Solar Radiation

Insolation

• Energy received by the earth is known as incoming solar radiation. which in short is termed as insolation.
• Earth’s surface receives most of its energy in short wavelengths (Radiation received 1.94 calories per sq cm per minute at the top of its atmosphere).

Aphelion and Perihelion.

• Aphelion - The earth is farthest from the sun (152 million km) on 4th July.
• Perihelion - On 3rd January, the earth is the nearest to the sun (147 million km).

Variability of Insolation at the surface of the Earth.

The factors that cause these variations in insolation are:

1. The rotation of earth on its axis - Earth’s axis makes an angle of 66½ with the plain of its orbit greater influence on insolation.
2. The angle of inclination of the sun’s rays - depends on the latitude of a place. Higher latitude less angle result slant sun rays.
3. The length of the day
4. The transparency of the atmosphere
5. The configuration of land in terms of its aspect.

Last two less influence

The Passage of Solar Radiation through the Atmosphere

• Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface.
• 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 result of scattering of light within the atmosphere.

Spatial Distribution of Insolation at the Earth’s Surface

• Insolation received
• 320 Watt/m2 in the tropics.
• 70 Watt/m2 in the poles.
• Equator receives comparatively less insolation than the tropics.
• Same latitude the insolation
• Continent insolation < Oceans insolation.
• In winter, the middle and higher latitudes receive less radiation than in summer.

Heating and cooling of atmosphere

Conduction

• 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 the land gets heated slowly and the upper layers in contact with the lower layers also get heated. This process is called conduction.
• Two bodies of unequal temperature (flow of energy from the warmer to cooler body).
• Transfer of heat continues until both the bodies attain the same temperature or the contact is broken.

Convection

• Air in contact with the earth rises vertically on heating in the form of currents and further transmits the heat of the atmosphere.
• Confined only to the troposphere.

Advection

• Transfer of heat through horizontal movement of air is called advection.
• Local wind ‘loo’ – Outcome of advection process.

Terrestrial Radiation

• Insolation received by the earth is in short waves forms and heats up its surface. After heated earth itself became a radiating body. Radiates energy to the atmosphere in the form of Long wave form.
• Energy heats up the atmosphere from below. This process is known as terrestrial radiation.
• Radiation absorbed by greenhouse gases. atmosphere is indirectly heated by the earth’s radiation.
• Amount of heat received from the sun is returned to space, thereby maintaining constant temperature.

Heat Budget of the Planet Earth

• The earth as a whole does not accumulate or loose heat. It maintains its temperature.
• Amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation.
• The reflected amount of radiation or The percentage of visible light reflected by an object is called the albedo of the earth.

Variation in the Net Heat Budget at the Earth’s Surface

• Some part of the earth has surplus radiation balance while the other part has deficit.
• Latitudinal variation in the net radiation balance of the earth - the atmosphere system.
• 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

• Interaction of insolation with the atmosphere and the earth’s surface creates heat which is measured in terms of temperature.
• Factors Controlling Temperature Distribution
1. The latitude of the place.
2. The altitude of the place - Temperature generally decreases with increasing height.
3. Distance from the sea.
• Sea gets heated slowly and loses heat slowly. Land heats up and cools down quickly.
• The places situated near the sea come under the moderating influence of the sea and land breezes which moderate the temperature.
4. Air-mass & Presence of warm and cold ocean currents.
• Influence of warm air masses experience higher temperature.
• Influence of cold air-masses experience low temperature.
• 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.
5. Local aspects.

Normal Lapse rate Vs Adiabatic lapse rate

Normal Lapse rate	Adiabatic lapse rate
Normal Lapse rate is when temperature   decreases with altitude It is 6.5°C per   1,000 m.	Adiabatic lapse rate is   when temperature of an air mass decreases as it expands while going up Adiabatic lapse rate for   dry atmosphere approximately 9.8 °C/1000 m (5.4 °F/1000 ft).

Distribution of Temperature

• Isotherms are generally parallel to the latitude.
• Deviation more pronounced in January than in July, especially in the northern hemisphere.
• Effects of land mass and the ocean currents are well pronounced.
• Northern hemisphere – In January the isotherms deviate to the north over the ocean and to the south over the continent.
• Southern hemisphere – Isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual than in the northern hemisphere.

• In July the isotherms generally run parallel to the latitude. The equatorial oceans record warmer temperature more than 27^O C Over the land more than 30°C is noticed in the subtropical continental region of Asia, along the 30° N latitude.

Inversion of Temperature

• Normal lapse rate is inverted. It is called Inversion of temperature.
• Inversion is usually of short duration but quite common nonetheless.
• Over polar areas, temperature inversion is normal throughout the year.
• Surface inversion promotes stability in the lower layers of the atmosphere.
• Smoke and dust particles get collected beneath the inversion layer and spread horizontally to fill the lower strata of the atmosphere, lasts for few hours until the sun comes up and beings to warm the earth.
• The inversion takes place in hills and mountains due to air drainage. Cold air at the hills and mountains, produced during night, flows under the influence of gravity. Being heavy and dense, the cold air acts almost like water and moves down the slope to pile up deeply in pockets and valley bottoms with warm air above. This is called air drainage. It protects plants from frost damages.

Latent Heat of Condensation    

• The latent heat of condensation is defined as the heat released when one mole of the substance condenses. The temperature does not change during this process, so heat released goes directly into changing the state of the substance. It is expressed as kg/mol or kJ/kg.