Category: Geography

Solar Insolation

Solar Insulation is the amount of solar radiation incident upon a unit horizontal surface over a specified time for a given place. It mostly depends on the solar zenith angle and also on the ratio (d/d_m) of the actual distance to the mean distance of the Earth from the Sun.

The surface of the earth receives solar radiation in form of short wavelengths. This energy received by the earth is known as Incoming Solar Radiation also called Insolation.

Insolation on Earth’s Surface

The earth gets a very small amount of solar energy. This is because the earth is a geoid that resembles a sphere. Due to this, the sun’s rays fall obliquely at the top of the atmosphere.

What time of year is insolation greatest?

The solar output received at the top of the atmosphere varies slightly in a year due to variations in the distance between the earth and the sun.

Aphelion

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

Perihelion

On 3rd January, the earth is the nearest to the sun (147 million km). This position is called perihelion. Therefore, 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 and sea and atmospheric circulation.

Hence, this variation in the solar output does not have a 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 :

• The rotation of the 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 in terms of its aspect.

But the transparency of the atmosphere and configuration of the land has less influence.

Rotation of the Earth on its Axis

The fact that the earth’s axis makes an angle of 66 with the plane of its orbit around the sun has a greater influence on the amount of insolation received at different latitudes.

The angle of inclination of the sun’s rays

The second factor that 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 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 gets distributed and the net energy received per unit area decreases.

Moreover, the slant rays are required to pass through a greater depth of the atmosphere resulting in more absorption, scattering, and diffusion.

Transparency of the Atmosphere

The atmosphere is largely transparent to short-wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth’s surface.

Within the troposphere water vapour, ozone, and other gases absorb much of the near-infrared radiation.

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

Spatial Distribution of Insolation at the Earth’s Surface

The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/m2 in the poles. Maximum insolation is received over the subtropical deserts, where the cloudiness is the least. The Equator receives comparatively less insolation than the tropics.

Generally, at the same latitude, the insolation is moreover the continent than over the oceans. In winter, the middle and higher latitudes receive less radiation than in summer.

FAQ

Why is less insolation received at the equator in June?

During June, especially around June 21 of the year, the position of the earth in its orbit is inclined directly towards the sun. This is called the Summer solstice in the Northern Hemisphere.

As a result during June, maximum insolation is received at the Tropic of Cancer, as Sun is directly perpendicular to the Tropic of Cancer. The Equator is below the Tropic of Cancer, and thereby receives less insolation at the equator in June.

The Composition of Atmosphere are Gases, Water, Vapour and Dust Particles and the below table shows details of different gases in the air particularly in the lower atmosphere. Next, we’ll look into its chemical composition.

Chemical composition of atmosphere

In the atmosphere of the earth, the air is a mixture of gases that contains 78% Nitrogen, 21% oxygen, and 1% other gases which include Carbon dioxide, Argon, Neon, Helium, etc and also water vapour.

Composition of gases in Atmosphere

The proportion of gases gets changed in higher layers of the Atmosphere, in a way that the oxygen gets almost negligible in quantity at the height of 120km.

Carbon Di Oxide

Carbon dioxide and water vapor are found only up to the height of 90km from the earth’s surface. Carbon dioxide is a very important gas in the Earth’s Atmosphere, as it is transparent to the incoming solar radiation and it is opaque to the outgoing solar radiation.

This nature of Carbon dioxide absorbs some part of terrestrial radiation and reflects it back to the earth’s surface and it is the main factor for the Green House Effect.

The volume of Carbon dioxide keep raising for the past few decades due to human activities and other gases’ volume remains constant. This increase in the volume of Carbon dioxide in the atmosphere increases the temperature of the air.

Ozone

Ozone is one of the important constituents in the atmosphere and it is found between 10 to 50 km above the earth’s surface. This gas acts as a filter and absorbs the ultraviolet ray from the sun rays and prevents it from entering the earth’s surface.

