Geography

 

 

• It is the long term change in the statistical distribution of weather patterns over periods of time
• Though it has been happening naturally for millions of years, in recent years it has accelerated due to anthropogenic causes and has been causing global warming.
• UNFCCC defines climate change as – “a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods”

 

 

• An increase in the average temperature of Earth’s near surface air and oceans since the mid-20^th century
• 4^th assessment report of IPCC: global temperature increased 74+0.18 degree C during the 20^th century.
• Caused by greenhouse gases
  • Water vapour, Co2, Methane, Nitrous Oxide, Ozone, CFCs (in order of abundance)
• Since the industrial revolution, the burning of fossil fuels has increased the levels of Co2 in the atmosphere from 280 ppm to 390 ppm.

 

The greenhouse effect is a natural process that warms the Earth’s surface. When the Sun’s energy reaches the Earth’s atmosphere, some of it is reflected back to space and the rest is absorbed and re-radiated by greenhouse gases. It is the process by which radiation from a planet’s atmosphere warms the planet’s surface to a temperature above what it would be without its atmosphere. If a planet’s atmosphere contains radioactively active gases (i.e., greenhouse gases) the atmosphere will radiate energy in all directions.

The greenhouse effect comes from molecules that are more complex and much less common. Water vapour is the most important greenhouse gas, and carbon dioxide (CO2) is the second-most important one. Methane, nitrous oxide, ozone and several other gases present in the atmosphere in small amounts also contribute to the greenhouse effect. In the humid equatorial regions, where there is so much water vapour in the air that the greenhouse effect is very large, adding a small additional amount of CO2 or water vapour has only a small direct impact on downward infrared radiation. However, in the cold, dry polar regions, the effect of a small increase in CO2 or water vapour is much greater. The same is true for the cold, dry upper atmosphere where a small increase in water vapour has a greater influence on the greenhouse effect than the same change in water vapour would have near the surface.

Green house effects changes are due to:-

• Energy;
•  Industry;
•  Agriculture;
•  Waste; and
• Land Use Land Use Change

 

Climatic Regions of India : Koeppen’s Classification
Climate Type	Climatic Region		Annual Rainfall in the Region
Amw (Monsoon type with shorter dry winter season)	Western coastal region, south of Mumbai		over 300 cm
As (Monsoon type with dry season in high sun period)	Coromandel coast = Coastal Tamil Nadu and adjoining areas of Andhra Pradesh	75 – 100 cm [wet winters, dry summers]
Aw (Tropical Savanah type)	Most parts of the peninsular plateau barring Coromandel and Malabar coastal strips	75 cm
BShw (Semi-arid Steppe type)	Some rain shadow areas of Western Ghats, large part of Rajasthan and contiguous areas of Haryana and Gujarat	12 to 25 cm
BWhw (Hot desert type)	Most of western Rajasthan	less than 12 cm
Cwg (Monsoon type with dry winters)	Most parts of the Ganga Plain, eastern Rajasthan, Assam and in Malwa Plateau	100 – 200 cm
Dfc (Cold, Humid winters type with shorter summer)	Sikkim, Arunachal Pradesh and parts of Assam	~200 cm
Et (Tundra Type)	Mountain areas of Uttarakhand The average temperature varies from 0 to 10°C	Rainfall varies from year to year.
E (Polar Type)	Higher areas of Jammu & Kashmir and Himachal Pradesh in which the temperature of the warmest month varies from 0° to 10°C	Precipitation occurs in the form of snow

 

 

 

 

 

Climatic Regions of India : Trewartha’s Classification

Climate Type	Climatic Region	Other Cliamatic Condtions
Am (Tropical Rain Forest)	Western coastal region, Sahayadris and parts of Assam	200 cm annual rainfall  & 18.2 C to 29 C temperature
Aw (Tropical Savanna)	Peninsular India except the semi arid zone	150 cm annual rainfall &  18 C-32 C temperature
Aw (Tropical Savanah type)	Most parts of the peninsular plateau barring Coromandel and Malabar coastal strips	75 cm
BS (Semi-arid Steppe type)	Runs southwards from central Maharashtra to Tamilnadu, Andhra Pradesh	40-75 cm annual rainfall & 20-32 C temperature
BSh (Tropical and subtropical Steppe)	Ranges from Punjab to Kutch	Annual temperature 35 C & 30-60 cm annual rainfall
BWh (Tropical Desert)	Western parts of Barmer, Jaiselmer and Bikaner and parts of Kutch	Annual Temperature 35 C & annual rainfall 25 cm
Caw (Humid Subtropical Climate with dry winters)	It ranges from Punjab to Assam	Rainfall from 100-150 cm
H (Mountain Climate)	Mountain areas of Himalayas including Jammu & Kashmir, Uttarakhand, Himachal Pradesh, Sikkim, Arunachal Pradesh	The average temperature varies from 0 to 10°C.

 

 

 

The atmospheric disturbances which involve a closed circulation about a low pressure centre,
anticlockwise in the northern atmosphere and clockwise in the southern hemisphere are called
cyclones. They fall into the following two broad categories: (a) Extra-tropical or Temperate and (b) tropical cyclones.

