The earth’s atmosphere is a dynamic entity. Large volumes of air move up and down and across the face of the Earth. The movement of air is due to the involvement of some energy. The atmosphere is not a closed system. It is in contact with both the Earth and space and receives energy from both directions. However, Earth directly contributes only a negligible amount of energy to the atmosphere, and its central role is to reflect energy from elsewhere.
The sole ultimate source of atmospheric energy is heat and light received through space from the Sun. This energy is known as Solar Insolation. Only two energy units out of 1,00,00,00,000 units of energy radiated by the sun reach the earth’s surface due to its small size and great distance from the Sun. The unit of measurement of this energy is Langley (Ly). At the height of its atmosphere, the earth receives 1.94 calories per square centimetre per minute (2 Langley).
Read More: Isotherms
Insolation Factors Effecting
On earth, not every location experiences the same amount of sunlight. Insolation varies from location to location, as well as throughout the day, season, and year. This variation is caused by numerous factors. The surface of Earth is not devoid of features. The main elements affecting the distribution of insolation are topographical changes. Strong local gradients of insolation are produced by variations in elevation, surface orientation (slope and aspect), and obstruction by nearby topographic features.
Read More: Heat Transfer
Gradients of Insolation
Strong local gradients of insolation are produced by variations in elevation, surface orientation (slope and aspect), and obstruction by nearby topographic features. Similarly, in the northern hemisphere, a south-facing slope (more open to sunlight and warm winds) will be warmer and dryer due to higher evapotranspiration levels than a north-facing slope.
For instance, in the Swiss Alps, farming is much more extensive on south-facing than on north-facing slopes. The Himalayas are a prime example of this phenomenon, with south-facing slopes being warm, wet, and forested, and north-facing slopes being cold, dry, and significantly more glaciated. On hills with somewhat moderate insolation, vegetation and human activity are more obvious.
Perihelion and Aphelion
The Earth orbits in an elliptical path around the sun. As a result, there are variations in the Sun’s distance from Earth. As a result, at the time when the Earth is closest to the Sun, known as the perihelion, the yearly insolation on the planet is somewhat higher than it is at the aphelion (the Earth is farthest from the Sun).
However, the effect of this solar output variation is balanced by other factors like the distribution of land and sea, atmospheric circulation, ocean currents, etc. Hence, this variation in the insolation does not significantly affect daily weather changes on the Earth’s surface.
Read More: Heat Waves
Axial Tilt of the Earth
The Earth rotates around its axis and makes an angle of 66½ with the plane of its orbit around the sun. This particular characteristic of the Earth has a significant amount of influence on the amount of insolation received at different latitudes. The seasonal changes that are seen in both hemispheres are the result of the axial tilt of the Earth and not because of the closeness of the Earth to the Sun. As the earth is round, the sun’s rays strike the surface at different angles as the earth is round. The angle formed by the sun’s rays with the tangent of the earth’s circle at a point is called the angle of incidence. It influences the insolation in two ways as follows:
- When the sun is almost overhead, the sun’s rays are vertical. Here the angle of incidence is large. Hence, they are localized in a smaller area and give more insolation at that place. If the sun’s rays are oblique, the angle of incidence is small and thus sun’s rays have to heat up a greater area and thus result in less insolation received there.
- The angle of inclination of solar radiation from the Sun depends on the latitude of a place. The higher the latitude is, the lesser angles they make with the Earth’s surface, resulting in slanting sun rays. The sun’s rays with small-angle travel more of the atmosphere than rays striking at a larger angle. The longer the path of the Sun’s rays, the amount of reflection and heat absorption by the atmosphere is greater.
Read More: Weathering
Insolation in Summer and Winter Seasons
The day’s duration is controlled partly by latitude and partly by the year’s season. The amount of insolation is closely related to the length of the day. It is because the insolation is received only during the day. Considering the other conditions are the same, the longer days result in greater insolation. In summer, longer days result in more insolation. Whereas in winter the days are shorter the insolation received is also less.
Read More: Physical Weathering
Insolation and Temperature on Earth
On account of the earth’s inclination on its axis at an angle of 23 ½ degrees, rotation and revolution, the day’s duration is not the same everywhere on the earth. At the equator, there are 12 hours day and night throughout the year. As one moves towards the poles, the days keep on increasing or decreasing. This explains why equatorial regions receive the most solar radiation.
The Earth’s atmosphere is nearly transparent to shortwave solar radiation that passes through the atmosphere before striking the earth’s surface. The transparency depends upon cloud cover, its thickness, water vapour and solid particles as they reflect, absorb or transmit insolation. The Ozone Layer absorbs high-energy ultraviolet rays. Thick clouds hinder the insolation from reaching the earth, while a clear sky helps it reach the surface.
Water vapour absorbs insolation, resulting in less insolation reaching the surface. The troposphere’s extremely small suspended particles disperse the visible spectrum both toward space and the earth’s surface.
Read More: Structure of the Atmosphere
Insolation Solar Variation
Solar variation refers to a change in the amount of radiation emitted by the Sun. These variations have periodic components, mainly the approximately 11- year sunspot cycle. Sunspots are temporary phenomena that appear in the Sun’s photosphere. It appears visibly as dark spots compared to surrounding regions. When there is an increase in sunspots it leads to an increase in the amount of solar radiation. But this change is almost negligible.
The insolation received at the surface differs from about 320 Watts/m2 in the tropics to about 70 watts/m2 in the poles. Maximum insolation is received in the subtropical deserts. The Equator receives comparatively less insolation due to the presence of clouds. The insolation is greater over the continent than over the oceans at the same latitude. This is because more clouds over the oceans reflect sun rays into space.
Isohels: Isohels are lines connecting points on the earth’s surface that receive equal amounts of sunshine. Isohels are more or less parallel to latitudes, especially in the southern hemisphere.
Read More: Chemical Weathering
The amount of solar energy that the planet receives or absorbs is known as insolation. Through volcanoes, springs, and geysers, some of the heat in the core and mantle is transmitted to the surface and the bottoms of the oceans. However, the amount of heat that the earth’s surface receives from its interiors pales in comparison to the heat that comes from the sun.
A crucial component of your UPSC preparation is geography. Both the UPSC Prelims and Mains General Studies Paper I include it. You may study everything about sun radiation, heat balance, and temperature for the IAS Exam in this post.