Water Vapour

The water vapour is also a variable gas in the atmosphere, that decreases with altitude and it accounts for up to 4% by volume in warm and wet tropics.

But its volume is less than 1% in the cold and dry desert and also in the polar regions. It is to be noted that the Volume of the Water Vapour decreases from the Equator to towards the poles.

Another property of Water Vapour is, it acts similar to Carbon dioxide, by absorbing the parts of the insolation from the sum and preserves the earth’s radiated heat.

Just like a blanket, it neither allows the earth to become too hot or too cold. It also contributes to the stability and instability in the air.

Dust Particles

The atmosphere has enough capacity to keep the small solid particles and it originates from various sources.

Some of these small solid particles include sea salts, fine soil, smoke soot, ash, dust, pollen, and dust from meteors. These small solid particles called Dust particles are normally concentrated in the lower layers of the atmosphere.

And sometimes, convectional air currents transport the dust particles to greater heights.

The concentration of dust particles is high in the subtropical and temperate regions and it is because dry winds are higher in these regions compared to the equatorial and polar regions.

The dust and salt particles act as hygroscopic nuclei, using which the water vapor condenses to produces clouds.

Distribution of alluvial soil in India

Alluvial soil is distributed widely in the Northern Plains of India, also in eastern coastal plains in India such as the Mahanadi delta, Godavari delta, Krishna delta and Kaveri delta.

In Northern Plains, it is found throughout the Indo-Gangetic plain and especially along with the lower courses of India’s major rivers such as their deltas.

Areas of Distribution

It is the highest with about 43%. The region which is rich in Alluvial soil is the Great Northern Plains such as the Plains of river Ganga. This is because of the Himalayas, the rivers from where it is originated.

The Ganga carries, transports the soils and fine sediments, by its flowing water and deposits the silt, fine sediment of soil and rocks along their banks.

By a narrow corridor in Rajasthan, they extend into the Gujarat plains.

In the peninsular regions, these are distributed along the eastern coast and river valley.

In the Upper and Middle Ganga plain, two varieties of Alluvial soils developed and they are Khadar and Bhangar. The Khadar is new alluvium, which is deposited annually and consists of fine silts soil.

Bhangar is older alluvium, and it is deposited away from the flood plains. Both these Khadar and Bhangar have calcareous concretions (Kankars). These soils are distributed along the lower and middle Ganga plains and Brahmaputra valley.

The Alluvial soil distribution in India, states are Assam, Bihar, Chandigarh, Delhi, Haryana, Himachal Pradesh, Jammu and Kashmir, Madhya Pradesh, West Bengal.

In the Peninsula, the deltas of Mahanadi, Godavari, Krishna and Kaveri rivers.

The Alluvium is very fertile has a good proportion of potash, phosphoric acid and lime which are ideal for the growth of sugarcane, paddy, wheat, and other cereals and pulses.

The erosional landforms of the river are River Valley Formation, Potholes and Plunge Pools, Incised or Entrenched Meanders, River Terraces, etc.

Valleys

The valley begins as small and narrow rills. The rills then slowly develop into long and wide gullies. Then the gullies get deepened, widened, and rise on becoming valleys.

Based on the dimensions and shape, the Valleys are classified as V-Shaped Valleys, Gorges, Canyons, etc.

V-Shaped Valleys

The V-Shaped Valley is formed by the river erosion over time and its shape is similar to the letter “V“.

Gorge

A Gorge is a narrow, deep valley running between the mountains or hills with very steep to straight sides. It is typically with steep rocky walls and rivers or streams running through it. Also, the canyon is characterized by steep step-like side slopes and might be as deep as the gorge.

It is almost equal in width at the top and as well the bottom. On the other hand, the Canyon is wide at the top than its bottom. Also, the fact, is that Canyon is a type of Gorge.

The valley depends on the structure and types of rock on which they form due to the erosion by running water.

For Example – Gorges are formed in Hard rocks and Canyons are formed in the horizontal bedded sedimentary rocks.