(a) Temperate Cyclones
Temperate cyclones are formed along a front in mid-latitudes between 35° and 65° N and S. They blow from west to east and are more pronounced in winter season.Temperate cyclones are mainly observed in Atlantic Ocean and North West Europe . They are generally extensive having a thickness of 9 to 11 kilometers and with 1040-1920 km short and long diametres respectively. Each such cyclone alternates with a high pressure anticyclone. The weather associated with the cyclone is drizzling rain and of cloudy nature for number of days. The anticyclone weather is sunny, calm and of cold waves.
(b) Tropical Cyclones
Tropical cyclones are formed along the zone of confluence of north-east and south-east trade winds. This zone is known as the Inter Tropical Convergence Zone (ITCZ). Cyclones generally occur in Mexico, South-Western and North Pacific Ocean, North Indian Ocean and South Pacific Ocean. These cyclones differ from temperate cyclones in many ways. There are no clear warm and cold
fronts as temperature seldom differs in Inter Tropical Convergence Zone. They do not have well-defined pattern of winds and are energised by convectional currents within them. Generally, these are shallow depressions and the velocity of winds is weak. These are not accompanied by anticyclones. The arrangement of isobars is almost circular. These are not extensive and have the diametres of 160-640km. However, a few of them become very violent and cause destruction in the regions of their influence. They are called hurricanes in the Carribean Sea, typhoons in the China, Japan and phillipines,

 

 

 

 

 

 Evaporation and Condensation: dew, frost, fog, mist and cloud, rainfall types

 Evaporation 

 

Evaporation is the process of which water changes from its liquid state to gaseous form. This process takes place at all places, at all times and at all temperatures except at dew point or when the air is saturated. The rate of evaporation is affected by several factors. Important among them are as under:
(i) Accessibility of water bodies :-The rate of evaporation is higher over the oceans than on the continents.
(ii) Temperature :-when the temperature of an air is high, it is capable of holding more moisture in its body than at a low temperature. It is because of this that the rate of evaporation is more in summers than in winters. That is why wet clothes dry faster in summers than in winters.
(iii) Air moisture :-If the relative humidity of a sample of air is high, it is capable of holding less moisture. On the other hand if the relative humidity is less, it can take more moisture. Hence, the rate of evaporation will be high. Aridity or dryness of the air also increases the rate of evaporation. During rainy days, wet clothes take more time to dry owing to the high percentage of moisture content in the air, than on dry days.
(iv) Wind :-If there is no wind, the air which overlies a water surface will get saturated through evaporation. This evaporation will cease once saturation point is reached. However, if there is wind, it will blow that saturated or nearly saturated air away from the evaporating surface and replace it with air of lower humidity. This allows evaporation to continue as long as the wind keep blowing saturated air away and bring drier air.
(v) Cloud cover :-The cloud cover prevents solar radiation and thus influences the air temperatures at a place. This way, it indirectly controls the process of evaporation.

Condensation

Condensation the process by which water vapor (gas) in the atmosphere turns into water (liquid state). It is the opposite of evaporation. This stage is very important because it is the cloud formation stage. Cool temperatures are essential for condensation to happen, because as long as the temperature in the atmosphere is high, it can hold the water vapor and delay condensation.

When a gas is cooled sufficiently or, in many cases, when the pressure on the gas is increased sufficiently, the forces of attraction between molecules prevent them from moving apart, and the gas condenses to either a liquid or a solid.

• Example: Water vapor condenses and forms liquid water (sweat) on the outside of a cold glass or can.
• Example: Liquid carbon dioxide forms at the high pressure inside a CO₂ fire extinguisher.

The temperature of the air falls in two ways. Firstly, cooling occurs around very small particles of freely floating air when it comes in contact with some colder object. Secondly, loss in air temperature takes place on a massive scale due to rising of air to higher altitudes. The condensation takes place around the smoke, salt and dust particles which attract water vapour to condense around them. They are called hygroscopic nuclei. When the relative humidity of an air is high, a slight cooling is required to bring the temperature down below dew point. But when the relative humidity is low and the temperature of the air is high, a lot of cooling of the air will be necessary to bring the temperature down below dew point. Thus, condensation is directly related to the relative humidity and the rate of cooling.

here are four types of condensation and the worst period for such problems is September to May:-

1. Surface condensation. This is the most familiar type of condensation, taking the form of water on window panes, cold wall surfaces and tiles.
2. Interstitial condensation. This is condensation forming between walls or within the building structure.
3. Reverse condensation. This is also called “Summer condensation”. If rains drenches a wall and strong sunlight then dries it, the heat can actually force water vapour into the wall. When it then meets an insulated surface, it forms condensation at that barrier.
4. Radiation condensation. This is sometimes called “clear night condensation“. If there is a sudden temperature drop at night, it can cause condensation on the underside of roof coverings, for example: often this drips onto the insulation quilting and causes a distinctive mottled effect upon the quilting.

 

Dew, Frost, Fog, Mist and Cloud

Dew: When the atmospheric moisture is condensed and deposited in the form of water droplets on cooler surface of solid objects such as grass blades, leaves of plants and trees and stones, it is termed as dew. Condensation in dew form occurs when there is clear sky, little or no wind, high relative humidity and cold long nights. These conditions lead to greater terrestrial radiation and the solid objects become cold enough to bring the temperature of air down below dew point. In this process the extra moisture of the air gets deposited on these objects. Dew is formed when dew point is above freezing point. Dew formation can be seen if the water is poured into a glass from the bottle kept in a refrigerator. The outer cold surface of the glass brings the temperature of the air in contact with the surface down below dew point and extra moisture gets deposited on the outer wall of the glass.
Frost: When the dew point is below freezing point, under above mentioned conditions, the condensation of extra moisture takes place in the form of very minute particles of ice crystals. It is called frost. In this process, the air moisture condenses directly in the form of tiny crystal of ice. This form of condensation is disastrous for standing crops such as potato, peas, pulses, grams, etc. It also creates problems for road transport system.
Mist and Fog: When condensation takes place in the air near the earth’s surface in the form of tiny droplets of water hanging and floating in the air, it is called mist. In mist the visibility is more than one kilometer and less than two kilometers. But when the visibility is reduced to less than one kilometer, it is called fog. Ideal conditions for the formation of mist and fog are clear sky, calm and cold winter nights.
Cloud: Clouds are visible aggregates of water droplets, ice particles, or a mixture of both along with varying amounts of dust particles. A typical cloud contains billions of droplets having diameters on the or- der 060.01 to 0.02 mm; yet liquid or solid water accounts for less than 10 parts per million of the cloud volume. Clouds are generally classified on the basis of their general form or appearance and alti- tude.

Rainfall types.

