GS Notes

1.5 - Geography Notes 21

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Elaborate Notes

INSOLATION, HEAT BUDGET, AND ALBEDO

  • Insolation (Incoming Solar Radiation):
    • Insolation is the primary source of energy for almost all natural processes on the Earth’s surface and in its atmosphere. It is defined as the solar radiation that reaches the Earth’s surface, measured by the amount of solar energy received per square centimetre per minute.
    • The Sun radiates a vast amount of energy, but the Earth, being a small and distant body, intercepts only a minuscule fraction of it—approximately one part in two billion.
    • Solar Constant: This is the average amount of solar radiation received at the outer edge of the Earth’s atmosphere on a surface perpendicular to the Sun’s rays when the Earth is at its mean distance from the Sun. Its value is approximately 1.94 calories per square centimetre per minute, often rounded to 1367 Watts per square meter. This value is not truly constant; it exhibits minor fluctuations due to solar activity, such as sunspots. The term was coined by Charles Greeley Abbot and Samuel Pierpont Langley of the Smithsonian Astrophysical Observatory in the early 20th century.
  • Wavelength of Radiation:
    • The Sun, with a surface temperature of about 6000 K, radiates energy primarily in the form of short-wavelength radiation (ultraviolet, visible, and near-infrared). This is governed by Wien’s Displacement Law, which states that the wavelength of maximum emission is inversely proportional to the object’s absolute temperature.
    • The Earth, with an average surface temperature of about 288 K (15°C), absorbs this shortwave radiation and re-radiates energy as long-wavelength infrared radiation, also known as Terrestrial Radiation. This fundamental difference in wavelengths is crucial to understanding the Greenhouse Effect.

GREEN HOUSE EFFECT

  • Mechanism: The Earth’s atmosphere is largely transparent to the incoming shortwave solar radiation, allowing it to pass through and heat the Earth’s surface. However, it is relatively opaque to the outgoing longwave terrestrial radiation.
  • Historical Context: The concept was first proposed by French mathematician Joseph Fourier in 1824, who argued that the atmosphere must act like the glass of a hothouse (greenhouse), letting in solar radiation but trapping the outgoing heat. In 1859, Irish physicist John Tyndall experimentally identified the gases responsible, demonstrating that water vapour (H₂O) and carbon dioxide (CO₂) were strong absorbers of infrared radiation. In 1896, Swedish scientist Svante Arrhenius was the first to quantify the effect, calculating that a doubling of atmospheric CO₂ could lead to significant global warming.
  • Greenhouse Gases (GHGs): These are the atmospheric gases that absorb and re-emit longwave radiation, thereby trapping heat. Key GHGs include:
    • Primary GHGs: Carbon Dioxide (CO₂), Methane (CH₄), Nitrous Oxide (N₂O), and Water Vapour (H₂O). Water vapour is the most abundant GHG, but its concentration is a function of temperature (feedback mechanism), while others are long-lived and drive climate change.
    • Fluorinated Gases: These are synthetic, potent GHGs, including Hydrofluorocarbons (HFCs), Perfluorocarbons (PFCs), Sulphur Hexafluoride (SF₆), and Nitrogen Trifluoride (NF₃). These are regulated under international agreements like the Kigali Amendment to the Montreal Protocol.
  • Atmospheric Heating and Normal Lapse Rate:
    • The primary source of heat for the troposphere is not direct solar radiation, but the terrestrial radiation emitted from the Earth’s surface. The atmosphere is heated from below.
    • Consequently, in the troposphere, air temperature generally decreases with an increase in altitude. This rate of temperature decrease is called the Normal Lapse Rate (or Environmental Lapse Rate), and its average value is 6.5°C per 1000 meters (or 1 km). This rate is an average and can vary based on location, season, and time of day.