Potholes and Plunge Pools

Over the rocky beds of hill-streams, circular depressions called potholes form because of stream erosion aided by the abrasion of rock fragments. Once a small and shallow depression forms, pebbles and boulders get collected in those depressions and get rotated by flowing water and consequently the depressions grow in dimensions.

A series of such depressions eventually join and the stream valley gets deepened. At the foot of waterfalls also, large potholes, quite deep and wide, form because of the sheer impact of water and rotation of boulders.

Such large and deep holes at the base of waterfalls are called plunge pools. These pools also help in the deepening of valleys. Waterfalls are also transitory like any other landform and will recede gradually and bring the floor of the valley above waterfalls to the level below.

Incised or Entrenched Meanders

In streams that flow rapidly over steep gradients, normally erosion is concentrated on the bottom of the stream channel. Also, in the case of steep gradient streams, lateral erosion on the sides of the valleys is not much when compared to the streams flowing on low and gentle slopes.

Because of active lateral erosion, streams flowing over gentle slopes, develop sinuous or meandering courses. It is common to find meandering courses over floodplains and delta plains where stream gradients are very gentle.

But very deep and wide meanders can also be found cut in hard rocks. Such meanders are called incised or entrenched meanders.

Meander loops develop over original gentle surfaces in the initial stages of development of streams and the same loops get entrenched into the rocks normally due to erosion or slow, continued uplift of the land over which they start.

They widen and deepen over time and can be found as deep gorges and canyons in hard rock areas. They give an indication of the status of the original land surfaces over which streams have developed.

River Terraces

River terraces are surfaces marking old valley floor or floodplain levels. They may be bedrock surfaces without any alluvial cover or alluvial terraces consisting of stream deposits.

River terraces are basically products of erosion as they result due to vertical erosion by the stream into its own depositional floodplain.

There can be a number of such terraces at different heights indicating former river bed levels. The river terraces may occur at the same elevation on either side of the rivers in which case they are called paired terraces. When a terrace is present only on one side of the stream and with none on the other side or one at quite a different elevation on the other side, the terraces are called unpaired terraces.

Unpaired terraces are typical in areas of slow uplift of land or where the water column changes are not uniform along both the banks.

The terraces may result due to:

(i) receding water after a peak flow

(ii) change in hydrological regime due to climatic changes;

(iii) tectonic uplift of land;

(iv) sea-level changes in the case of rivers closer to the sea.

Plain formed by Wind Deposition

The two features that are formed by wind deposition are Sand dunes and loess deposits. The dunes are also called Aeolian Landforms. The wind is a good sorting agent. Based on the velocity of the wind, varying sizes of grains are moved along the floor by saltation, or by rolling, carried in suspension.

During this process of transportation, the material gets sorted. The lighter material is carried long, and heavy materials stay grounded.

When the wind slows or begins to die down, based on the sizes of grains and their critical velocities, the grains will begin to settle.

So, in depositional landforms made by wind, good sorting of grains can be found. Since wind is there everywhere and wherever there is a good source of sand and with constant wind directions, depositional features in arid regions can develop anywhere.

Sand Dunes

Dry hot deserts are good places for sand dune formation. Obstacles to initiating dune formation are equally important. There can be a great variety of dune forms.

Crescent-shaped dunes called barchans with the points or wings directed away from wind direction i.e., downwind, form where the wind direction is constant and moderate and where the original surface over which sand is moving is almost uniform.

Parabolic dunes form when sandy surfaces are partially covered with vegetation. That means parabolic dunes are reversed barchans with wind direction being the same.

Seif is similar to barchan with a small difference. Seif has only one wing or point. This happens when there is a shift in wind conditions.

The lone wings of seifs can grow very long and high. Longitudinal dunes form when the supply of sand is poor and wind direction is constant. They appear as long ridges of considerable length but are low in height.

Transverse dunes are aligned perpendicular to the wind direction. These dunes form when the wind direction is constant and the source of sand is an elongated feature at right angles to the wind direction. They may be very long and low in height.