Precipitation or Rainfall is defined as water in liquid or solid forms falling to the earth. It happens when continuous condensation in the body of air helps the water droplets or ice crystals to grow in size and weight that the air cannot hold them and as a result these starts falling on the ground under the force of gravity.

Different types of Rainfall are:-

• Convectional Rainfall :-Excessive heating of the earth’s surface in tropical region results in the vertical air currents. These currents, lift the warm moist air to higher strata of atmosphere. When-the temperature of such a humid air starts falling below dew point continuously, clouds are formed. These clouds cause heavy rainfall which is associated with lightning and thunder. This type of rainfall is called conventional rainfall. It is very common in equatorial region where it is a daily phenomenon in the afternoon
  (b) Orographic or Relief Rainfall :-Orographic rainfall on formed where air rises and cools because of a topographic barrier. When their temperature fall below dew point, clouds are formed. These clouds cause widespread rain on the windward slopes of the mountain range. This type of rain is called orographic rainfall. However when these winds cross over the mountain range and descend along the leeward slopes, they get warm and cause little rain. Region lying on the leeward side of the mountain receiving little rain is called rainshadow area (see figure 12.4). A famous example of orographic rainfall is Cherrapunji on the southern margin of the Khasi Hills in Meghalaya India.
  (c) Convergence or Cyclonic Rainfall:-Convergence rainfall, produced where air currents converge and rise. In tropical regions where opposing air currents have comparable temperatures, the lifting is more or less vertical and is usually accompanied by con- vention. Convectioned activity frequently occurs along fronts where the temperature of the air masses concerned are quite different. Mixing of air along the front also probably contributes to condensation and therefore to the frontal rainfall. When two large air masses of different densities and temperature meet, the warmer moist air mass is lifted above the colder one. When this happens, the rising warm air mass condenses to form clouds which cause extensive down pour. This rainfall is associated with thunder and lightning. ‘This type of rainfall is also called frontal rainfall. This type of rainfall is associated with both warm and cold fronts, (fig. 12.5) It is gener- ally steady and may persist for a whole day or even longer.

 

 

 

Airmasses

 

An airmass is a large body of air with relatively uniform thermal and moisture characteristics. Airmasses cover large regions of the earth, typically several hundred thousand square kilometers. Airmasses can be as deep as the depth of the troposphere or as shallow as 1 to 2 km.
Airmasses form when air remains over a relatively flat region of the earth* with homogeneous surface characteristics for an extended period of time. ( Canadian and Siberian plains, cool oceanic regions such as the North Atlantic and Pacific, deserts, such as the Sahara and the American southwest, and tropical oceanic regions including the equatorial Atlantic and Pacific, and smaller water bodies such as the Caribbean Sea and the Gulf of Mexico).

Polar air masses, containing little moisture and low temperatures move downward from the poles.  Air masses that form over water are generally moist, and those that form over the tropical oceans are both moist and warm. Because of the Coriolis effect due to the Earth’s rotation, air masses generally move across North America from west to east.  But, because of the differences in moisture and heat, the collision of these air masses can cause instability in the atmosphere.

Polar air mass is cold and tropical air mass is warm. When cold air mass and warm air mass blow against each other, the boundary line of convergence separating the two air masses is termed as front. When the warm air mass, moves upward over the cold air mass the front formed in such a situation is called warm front. On the contrary, when the cold air mass advances faster and undercuts the warm air mass and forces the warm air upwards, the front so formed is called cold front. The frontal surface of cold front is steeper than that of a warm front . A prevailing air mass in any region – polar, tropical, maritime or continental largely controls the regions general weather.

Different air masses are:-

1. Maritime tropical (mT)
   ii. Continental tropical (cT)
   iii. Maritime polar (mP)
   iv. Continental polar (cP)
   v. Continental arctic (cA).

Where ‘m’ stands for Maritime; ‘c’ stands for continental; ‘T’ stands for tropical; ‘P’ stands for polar and ‘A’ stands for arctic region.

Fronts

An important properties of air is that it is a poor conductor of energy. This means that when two different bodies of air come together, they do not readily mix. Rather, each body of air will retain its individual properties, and a boundary forms between them. When two large air masses meet, the boundary that separates them is called a front. Fronts represent fairly abrupt transitions between two large air masses. The warm, moist air might dominate an area hundreds of miles across, while in another part of the continent a cold, dry air mass holds sway over an equally large region. However, where the two air masses meet, the transition layer between them may be only a few tens of miles across, clearly a sharp transition between two massive bodies of air.

Fronts are recognized by the following properties:-

• Sharp temperature changes over a relatively short distance. Sometimes change of 10 to 20 C may be observed.
• Change in moisture content
• Rapid shifts in wind direction
• Pressure changes
• Clouds and precipitation patterns

Types of Fronts:-

Warm Fronts: A warm front occurs when a warm air mass advances and replaces a cold air mass. On a weather map, a warm front is depicted as a red arc, with red semicircles pointing in the direction of the advancing warm air.

Cold Fronts :-A cold front occurs when a mass of cold air advances into a region of warmer air.

Stationary Fronts:- A stationary front forms when a cold front or warm front stops moving. This happens when two masses of air are pushing against each other but neither is powerful enough to move the other. Winds blowing parallel to the front instead of perpendicular can help it stay in place.

Occluded Fronts:- Sometimes a cold front follows right behind a warm front. A warm air mass pushes into a colder air mass (the warm front) and then another cold air mass pushes into the warm air mass (the cold front). Because cold fronts move faster, the cold front is likely to overtake the warm front. This is known as an occluded front

 

 

 

The temperature is the measurement in degrees of how hot (or cold) a thing (or a place) is.
The temperature of the atmosphere is not same across the Earth. It varies in spatial and temporal dimensions. The temperature of a place depends largely on the insolation received by that place. The interaction of insolation with the atmosphere and the earth’s surface creates heat which is measured in terms of temperature. It is important to know about the temperature distribution over the surface of the earth to understand the weather, climate, vegetation zones, animal and human life etc. following factors determine the temperature of air at any place.

