The sun is the primary source of energy on the earth. This energy is radiated in all directions into space through short waves. It is known as solar radiation. 

The earth receives most of its energy in the form of short waves and the energy which is received by earth is known as incoming solar radiation (insolation). The amount of insolation received by earth varies slightly in a year due to earth’s revolution around the sun. The earth is farthest from sun on 4^th July, which is known as aphelion and on 3^rd January, the earth is closest from the sun, and this position is called perihelion. Hence, the amount of insolation received by earth during perihelion is slightly more than during aphelion.

VARIABILITY OF INSOLATION AT THE SURFACE OF EARTH

The amount of insolation received on earth’s surface is not uniform everywhere. It varies from place to place and time to time. The tropical zone receive maximum annual insolation and it decreases towards poles.

Factors influencing the variations in insolation:

• The rotation of earth on its axis
• The angle of inclination of the sun’s rays
• The length of the day
• The transparency of the atmosphere
• The configuration of land

Earth’s axis makes an angle of 66½ with its orbital plane around the sun, has greater influence on the amount of insolation received at different latitudes. Further, the angle of inclination of rays is another important factor which determines the amount of insolation. For example, the higher the latitude the less is the angle of inclination, resulting into slant rays. The area covered by slant rays is always more than the vertical rays and hence the energy gets distributed (the net energy per unit area decreases).

DURING SUMMER SOLSTICE

THE PASSAGE OF SOLAR RADIATION THROUGH THE ATMOSPHERE

Solar radiation passes through the atmosphere before reaching the Earth’s surface, where various gases, such as water vapor and ozone, absorb a significant amount of it. Suspended particles in the Troposphere scatter the visible spectrum of light, both towards space and towards the Earth, creating the color in the sky. The red color of the rising and setting sun and the blue color of the sky are caused by the scattering of light within the atmosphere.

SPATIAL DISTRIBUTION OF INSOLATION AT THE EARTH’S SURFACE

Insolation received by earth varies from equator to poles. Maximum insolation received over subtropical deserts and equator receives comparatively less insolation than tropics. Moreover, at the same latitude the insolation is more over the continent than over the oceans.

HEATING & COOLING OF ATMOSPHERE

There are four heating processes directly responsible for heating the atmosphere: 

1. Radiation: It is a process by which solar energy reaches the earth and the earth loses energy to outer space. When the source of heat transmits heat directly to an object through heat waves, it is known as radiation process. 
2. Conduction: When two objects of unequal temperature meet each other, heat energy flow from the warmer object to the cooler object and this process of heat transfer is known as conduction. The flow continues till temperature of both the objects becomes equal or the contact is broken. The conduction in the atmosphere occurs at zone of contact between the atmosphere and the earth’s surface.
3. Convection: The air of the lower layers of the atmosphere gets heated either by the earth’s radiation or by conduction and results into its expansion. Its density decreases and it moves upwards. Continuous ascent of heated air creates vacuum in the lower layers of the atmosphere. As a consequence, cooler air comes down to fill the vacuum, leading to convection. It is confined only to the troposphere.
4. Advection: It is the process of horizontal heat transfer by winds. In middle latitudes, it is responsible for most of the daily weather variations caused by day and night. For example, in northern India, during the summer season, the local winds called ‘Loo’ are caused by advection.

TERRESTRIAL RADIATION

Insolation refers to the amount of solar radiation that reaches the Earth’s surface in the form of shortwaves. The Earth, after absorbing this energy, then radiates it back into the atmosphere as longwave. Much of this radiation is absorbed by atmospheric gases such as water vapor, carbon dioxide, and ozone, which play a crucial role in heating up the atmosphere and sustaining life on Earth. This process is known as terrestrial radiation.

HEAT BUDGET OF PLANET EARTH

The balance between the amount of solar radiation received by the Earth (insolation) and the amount of radiation emitted by the Earth back into the atmosphere (terrestrial radiation) helps maintain a constant annual mean temperature on the Earth’s surface. This balance is known as the Earth’s heat budget. The reflected amount of radiation is called the albedo of the earth. The earth does not accumulate or loose 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. Although the earth, maintains balance between incoming solar radiation and outgoing terrestrial radiation. But this is not true what we observe at different latitudes. The amount of insolation received is directly related to latitudes. 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 known as latitudinal heat balance.

HEAT BUDGET OF EARTH