GS Notes

1.5 - Geography Notes 22

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Elaborate Notes

Temperature and Factors Affecting Temperature

  • Fundamental Concepts

    • Temperature: A measure of the degree of hotness or coldness of a body or environment, corresponding to the average kinetic energy of its atoms or molecules. In climatology, it specifically refers to the sensible heat in the atmosphere.
    • Measurement: Temperature is measured using a thermometer. Historical development includes the Fahrenheit scale (Daniel Gabriel Fahrenheit, 1724), the Celsius scale (Anders Celsius, 1742), and the Kelvin scale (William Thomson, Lord Kelvin, 1848), which is the SI base unit of temperature.
    • Key Temperature Ranges:
      • Diurnal Range of Temperature: This is the difference between the maximum and minimum temperatures recorded within a 24-hour period. It is highest in hot, dry deserts (e.g., Sahara Desert) due to clear skies and low humidity, and lowest in humid, equatorial regions (e.g., Amazon Basin) and over large water bodies due to cloud cover and the high specific heat of water.
      • Mean Daily Temperature: Calculated as the average of temperature readings taken over a 24-hour period, or more simply, as (Maximum Temperature + Minimum Temperature) / 2.
      • Mean Monthly Temperature: The average of the mean daily temperatures for a given month.
      • Annual Range of Temperature: The difference between the mean monthly temperature of the warmest month and the coldest month of the year. This range is minimal near the equator and increases significantly towards the poles, especially over continental interiors (continentality).

  • Global Temperature Extremes

    • Highest Recorded Temperature: The World Meteorological Organization (WMO) officially recognizes 56.7°C (134°F) recorded on 10 July 1913 at Furnace Creek Ranch, Death Valley, California, USA. The previously held record of 58°C in Al Azizia, Libya (1922) was invalidated by the WMO in 2012 due to issues with instrumentation and observation practices.
    • Lowest Recorded Temperature: The lowest confirmed temperature is -89.2°C (-128.6°F) recorded on 21 July 1983 at the Soviet Vostok Station in Antarctica.

  • Distribution of Temperature and Isotherms

    • Isotherms: These are imaginary lines on a map connecting points that have the same temperature at a given time or on average over a given period. The concept was pioneered by Prussian geographer and naturalist Alexander von Humboldt in 1817 to represent temperature distribution visually.
    • Latitudinal Variation: The primary control on temperature distribution is latitude, as it determines the angle of incidence of solar radiation (insolation). However, the distribution is not uniform along a given latitude due to factors like altitude, continentality, ocean currents, and prevailing winds.
    • Seasonal Shifting of Isotherms:
      • The isotherms shift northwards during the Northern Hemisphere’s summer (around July) and southwards during its winter (around January), following the apparent migration of the sun.
      • This seasonal shift is much more pronounced over the Northern Hemisphere compared to the Southern Hemisphere. This is a direct consequence of the differential heating of land and water; the Northern Hemisphere has a much larger landmass, which heats up and cools down more rapidly than the vast oceanic expanses of the Southern Hemisphere.
    • Behavior of Isotherms over Land and Oceans:
      • Isotherms bend when they cross from a land surface to an ocean surface.
      • In January (Northern Winter): Oceans are relatively warmer than continents. Therefore, isotherms bend poleward when moving over the oceans, indicating warmer conditions extending to higher latitudes.
      • In July (Northern Summer): Oceans are relatively cooler than continents. Consequently, isotherms bend equatorward over the oceans, indicating that cooler conditions are found at lower latitudes over water compared to land.
    • Annual Temperature Range Patterns: The annual range of temperature is lowest near the equator (typically < 5°C) and increases towards the poles. The highest annual ranges are found in the continental interiors of the mid-to-high latitudes, such as in Siberia (e.g., Verkhoyansk), where the range can exceed 60°C.

Temperature Inversion

  • Definition: Under normal atmospheric conditions, temperature decreases with an increase in altitude in the troposphere. This is known as the Normal Lapse Rate, averaging about 6.5°C per 1000 meters. A temperature inversion is a deviation from this normal pattern, where a layer of cooler air at the surface is overlain by a layer of warmer air. This phenomenon creates a highly stable atmospheric condition.