1. The latitude of the place – Intensity of insolation depends on the latitude. The amount of insolation depends on the inclination of sun rays, which is further depends upon the latitude of the place. At the equator sun’s rays fall directly overhead throughout the year. Away from the equator towards poles, the inclination of the Sun’s rays increases. In conclusion, if other things remain the same, the temperature of air goes on decreasing from the equator towards poles.
2. The altitude of the place – the atmosphere is largely heated indirectly by re-radiated terrestrial radiation from the earth’s surface. Therefore, the lower layers of the atmosphere are comparatively warmer than the upper layers, even in the same latitudes. For example, Ambala (30 21’ N) and Shimla (31 6’) are almost at the same latitude. But the average temperature of shimla is much lower than the Ambala. It is because Ambala is located in plain at an altitude of 272 m above sea level whereas Shimla is located at an altitude of 2202 m above sea level. In other words, the temperature generally decreases with increasing height (figure 6(a)). The rate of decrease of temperature with height is termed as the normal lapse rate. It is 6.5°C per 1,000 m. That’s why, the mountains, even in the equatorial region, have snow covered peaks, like Mt. Kilimanjaro, Africa.
3. Distance from the Sea – the land surface is heated at a faster rate than the water N surface. Thus the temperature of the air over land and water surfaces is not the same Student Notes: at a given time. In summers, the sea water is cooler than the land and in winters, land is much colder than the sea water. The coastal areas experience the sea breezes during the daytime and the land breezes during the night time. This has a moderating influence on the temperature of the coastal areas. Against this the places in the interior, far away from the sea, have extreme climate. The daily range of temperature is less near the coastal area and it increases with increase in distance from the sea coast (figure 6(b)). The low daily range of temperature is the characteristic of marine climate. That’s why, the people of Mumbai have hardly any idea of extremes of temperature.

(a) Horizontal Distribution of Temperature
Distribution of temperature across the latitudes over the surface of the earth is called its horizontal distribution. On maps, the horizontal distribution of temperature is commonly shown by “Isotherms”, lines connecting points that have equal temperatures. An isotherm is made of two words ‘iso’ and ‘therm’, ‘Iso’ means equal and ‘therm’ means” temperature. If you study an isotherm map you will find that the distribution of temperature is uneven. The factors responsible for the uneven distribution of temperature are as follows:
(i) Latitude
(ii) Land and Sea Contrast
(iii) Relief and Altitude
(iv) Ocean Currents
(v) Winds
(vi) Vegetation Cover
(vii) Nature of the soil
(viii) Slope and Aspect

(b) Vertical Distribution of Temperature
The permanent snow on high mountains, even in the tropics, indicate the decrease of temperature with altitute. Observations reveals that there is a fairly regular decrease in temperature with an increase in altitude. The average rate of temperature decrease upward in the troposphere is about 6 C per km, extending to the tropopause. This vertical gradient of temperature is commonly referred to as the standard atmosphere or normal lapse rate, but is varies with height, season, latitude and other factors. Indeed the actual lapse rate of temperature does not always show a decrease with altitude.

Temperature Inversion

Temperature decreases with increase in altitude. In normal conditions, as we go up, temperature decreases with normal lapse rate. It is 6.5°C per 1,000 m. Against this normal rule sometimes, instead of decreasing, temperature may rise with the height gained. The cooler air is nearer the earth and the warmer air is aloft. This rise of temperature with height is known as Temperature inversion. Temperature inversion takes place under certain specific conditions. These are discussed below:

•  Long winter nights – if in winters the sky is clear during long nights, the terrestrial radiation is accelerated. The reason is that the land surface gets cooled fairly quickly. The bottom layer of atmosphere in contact with the ground is also cooled and the upper layer remains relatively warm.
• Cloudless clear sky – The clouds obstruct the terrestrial radiation. But this radiation does not face any obstacles for being reflected into space when the sky is clear. Therefore the ground is cooled quickly and so is the air in contact with it cooled.
• Dry air – humid air absorbs the terrestrial radiation but dry air is no obstruction to terrestrial radiation and allows the radiation to escape into space.
• Calm atmosphere – the blowing of winds bring warm and cold air into contact. Under conditions of calm atmosphere the cold air stays put near the ground.
• Ice covered surface – in ice covered areas due to high albedo less insolation is received. During night due to terrestrial radiation most of the heat is lost to atmosphere and the surface is cooled. The air in contact with it is also cooled but the upper layer remains warm.

 

 

 

The ultimate source of atmospheric energy is in fact heat and light received through space from the Sun. This energy is known as solar insolation. The Earth receives only a tiny fraction of the total amount of Sun’s radiations. Only two billionths or two units of energy out of 1,00,00,00,000 units of energy radiated by the sun reaches the earth’s surface due to its small size and great distance from the Sun. The unit of measurements of this energy is Langley (Ly). On an average the earth receives 1.94 calories per sq. cm per minute (2 Langley) at the top of its atmosphere.

Incoming solar radiation through short waves is termed as insolation. The amount of insolation received on the earth’s surface is far less than that is radiated from the sun because of the small size of the earth and its distance from the sun. Moreover water vapour, dust particles, ozone and other gases present in the atmosphere absorb a small amount of insolation.

