27 units are reflected back from the top of the clouds and 2 units from the snow and ice-covered areas of the earth.

Albedo

i) When the solar radiation passes through the atmosphere, some amount of it is reflected, scattered and absorbed. The reflected amount of radiation is called the albedo of the earth. The value of albedo will be different for different surfaces.

ii) Because of the effect of albedo, highly developed areas such as urban cities can experience higher average temperatures than the surrounding suburban or rural areas, a phenomenon known as the Urban heat island effect. The higher average temperature can be attributed to less vegetation, higher population densities, and more infrastructures with dark surfaces (asphalt roads, brick buildings, etc.).

Variation In The Net Budget At The Earth’s Surface

i) There are variations in the amount of radiation received at the earth’s surface. Some part of the earth has surplus radiation balance while the other part has deficit.

ii) The surplus heat energy from the tropics is redistributed pole wards and as a result the tropics do not get progressively heated up due to the accumulation of excess heat or the high latitudes get permanently frozen due to excess deficit.

Temperature

i) The interaction of insolation with the atmosphere and the earth’s surface creates heat which is measured in terms of temperature.

ii) While heat represents the molecular movement of particles comprising a substance, the temperature is the measurement in degrees of how hot (or cold) a thing (or a place) is.

Factors Controlling Temperature Distribution

The temperature of air at any place is influenced by the latitude and altitude of the place, distance from sea, ocean currents and other local aspects.

The Latitude : The temperature of a place depends on the insolation received. Insolation varies according to the latitude hence the temperature also varies accordingly.

The Altitude : The atmosphere is indirectly heated by terrestrial radiation from below. Therefore, the places near the sea-level record higher temperature than the places situated at higher elevations. In other words, 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°C per 1,000 m.

Distance From The 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.

The places situated near the sea come under the moderating influence of the sea and land breezes which moderate the temperature.

Air-mass And Ocean Currents :

Like the land and sea breezes, the passage of air masses also affects the temperature. The places, which come under the influence of warm air-masses experience higher temperature and the places that come under the influence of cold air- masse experience low temperature.

Similarly, the places located on the coast where the warm ocean currents flow record higher temperature than the places located on the coast where the cold currents flow.

Distribution Of Temperature

Horizontal Distribution of Temperature in January

i) In January, the sun shines vertically overhead near the tropic of Capricorn. Hence, it is summer in the southern hemisphere and winter in the northern hemisphere.

ii) A high temperature is found over the landmasses mainly in three regions of the southern hemisphere. These regions are North-West Argentina, East and Central Africa, and Central Australia. Isotherm of 30°C closes them.

iii) In the northern hemisphere, land masses are cooler than the oceans. As the air is warmer over the oceans than over landmasses in the northern hemisphere, the isotherms bend towards the north (poles) when they cross the oceans and to the south (equator) over the continents.

iv) This can be clearly visible over the North Atlantic Oceans. The presence of warm ocean currents (Gulf Stream and North Atlantic Drift) make the Northern Atlantic Ocean warmer and the isotherms bend towards the poles. Over the land, the temperature decreases sharply and the isotherms bend towards the equator in Europe.

v) In the southern hemisphere, the effect of the oceans is well pronounced (due to few landmasses).

vi) Here, the isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual than in the northern hemisphere.

Horizontal Distribution of Temperature in July

i) In July, the sun shines vertically overhead near the Tropic of cancer. Hence, high temperatures are found in the entire northern hemisphere. The regions having high temperature include South Western USA, the Sahara, the Arabia, Iraq, Iran, Afghanistan, desert region of India and China.

ii) However, the lowest temperature of 0°C is also noticed in the Northern Hemisphere during summer in the central part of Greenland.

iii) During summer in the northern hemisphere, isotherms bend towards the equator while crossing oceans and towards the poles while crossing landmasses.

iv) Isotherms are wide spaced over oceans while they are closely spaced over landmasses.

v) In July, the deviation of isotherms is not that much pronounced as in January.

The Range Of Temperature between January and July : The highest range of temperature is more than 60°C over the north-eastern part of Eurasian continent. This is due to continentality. The least range of temperature, 3°C, is found between 20° S and 15° N.

Inversion Of Temperature : Temperature decreases with increase in elevation is called Normal Lapse Rate. When the normal lapse rate is inverted then it is called Inversion of Temperature. Inversion is usually short duration but common.

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Saturday, 23 October 2021

Solar Radiation, Heat Balance And Temperature