HEAT TRANSFER

The Earth’s atmosphere and oceans are in constant motion, redistributing the surplus heat from the tropics to the deficit regions of the poles. This is achieved through several mechanisms:

  • (1) Radiation: This is the transfer of heat through electromagnetic waves. It does not require a medium. Both incoming solar radiation and outgoing terrestrial radiation are forms of radiative heat transfer. For example, the ground cools at night by radiating heat back into space.
  • (2) Conduction: This is the transfer of heat through direct molecular contact. It is most effective in solids. In the context of atmospheric science, conduction is significant only in the thin layer of air directly in contact with the Earth’s surface. During the day, the ground heats the air layer above it via conduction.
  • (3) Convection: This involves the transfer of heat through the vertical movement of a fluid (liquid or gas). When the air near the surface is heated, it expands, becomes less dense, and rises. Cooler, denser air from above sinks to take its place, creating a convective cell. This process is fundamental to the formation of clouds (e.g., cumulus clouds) and thunderstorms, especially in equatorial regions where intense surface heating drives strong convective currents.
  • (4) Advection: This is the horizontal transfer of heat by the movement of air (winds) or water (ocean currents). Advection is a large-scale process responsible for significant heat redistribution across the globe.
    • Example: The planetary winds, like the Westerlies, carry warm air from lower latitudes to higher latitudes. Similarly, ocean currents like the Gulf Stream transport vast amounts of heat from the Gulf of Mexico across the Atlantic, moderating the climate of Western Europe.

FACTORS AFFECTING INSOLATION

The amount of insolation reaching the Earth’s surface at any given location is not uniform and is influenced by several factors:

  • (1) Transparency of the Atmosphere: The atmosphere is not perfectly transparent. Insolation can be reflected, scattered, or absorbed by atmospheric constituents.
    • Clouds: Cloud cover is the most significant factor. Thick clouds can reflect a large portion of incoming radiation back to space.
    • Aerosols: Dust particles, salt, pollen, and pollutants (aerosols) scatter and absorb solar radiation. Major volcanic eruptions, like that of Mount Pinatubo in 1991, can inject large amounts of sulphur dioxide into the stratosphere, forming sulphate aerosols that increase atmospheric reflection and cause a temporary global cooling effect.
  • (2) Latitude (Angle of Incidence):
    • The angle at which the Sun’s rays strike the Earth’s surface is a primary determinant of insolation intensity. In tropical regions (between 23.5°N and 23.5°S), the sun is overhead or nearly overhead for much of the year. The vertical rays concentrate energy on a smaller surface area, leading to more intense heating.
    • In temperate and polar regions, the sun’s rays strike the surface at an oblique (low) angle. The same amount of energy is spread over a larger area, resulting in less intense heating. Furthermore, oblique rays must travel through a greater thickness of the atmosphere, leading to more absorption and scattering.
  • (3) Length of the Day (Duration of Sunlight):
    • The duration of daylight varies significantly with latitude and season due to the Earth’s axial tilt (23.5°). In the summer solstice, the respective hemisphere experiences its longest day, allowing for a longer period of solar heating. Conversely, winter days are shorter, reducing the total insolation received. At the poles, this effect is extreme, with up to six months of continuous daylight followed by six months of darkness.

HEAT BUDGET

  • The Earth’s Heat Budget is an accounting of the balance between incoming solar radiation and outgoing terrestrial radiation. Over long periods, the Earth maintains a stable average temperature because the total energy gained is equal to the total energy lost. This state of equilibrium is essential for a stable climate.
  • Breakdown of the Budget (based on the provided image):
    • Assume 100 units of solar radiation reach the top of the atmosphere.
    • Incoming Path (Shortwave):
      • 35 units are reflected back to space (Albedo): 27 units by clouds, 2 units by the Earth’s surface (snow, ice), and 6 units by atmospheric scattering.
      • 65 units are absorbed: 14 units by the atmosphere (gases, water vapour) and 51 units by the Earth’s surface.
    • Outgoing Path (Longwave):
      • The Earth’s surface, having absorbed 51 units, radiates this energy back as longwave radiation.
      • 17 units are radiated directly to space through the “atmospheric window” (wavelengths where absorption by GHGs is minimal).
      • 34 units are absorbed by the atmosphere.
      • The atmosphere, now holding 14 units (from direct absorption) + 34 units (from terrestrial radiation), radiates a total of 48 units. This radiation goes both upwards to space and downwards back to the surface (counter-radiation, part of the greenhouse effect). The image simplifies this by showing 48 units radiated to space from the atmosphere.
    • Final Balance: 35 units (reflected) + 17 units (radiated from surface) + 48 units (radiated from atmosphere) = 100 units lost to space. This balances the 100 units of incoming radiation.
    • The heat budget is not balanced at every latitude. There is a net energy surplus in the tropics and a net energy deficit in the polar regions. This imbalance drives global atmospheric and oceanic circulation.