The amount of insolation received on the earth’s surface is not uniform everywhere. It varies from place to place and from time to time. The tropical zone receive the maximum annual insolation. It gradually decreases towards the poles. Insolation is more in summers and less in winters.
The following factors influence the amount of insolation received.
(i) The angle of incidence:-The angle formed by the sun’s ray with the tangent of the earth’s circle at a point is called
angle of incidence. It influences the insolation in two ways. First, when the sun is almost overhead, the rays of the sun are vertical. The angle of incidence is large hence, they are concentrated in a smaller area, giving more amount of insolation at that place. If the sun’s rays are oblique, angle of incidence is small and sun’s rays have to heat up a greater area, resulting in less amount of insolation received there. Secondly, the sun’s rays with small angle, traverse more of the atmosphere, than rays striking at a large angle. Longer the path of sun’s rays, greater is the amount of reflection and absorption of heat by atmosphere. As a result the intensity of insolation at a place is less.
(ii) Duration of the day. (daily sunlight period) :-The duration of day is controlled partly by latitude and partly by the season of the year. The amount of insolation has close relationship with the length of the day. It is because insolation is received only during the day. Other conditions remaining the same, the longer the days the greater is the amount of insolation. In summers, the days being longer the amount of insolation received is also more. As against this in winter the days are shorter the insolation received is also less. On account of the inclination of the earth on its axis at an angle of 23 ½ , rotation and revolution, the duration of the day is not same everywhere on the earth. At the equator there is 12 hours day and night each throughout the year. As one moves towards poles duration of the days keeps on increasing or decreasing. It is why the maximum insolation is received in equatorial areas.

(iii) Transparency of the atmosphere.Transparency of the atmosphere: Transparency of the atmosphere also determines the amount of insolation reaching the earth’s surface. The transparency depends upon cloud cover, its thickness, dust particles and water vapour, as they reflect, absorb or transmit insolation. Thick clouds hinder the insolation to reach the earth while clear sky helps it to reach the surface. Water vapour absorb insolation, resulting in less amount of insolation reaching the surface.

Heat Budget

Energy emitted by the Earth’s climate system tends to maintain a balance with solar energy coming into the system. This balance, known as the radiation budget, allows the Earth to maintain the moderate temperature range essential for life as we know it.
There is positive radiation balance between 35°S and 40°N, which drives the weather systems. Ocean currents even out the difference
When incoming short-wave solar radiation (Figure 3), known as insolation, enters the Earth’s climate system, a portion of it is absorbed at the Earth’s surface, causing the surface to heat up. Some of the absorbed energy is then radiated outward in the form of long-wave infrared radiation. Cloud layers trap some of the radiation from the Earth’s surface, and then emit long-wave radiation, both outward and back to the surface. The temperature of the Earth’s surface is about 33°C higher due to long-wave radiation contribution from the atmosphere .
The amount of radiation emitted by the Earth’s surface that makes it back to space is the result of many interrelated influences, such as the amount of cloud cover, cloud heights, characteristics of cloud droplets, amount and distribution of water vapor and other greenhouse gases, land features, surface temperature, and the transparency of the atmosphere. In the warm tropical areas, low values of outgoing longwave radiation (OLR) correspond to large amounts of high, cold clouds while high values of OLR correspond to relatively clear areas or cloudy areas with low, warm clouds. In the extra-tropics OLR values typically decrease with decreasing temperature.

Let us suppose that the total heat (incoming solar radiation) received at the top of the atmosphere is 100 units (see fig. 10.2) Roughly 35 units of it are reflected back into space even before reaching the surface of the earth. Out of these 35 units, 6 units are reflected back to space from the top of the atmosphere, 27 units reflected by clouds and 2 units from the snow and ice covered surfaces.
Out of the remaining 65 units (100-35), only 51 units reach the earth’s surface and 14 units are absorbed by the various gases, dust particles and water vapour of the atmosphere.
The earth in turn radiates back 51 units in the form of terrestrial radiation. Out of these 51 units of terrestrial radiation, 34 units are absorbed by the atmosphere and the remaining 17 units directly go to space. The atmosphere also radiates 48 units (14 units of incoming radiation and 34 units of outgoing radiation absorbed by it) back to space. Thus 65 units of solar radiation entering the atmosphere are reflected back into the space. This account of incoming and outgoing radiation always maintains the balance of heat on the surface of the earth.

 

An atmosphere is a layer of gases surrounding a planet or other material body, that is held in place by the gravity of that body. Many of the planets in this solar system have atmospheres, but none that we know of have an atmosphere quite like ours – one that can support life.

The air is a mixture of several gases. The air encompasses the earth from all sides. The air surrounding the Earth is called the atmosphere. The atmosphere is an integral part of our Earth. It is connected with the earth due to the gravitational force of the earth. It helps in stopping the ultra violet rays harmful for the life and maintain the suitable temperature necessary for life. The air is essential for the survival of all forms of life on the earth.

Composition of the atmosphere

 

The atmosphere is made up of different types of gases, water vapors and dust particles. The composition of the atmosphere is not static. It changes according to the time and place.

• Nitrogen N2  78%
• Oxygen O2 20.9%
• Argon Ar 9.34%
• Carbon dioxide CO2 3.84 %
• Neon
• Helium
• Methane
• Krypton
• Hydrogen
• Nitrous oxide
• Xenon
• Ozone

Water vapor is unique in that its concentration varies from 0-4% of the atmosphere depending on where you are and what time of the day it is.  In the cold, dry artic regions water vapor usually accounts for less than 1% of the atmosphere, while in humid, tropical regions water vapor can account for almost 4% of the atmosphere.  Water vapor content is very important in predicting weather.

Greenhouse gases whose percentages vary daily, seasonally, and annually have physical and chemical properties which make them interact with solar radiation and infrared light (heat) given off from the earth to affect the energy balance of the globe.

The atmosphere also change composition with height and can be divided into two layers. The lower layer is called the homosphere and has the composition we talked about earlier. It’s top is approximately the mesopause.

Above the homosphere lies the heterosphere, a layer in which the gases are stratified into four shells. The lowermost shell is dominated by molecular nitrogen (N2); next, a layer of atomic oxygen (O) is encountered, followed by a layer dominated by helium atoms (He), and finally, a layer consisting of hydrogen atoms (H).

Importance of various components of atmosphere are:-

(i) Oxygen is very important for the living beings.
(ii) Carbon dioxide is very useful for the plants.
(iii) Dust particles present in the atmosphere create suitable conditions for the precipitation.
(iv) The amount of water vapour in the atmosphere goes on changing and directly affects the plants and living beings.
(v) Ozone protects all kinds of life on the earth from the harmful ultra violet rays of the sun.