When sand is plenty, quite often, the regular shaped dunes coalesce and lose their individual characteristics. Most of the dunes in the deserts shift and a few of them will get stabilised especially near human habitations.

Conclusion

Deposition landforms of Wind such as a Sand Dune is a mound of sand that is deposited by wind. The landforms vary in size and shape. The Layers of sand and silt which are deposited in the same area are called Loess, it is very fertile.

The wind is one of the two dominant agents in hot deserts. The desert floors get heated up too much and too quickly because of are dry and barren.

The heated floors heat up the air directly above them and result in upward movements in the hot lighter air with turbulence, and any obstructions in its path set up eddies, whirlwinds, updrafts, and downdrafts.

Winds also move along the desert floors with great speed and the obstructions in their path create turbulence. Of course, there are storm winds that are very destructive.

Winds cause deflation, abrasion, and impact. Deflation includes lifting and removal of dust and smaller particles from the surface of rocks.

In the transportation process sand and silt act as effective tools to abrade the land surface. The impact is simply the sheer force of momentum which occurs when sand is blown into or against a rock surface. It is similar to sandblasting operation.

The wind action creates a number of interesting erosional and depositional features in the deserts.

In fact, many features of deserts owe their formation to mass wasting and running water as sheet floods. Though rain is scarce in deserts, it comes down torrentially in a short period of time.

The desert rocks devoid of vegetation, exposed to mechanical and chemical weathering processes due to drastic diurnal temperature changes, decay faster and the torrential rains help in removing the weathered materials easily.

That means the weathered debris in deserts is moved by not only wind but also by rain/sheet wash.

The wind moves fine materials and general mass erosion is accomplished mainly through sheet floods or sheet wash. Stream channels in desert areas are broad, smooth, and indefinite and flow for a brief time after rains.

Erosional landforms of wind

Pediments and Pediplains Landscape evolution in deserts are primarily concerned with the formation and extension of pediments. Gently inclined rocky floors close to the mountains at their foot with or without a thin cover of debris are called pediments.

Such rocky floors form through the erosion of the mountain front through a combination of lateral erosion by streams and sheet flooding.

Erosion starts along the steep margins of the landmass or the steep sides of the tectonically controlled steep incision feature over the landmass.

Once, pediments are formed with a steep wash slope followed by a cliff or free face above it, the steep wash slope and free face retreat backward.

This method of erosion is termed a parallel retreat of slopes through backwashing. So, through parallel retreat of slopes, the pediments extend backward at the expense of the mountain front, and gradually, the mountain gets reduced leaving an inselberg which is a remnant of the mountain.

That’s how the high relief in desert areas is reduced to low featureless plains called pediplains.

Playas

Plains are by far the most prominent landforms in the deserts. In basins with mountains and hills around and along, the drainage is towards the centre of the basin, and due to gradual deposition of sediment from basin margins, a nearly level plain forms at the centre of the basin.

In times of sufficient water, this plain is covered up by a shallow water body. Such types of shallow lakes are called playas where water is retained only for a short duration due to evaporation and quite often the playas contain good deposition of salts.

The playa plain covered up by salts is called alkali flats.

Deflation Hollows and Caves Weathered mantle from over the rocks or bare soil, gets blown out by the persistent movement of wind currents in one direction.

This process may create shallow depressions called deflation hollows. Deflation also creates numerous small pits or cavities over rock surfaces.

The rock faces suffer impact and abrasion of wind-borne sand and the first shallow depressions called blowouts are created, and some of the blowouts become deeper and wider fit to be called caves.

Mushroom, Table, and Pedestal Rock Many rock-outcrops in the deserts easily susceptible to wind deflation and abrasion are worn out quickly leaving some remnants of resistant rocks polished beautifully in the shape of mushroom with a slender stalk and a broad and rounded pear-shaped cap above.

Sometimes, the top surface is broad like a tabletop and quite often, the remnants stand out like pedestals.