ALBEDO

  • Albedo is the measure of the reflectivity of a surface. It is expressed as a percentage or a decimal (Reflection Coefficient) representing the ratio of reflected radiation to incident radiation.
  • Average Albedo: The Earth’s average planetary albedo is about 30-35%. This means about one-third of the incoming solar radiation is reflected back to space without contributing to the heating of the Earth system.
  • Albedo of Different Surfaces:
    • Fresh Snow/Ice: 80-95% (highly reflective)
    • Clouds (thick): 70-80%
    • Desert Sand: 30-40%
    • Forests: 10-20%
    • Asphalt/Dark Surfaces: 5-10%
    • Water Bodies: Varies with sun angle; low for vertical rays (less than 10%), high for oblique rays.
  • Albedo plays a crucial role in climate feedback loops. For instance, the ice-albedo feedback: as global temperatures rise, ice and snow melt, exposing darker ocean or land surfaces. These darker surfaces have a lower albedo, absorb more solar radiation, and cause further warming, leading to more melting.

TEMPERATURE AND FACTORS AFFECTING IT

Temperature is a measure of the degree of sensible heat or coldness of a body or an environment. It is a direct result of the energy balance at a given location.

  • (1) Insolation: The primary factor. Higher insolation generally leads to higher temperatures. However, the location with the highest insolation may not be the hottest. For example, the sub-tropics often record higher temperatures than the equator because of clearer skies and less cloud cover, which allows more direct insolation to reach the surface.
  • (2) Albedo: As discussed, surfaces with high albedo (e.g., ice caps) remain cooler because they reflect more sunlight, while low-albedo surfaces (e.g., tarmac roads) absorb more energy and become hotter.
  • (3) Nature of the Surface (Specific Heat):
    • Specific heat is the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius.
    • Water has a very high specific heat capacity compared to land (rock, soil). This means water requires more energy to heat up and also cools down more slowly.
    • Consequently, land surfaces (low specific heat) experience rapid and intense heating and cooling, leading to large diurnal (daily) and annual temperature ranges. Water bodies (high specific heat) have a moderating effect, with smaller temperature fluctuations.
  • (4) Distance from the Sea (Continentality): Locations in the interior of large continents are far from the moderating influence of oceans. They exhibit extreme temperature conditions, with very hot summers and very cold winters. This is known as continentality. Coastal areas, by contrast, experience a maritime climate with milder temperatures throughout the year. For instance, Delhi has a much higher annual range of temperature than Mumbai.
  • (5) Distribution of Continents: The Northern Hemisphere is often called the “land hemisphere” due to its greater proportion of continental landmass, while the Southern Hemisphere is the “water hemisphere.” This distribution causes the Northern Hemisphere to experience greater temperature extremes and a higher average annual temperature range than the Southern Hemisphere.
  • (6) Altitude: Due to the Normal Lapse Rate, temperature decreases with increasing altitude in the troposphere. This is why high-altitude locations like Shimla are cooler than plains locations like Chandigarh, even if they are at a similar latitude.
  • (7) Winds (Advection): Winds are agents of heat transport. Onshore winds can bring cool, moist air to a coastal region in summer, lowering temperatures. Offshore winds can do the opposite. Large-scale winds like the Loo in Northern India transport intense heat from the desert regions, causing a sharp rise in temperature during summer.
  • (8) Ocean Currents: Ocean currents act as global conveyor belts of heat. Warm currents, like the North Atlantic Drift, carry warm equatorial waters poleward, making regions like Western Europe significantly warmer than they would be otherwise at their latitude. Cold currents, like the Peru (Humboldt) Current, bring cold polar waters equatorward, cooling the adjacent coastlines.