 

Structure  and stratification of the atmosphere

Variations of temperature, pressure and density are much larger in vertical directions than in horizontal. This strong vertical variations result in the atmosphere being stratified in layers that have small horizontal variability compare to the variations in the vertical.

The atmosphere can be divided into five layers according to the diversity of temperature and density.
(a) Troposphere :-It is the lowest layer of the atmosphere. The height of this layer is about 18 kms on the equator and 8 kms on the poles. The main reason of higher height at the equator is due to presence of hot convection currents that push the gases upward.
This is the most important layer of the atmosphere because all kinds of weather changes take place only in this layer. Due to these changes development of living world take place on the earth. The air never remains static in this layer. Therefore this layer is called changing sphere or troposphere.
The environmental temperature decreases with increasing height of atmosphere. It decreases at the rate of 1 C at the height of 165 metre. This is called Normal lapse rate.
The upper limit of the troposphere is called tropopause. This is a transitional zone. In this zone characteristics of both the troposphere and ionosphere are found.

(b) Stratosphere :-This layer lies above the troposphere and spread upto the height of 50 kms from the Earth’s surface. Its average extent 40 kms.
The temperature remains almost the same in the lower part of this layer upto the height of 20 kms. After this the temperature increases slowly with the increase in the height. The temperature increases due to the presence of ozone gas in the upper part of this layer.
Weather related incidents do not take place in this layer. The air blows horizontally here. Therefore this layer is considered ideal for flying of aircrafts.

(c) Mesosphere :-It spreads above the stratosphere upto the height of 80 kms. from the surface of the earth. It’s extent is 30 kms. Temperature goes on decreasing and drops upto – 100 C.

(d) Ionosphere :-The ionosphere lies from about 80-400 km in height and is electrically charged as short wave solar radiation ionizes the gas molecules. The electrical structure of the atmosphere is not uniform and is arranged into three layers, D, E, and F. Since the production of charged particles requires solar radiation, the thickness of each layer, particularly the D and E layers, changes from night to day. The layers weaken and disappear at night and reappear during the day. The F layer is present during both day and night. This change in height of the various electrically charged layers doesn’t effect the weather, but does effect radio signals.

The auroras also take place in the ionosphere since this is the electrically charged layer. The aurora borealis (northern lights) and aurora australis (southern lights) is closely correlated to solar flare activity.

(e) Exosphere:-This is the last layer of the atmosphere located above ionosphere and extends to beyond 400 km above the earth.  Gases are very sparse in this sphere due to the lack of gravitational force. Therefore, the density of air is very less here.

 

Table of content

• Weathering
• Mass Wasting
• Erosion and Deposition
• Soil Formation
• Landscape (Geological) Cycles
• Davis Cycle
• Penck Cycle

Weathering

 

Weathering is the general term applied to the combined action of all processes that cause rock to disintegrate physically and decompose chemically because of ex- posure near the Earth’s surface through the elements of weather. Among these elements temperature, rainfall, frost, fog and ice are the important ones. Weathering begins as soon as rocks come in contact with one or more than one elements of weather on the surface of the earth. In nature, generally both the disintegration and decomposition act together at the sametime and assist each other. We must remember that the weathered material (i.e. disintegrated and decomposed) lie in situ (i.e. at its original position). In this process no transportation or movement of material is involved other than its falling down under the force of gravity.

Weathering is the response of rocks to a changing environment. For example, plutonic rocks form under conditions at high pressures and temperatures. At the Earth’s surface they are not as stable as the conditions under which they formed. In response to the environmental change, they gradually weather (transform to more stable minerals).

Different types of Weathering are:-

1. Physical Weathering :-The mechanical breakup or disintegration of rock doesn’t change mineral makeup. It creates broken fragments or “detritus.” which are classified by size:

• Coarse-grained – Boulders, Cobbles, and Pebbles.
• Medium-grained – Sand
• Fine-grained – Silt and clay (mud).

Various process of Physical weathering are:-

• Development of Joints – Joints are regularly spaced fractures or cracks in rocks that show no offset across the fracture (fractures that show an offset are called faults).
• Crystal Growth – As water percolates through fractures and pore spaces it may contain ions that precipitate to form crystals. As these crystals grow they may exert an outward force that can expand or weaken rocks.
• Thermal Expansion – Although daily heating and cooling of rocks do not seem to have an effect, sudden exposure to high temperature, such as in a forest or grass fire may cause expansion and eventual breakage of rock. Campfire example.
• Root Wedging – Plant roots can extend into fractures and grow, causing expansion of the fracture. Growth of plants can break rock – look at the sidewalks of New Orleans for example.
• Animal Activity – Animals burrowing or moving through cracks can break rock.
• Frost Wedging – Upon freezing, there is an increase in the volume of the water (that’s why we use antifreeze in auto engines or why the pipes break in New Orleans during the rare freeze). As the water freezes it expands and
  exerts a force on its surroundings. Frost wedging is more prevalent at high altitudes where there may be many freeze-thaw cycles.

2. Chemcial weathering :-involves a chemical transformation of rock into one or more new compounds.  A group of weathering processes viz; solution , carnonation, hydration , oxidation and reduction acts on the roks to decompose, dissolve orreduce them to a fine clastic state through chemical reactions by oxygen ,surface /soil water and other acids. Water and air along with heat must be present to speed up all chemical reactions. Over and above the carbon dioxide present in the air, decomposition of plants and animals increases the quanitity of carbon dioxide underground . Chamical weathering involves four major processes:

• Oxidation is the process in which atmospheric oxygen reacts with the rock to produce oxides. The process is called oxidation. Greatest impact of this process is observed on ferrous minerals. Oxygen present in humid air reacts with iron grains in the rocks to form a yellow or red oxide of iron. This is called rusting of the iron. Rust decomposes rocks completely with passage of time.
• Carbonation is the process by which various types of carbonates are formed. Some of these carbonates are soluble in water. For example, when rain water con- taining carbon dioxide passes through pervious limestone rocks, the rock joints enlarge due to the action of carbonic acid. The joints enlarge in size and lime is removed in solution. This type of breakdown of rocks is called carbonation.
• Hydration is the process by which water is absorbed by the minerals of the rock. Due to the absorption of water by the rock, its volume increases and the grains lose their shape. Feldspar, for example, is changed into kaolin through hydration. Kaolin on Vindhyan Hills near Jabalpur has been formed in this manner.
• Solution is the process in which some of the minerals get dissolved in water. They are therefore removed in solution. Rock salt and gypsum are removed by this process.