Prelims Pointers

  • Insolation: Incoming Solar Radiation.
  • Earth intercepts about 1 part in 2 billion of the Sun’s total energy output.
  • Solar Constant: The rate at which energy is received at the outer edge of the atmosphere. Its value is approximately 1.94 calories/cm²/minute or 1367 W/m².
  • Incoming solar radiation is shortwave radiation.
  • Outgoing radiation from the Earth’s surface is longwave radiation (Terrestrial Radiation).
  • Greenhouse Effect: The process of warming the atmosphere by trapping outgoing longwave radiation.
  • Greenhouse Gases (GHGs):
    • Primary: CO₂, CH₄, N₂O, Water Vapour (H₂O).
    • Synthetic: HFCs, PFCs, SF₆, NF₃.
  • The atmosphere is primarily heated from below by terrestrial radiation.
  • Normal Lapse Rate: The average rate of temperature decrease with altitude in the troposphere is 6.5°C per km.
  • Methods of Heat Transfer:
    1. Radiation: Transfer through electromagnetic waves.
    2. Conduction: Transfer through molecular contact.
    3. Convection: Vertical transfer of heat by mass movement of a fluid.
    4. Advection: Horizontal transfer of heat by mass movement (e.g., winds, ocean currents).
  • Albedo: The ratio of reflected radiation to the total incident radiation. Also called the Reflection Coefficient.
  • The average planetary albedo of Earth is ~35%.
  • Highest Albedo: Fresh snow (80-95%).
  • Lowest Albedo: Asphalt, dark soil (5-10%).
  • Land heats up and cools down faster than water due to its lower specific heat capacity.
  • Continentality: The effect of a location’s distance from the sea, leading to extreme temperatures in continental interiors.
  • The Northern Hemisphere has a higher annual range of temperature than the Southern Hemisphere due to a larger landmass.

Mains Insights

GS Paper I (Geography)

  • Inter-relationship between Insolation, Heat Budget, and Climate: The differential heating of the Earth’s surface, caused by variations in insolation due to latitude and surface characteristics, is the fundamental driver of global atmospheric circulation. The surplus heat in the tropics and deficit at the poles creates global pressure belts and planetary wind systems (e.g., Trade Winds, Westerlies) and ocean currents, which are nature’s mechanism to balance the global heat budget. Understanding this link is crucial to explaining global climate patterns and the distribution of biomes.
  • Continentality and Human Settlements: The concept of continentality explains why coastal regions (e.g., Mumbai, London) have moderate climates and are often densely populated, while continental interiors (e.g., Delhi, Central Siberia) face extreme climates that pose challenges to agriculture and human life. This physical factor has historically shaped patterns of settlement, economic activity, and cultural adaptations.
  • Altitude as a Climate Modifier: Altitude creates unique microclimates and vertical zonation of vegetation and human activities. For example, in tropical regions, high mountains can support temperate or even alpine ecosystems, a phenomenon described by Alexander von Humboldt. This allows for the cultivation of different crops at different elevations and influences settlement patterns.

GS Paper III (Environment & Climate Change)

  • The Greenhouse Effect: Natural vs. Anthropogenic: It is vital to differentiate between the natural greenhouse effect, which keeps the Earth’s average temperature at a habitable 15°C (without it, it would be -18°C), and the enhanced greenhouse effect caused by anthropogenic emissions. The Mains answer should clearly articulate that human activities since the industrial revolution have drastically increased the concentration of long-lived GHGs like CO₂, upsetting the Earth’s delicate heat budget and leading to global warming.
  • Albedo and Climate Feedback Loops: Albedo is not a static factor. Changes in land use (deforestation), urbanization (creating “urban heat islands” with low-albedo surfaces), and the melting of polar ice caps alter the Earth’s surface albedo. The ice-albedo feedback is a critical positive feedback loop that accelerates warming. Such concepts are essential for explaining the complexities and non-linear nature of climate change.
  • Impact on Economic Sectors: The disruption of the global heat budget manifests as climate change, which has direct and severe consequences for the economy.
    • Agriculture: Changes in temperature and precipitation patterns affect crop suitability and yields.
    • Infrastructure: Sea-level rise threatens coastal cities and infrastructure, while extreme weather events damage property and disrupt supply chains.
    • Energy: Changes in heating and cooling demands, as well as impacts on hydropower and renewable energy generation.

GS Paper IV (Ethics, Integrity and Aptitude)

  • Climate Justice and Equity: The alteration of the global heat budget by anthropogenic GHGs raises profound ethical questions. Historically, developed nations have been the largest emitters, yet the most severe impacts of climate change are disproportionately borne by developing and least developed countries (LDCs) which have contributed the least to the problem. This leads to the principle of “Common But Differentiated Responsibilities and Respective Capabilities” (CBDR-RC) in international climate negotiations.
  • Inter-generational Equity: The current generation’s actions in altering the climate system have long-lasting consequences for future generations. The concept of sustainable development is rooted in the ethical responsibility to ensure that our pursuit of economic growth does not compromise the ability of future generations to meet their own needs and live on a habitable planet. This involves a moral obligation to mitigate climate change and preserve the Earth’s natural systems.

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