3. Biotic weathering :- is a type of weathering that is caused by living organisms. Most often the culprit ofbiotic weathering are plant roots. These roots can extend downward, deep into rock cracks in search of water, and nutrients. In the process they act as a wedge, widening and extending the cracks.

Mass Wasting

 

Mass wasting is defined as the down slope movement of rock and regolith near the Earth’s surface mainly due to the force of gravity.   Mass movements are an important part of the erosional process, as it moves material from higher elevations to lower elevations where transporting agents like streams and glaciers can then pick up the material and move it to even lower elevations.   Mass movement processes are occurring continuously on all slopes; some act very slowly, others occur very suddenly, often with disastrous results.  Any perceptible down slope movement of rock or regolith is often referred to in general terms as a landslide.  Landslides, however, can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. Mass wasting can be classified as:-

 

• Slope Failures – a sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping.
• Sediment Flows – debris flows down hill mixed with water or air.

 

Erosion and Deposition

 

Soil erosion is the deterioration of soil by the physical movement of soil particles from a given site. Wind, water, ice, animals, and the use of tools by man are usually the main causes of soil erosion. It is a natural process which usually does not cause any major problems. It becomes a problem when human activity causes it to occur much faster than under normal conditions.The removal of soil at a greater rate than its replacement by natural agencies (water, wind etc.) is known as soil erosion.
Soil erosion is of four types which are as follows:-

• Wind Erosion :-Winds carry away vast quantity of fine soil particles and sand from deserts and spread it over adjoining cultivated land and thus destroy their fertility. This type of erosion is known as wind erosion. It takes place in and around all desert regions of the world. In India, over one lakh kilometers of land is under Thar Desert, spread over parts of Gujarat, Haryana, Punjab and Rajasthan states. These areas are subject to intense wind erosion.
• Sheet Erosion :-Water when moves as a sheet takes away thin layers of soil. This type of erosion is called sheet erosion. Such type of erosion is most common along the river beds and areas affected by floods. In the long run, the soil is com- pletely exhausted due to removal of top soil and becomes infertile.
• Rill Erosion :-The removal of surface material usually soil, by the action of running water. The processes create numerous tiny channels (rills) a few centimeters in depth, most of which carry water only during storms.
• Gully Erosion :-When water moves as a channel down the slope, it scoops out the soil and forms gullies which gradually multiply and in the long run spread over a wide area. This type of erosion is called gully erosion. The land thus dissected is called bad lands or ravines. In our country, the two rivers Chambal and Yamuna are famous for their ravines in U.P. and M.P. states.

Deposition / Sedimentation – occurs when sediment settles out as winds/water current die down, or as glaciers melt. When sediment is transported and deposited, it leaves clues to the mode of transport and deposition. For example, if the mode of transport is by sliding down a slope, the deposits that result are generally chaotic in nature, and show a wide variety of particle sizes. Grain size and the interrelationship between grains gives the resulting sediment texture. Thus, we can use the texture of the resulting deposits to give us clues to the mode of transport and deposition. Sorting – The degree of uniformity of grain size. Particles become sorted on the basis of density, because of the energy of the transporting medium. High energy currents can carry larger fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density.

Soil Formation

 

Soil consists of rock and sediment that has been modified by physical and chemical interaction with organic material and rainwater, over time, to produce a substrate that can support the growth of plants.Soil is the uppermost layer of the land surface that plants use and depend on for nutrients, water and physical support.

Factors of soil formation are:-

• Parent material: soil formation depends on the mineral material, or organic material from which the soil is formed. Soils will carry the characteristics of its parent material such as color, texture, structure, mineral composition and so on. For example, if soils are formed from an area with large rocks (parent rocks) of red sandstone, the soils will also be red in color and have the same feel as its parent material.
• Time: Soils can take many years to form. Younger soils have some characteristics from their parent material, but as they age, the addition of organic matter, exposure to moisture and other environmental factors may change its features. With time, they settle and are buried deeper below the surface, taking time to transform. Eventually they may change from one soil type to another.
• Climate:Two important climatic components, temperature and precipitation are key. They determine how quickly weathering will be, and what kind of organic materials may be available on and inside of the soils. Moisture determines the chemical and biological reactions that will occur as the soils are formed. Warmer climate with more rainfall means more vegetative cover and more animal action. It also means more runoff, more percolation and more water erosion. They all help to determine the kind of soils in an area.
• Relief:i.e. the landscape position and the slopes it has. Steep, long slopes mean water will run down faster and potentially erode the surfaces of slopes. The effect will be poor soils on the slopes, and richer deposits at the foot of the slopes. Also, slopes may be exposed to more direct sunlight, which may dry out soil moisture and render it less fertile.
• Organisms:The source and richness of organic matter is down to the living things (plants and animals) that live on and in the soils. Plants in particular, provide lots of vegetative residue that are added to soils. Their roots also hold the soils and protect them from wind and water erosion. They shelter the soils from the sun and other environmental conditions, helping the soils to retain the needed moisture for chemical and biological reactions. Fungi, bacteria, insects, earthworms, and burrowing animals help with soil aeration. Worms help breakdown organic matter and aid decomposition. Animal droppings, dead insects and animals result in more decaying organic matter. Microorganisms also help with mineral and nutrient cycling and chemical reactions.

 

Davis Cycle

 

After the upliftment of landmass by the tectonic forces the process of denudation is started. The rivers, valleys and associated landforms passes through distinctive stages, provided that there has been no significant interference by earth movements or by changes of sea-level or climate. This idealized concept of landscape evolution was introduced to geomorphology more than sixty years ago by W.M. Davis, who referred to the whole sequence of stage as a Cycle of Erosion.

The basic goal of Davisian model of geographical cycle and general theory of landform development was to provide basis for a systematic descriptions and genetic classification of landforms. According to this concept a landscape has a definite life history, and as the processes of land structure operate on it the surface features are marked by several changes in its life time. Thus, the evolution of landscape passes through a cycle, and cycle follows a definite sequence of development.

The successive stage of developmental sequences can be divided into three parts and may be identified as youth, maturity and old age. Davis presentation of scheme was both vigorous and vivid and his colourful analogy of the human life and landscapes both passing through the stages of youth, maturity and old age caught the imagination of scientific world.

• Youth:The uplift is complete and has stopped. Immediately erosion of the uplifted block sets in. The streams follow initial irregularities available without adjusting to the structure. These are consequent streams. The floors of the valley suffer down cutting while the summits remain almost unaffected. Increased relief heralds the beginning of mature age
• Maturity:At this stage, the vertical erosion slows down and the horizontal action increases. A characteristic feature is the erosion of mountain tops at a faster rate than lowering of the valley floor. The coming closer of lines ‘A’ and ‘B’ indicates emergence of a gentle slope. The subsequent streams gain importance now.
• Old Age:A gentle gradient, accentuated by horizontal action and deposition, reduces the erosion intensity. A thick layer of sediment represents the earlier erosion activity. The landforms get mellowed—lines ‘A’ and ‘B’ run parallel to each other. Relicts of mountains or monad knocks are dotting the water divides and a featureless plain—peneplane is produced.

In order to understand the evolution of a particular landscape it is extremely important to know the stage of development. But the geographical structure and the nature of rocks also exert an important influence on the fashioning of landscapes is a function of structure, process and time (as called as stage by the followers of Davis). These three factors are called as ‘Trio of Davis’.

Structure :means lithological (rock types) and structural characteristics (folding, faulting, joints etc.) of rocks. Time was not only used in temporal context but it was also used as a process itself leading to an inevitable progression of change of landform. Process means the agent of denudation including both, weathering and erosion (running water in the case of geographical cycle).

Process:Implies the factors or agents responsible for weathering and erosion.

Time:Implies the stage at which the cycle is—youth, maturity or old age.

Penck Cycle 

 

According to German geomorphologist Walther Penck, the characteristics of landforms of a given region are related to the tectonic activity of that region. Contrary to the Davisian concept that “landscape is a function of structure, process and time (stage)”, Penck put forward his view that geomorphic forms are an expression of the phase and rate of uplift in relation to the rate of degradation, where it is assumed that interaction between the two factors, uplift and degradation, is continuous. According to Penck’s view the landforms observed at any given site give expression to the relation between the two factors of uplift and degradation that has been or is in effect, and not to a stage in a progressive sequence.

Penck proposed three types of valley slopes on the basis of erosional intensity acting on crustal movements.

1. Straight slope:Indicating uniform erosion intensity and a uniform development of landforms or ‘Gleichformige Entwickelung’ in German.
2. Convex slope:Indicating waxing erosion intensity and a waxing development of landforms or ‘Aufsteigende Entwickelung.
3. Concave slope:Indicating waning erosion intensity and a waning development of landforms or ‘Absteigende Entwickelung.’

Different Phases according to Penck are:-

(a) Phase of waxing rate of landform development (Aufsteigende Entwickelung)
Endogenetic forces cause the slow rise of the initial land surface (Primarumpf) but later on the upliftment is rapid.
In this phase, because of upliftment and the increase in the channel gradient and stream velocity rivers continue to degrade their valleys with accelerated rate of valley deepening.
The rate of upliftment is faster than the rate of down-cutting. It results in the formation of gorges and narrow V-shaped valleys. Since the upliftment of landmass far exceeds the valley deepening, the absolute height goes on increasing.
Altitude of the summit of interfluves and valley bottom continues to increase due to the faster rate of upliftment than that of the vertical erosion.
This phase is characterized by the maximum altitude and the maximum relief (relative heights of the valley floors).

(b) Phase of uniform development of land form (Gleichformige Entwickelung)
This phase may be divided into three sub-phases on the basis of upliftment and consequent degradation

(i) The first sub-phase is characterised by the continuance of accelerated rate of uplift. The absolute height continues to increase because the rate of upliftment is still greater than the rate of down-cutting.
The maximum altitude or the absolute relief is achieved, but relative relief remains unaffected because the rate of valley deepening is almost equal to the rate of lowering of the summits of stream interfluves.
The valley walls are steep. This is known as the phase of uniform development because of uniformity in the rate of valley deepening and lowering of divide summits.
(ii) In the second sub-phase the absolute relief neither increases nor decreases. This is due to the fact that rate of upliftment and the rate of erosion are the same. However, in this phase the absolute height and the relative relief’s are unchanged. So this may be called the phase of uniform development of landforms.
(iii) In this sub-phase there is no more upliftment of land.

(c) Phase of Wanning development of landscape (Absteigende Entwickelung)
The erosional processes dominate in this phase. The lateral erosion rather than vertical erosion is more important. There is progressive decrease in the height of the landforms. In other words, the absolute and the relative relief decline.
The valley side slope consists of two parts, the upper and the lower. The upper segment continues to have steep angle which is called as gravity slope.
The lower segment of the slope is called wash slope. The wash slope is composed of talus materials of lower inclination which is formed at the base of valley sides.
The later part of this phase is marked by the presence of inselbergs and a series of concave wash slopes.
This type of extensive surface produced at the fag end of absteigende entwickelung has been labelled is endrumpf which may be equivalent to peneplain as envisaged by Davis in his cycle concept. Thus, the cycle of landscape development as envisaged by Penck ends in endrumpf.