  • Types of Temperature Inversion

    • Radiation Inversion (Surface Inversion):
      • Mechanism: Occurs when the ground rapidly radiates and loses heat during the night, a process known as terrestrial radiation. This cools the land surface and, through conduction, the layer of air immediately in contact with it. The air above this layer remains relatively warm.
      • Favorable Conditions:
        1. Long Winter Nights: Allows for a prolonged period of outgoing radiation without incoming solar radiation.
        2. Cloudless, Clear Skies: Clouds act like a blanket, reflecting outgoing longwave radiation back to the surface, thus preventing significant cooling. Clear skies permit maximum heat loss.
        3. Calm, Stable Air: Wind mixes the air, preventing the formation of a distinct cold layer near the surface. Calm conditions allow the stratification of air layers.
        4. Dry Air: Water vapor absorbs terrestrial radiation, so dry air near the ground promotes more rapid cooling.
    • Air Drainage Inversion (Katabatic or Valley Inversion):
      • Mechanism: Common in mountainous or hilly terrain. During the night, the air on the upper slopes cools down rapidly through radiation. This cold air, being denser, drains downslope under the influence of gravity, displacing the warmer, lighter air in the valley floor upwards. This results in the valley floor being filled with cold air while the warmer air forms a layer above it on the slopes. This downward flow of cold air is known as a katabatic wind.
    • Advection Inversion:
      • Mechanism: This occurs when a warm air mass moves horizontally (advection) over a cold surface, such as a snow-covered landscape or a cold ocean current. The lower layers of the warm air are cooled by contact with the cold surface, creating an inversion. This is common in coastal areas where warm continental air moves over cold ocean currents (e.g., California coast with the cold California Current).
    • Frontal Inversion (Subsidence Inversion):
      • Mechanism: Occurs at the boundary (front) between two air masses of different temperatures and densities. In a cold front or a warm front, the warmer, less dense air is forced to rise over the colder, denser air. This creates a situation where warmer air is positioned above colder air, forming an inversion along the frontal surface. This concept was extensively developed by the Bergen School of Meteorology, founded by Vilhelm Bjerknes in Norway around 1917.

Significance of Temperature Inversion

  • Atmospheric Stability and Weather:
    • Inversions create extremely stable atmospheric conditions. The warmer, lighter air on top acts as a ‘lid’, preventing vertical air movement (convection). This inhibits cloud formation and precipitation.
    • It leads to the formation of fog or mist, as the cold surface air cools to its dew point, causing water vapor to condense. Radiation fog is a classic example.
  • Air Pollution:
    • The stable ‘lid’ traps pollutants (like particulate matter, SOx, NOx) close to the ground, preventing their dispersal. This leads to a significant deterioration in air quality, especially in urban and industrial areas.
    • Example: The severe winter air pollution in Delhi and the broader Indo-Gangetic Plain is exacerbated by radiation inversions. The trapping of pollutants leads to the formation of dense smog (smoke + fog). Historically, the Great Smog of London in 1952 was a deadly air pollution event caused by a combination of coal smoke and a persistent temperature inversion, leading to thousands of deaths.
  • Transportation: The formation of dense fog drastically reduces visibility, causing major disruptions to air, road, and rail transport.
  • Settlement and Agriculture:
    • In mountainous regions, air drainage inversion makes valley floors prone to severe frost (‘frost pockets’). Consequently, human settlements and agricultural activities are often located on the middle slopes (known as the thermal belt), which remain warmer and frost-free.
    • Examples: Coffee plantations in the highlands of Brazil (fazendas) and fruit orchards (apples, apricots) in Himachal Pradesh and Uttarakhand in India are typically situated on slopes to avoid the cold air that settles in the valleys.

Pressure

  • Fundamental Concepts
    • Atmospheric Pressure: The force exerted per unit area by the weight of the column of air above that area. It is measured in millibars (mb) or Pascals (Pa). Standard sea-level pressure is defined as 1013.25 mb.
    • Measurement: Atmospheric pressure is measured using a barometer. The mercury barometer was invented by Evangelista Torricelli in 1643. Modern instruments include the aneroid barometer.
  • Factors Causing Variation in Pressure
    • Temperature (Thermal Factor): Air expands when heated, becoming less dense and exerting lower pressure. Conversely, cold air contracts, becomes denser, and exerts higher pressure. This creates a fundamental inverse relationship: High Temperature → Low Pressure; Low Temperature → High Pressure.
    • Altitude: Atmospheric pressure decreases with an increase in altitude because the mass of the overlying air column decreases.
    • Air Motion (Dynamic Factor):
      • Convergence and Subsidence: When air converges at high altitudes, it is forced to sink (subsidence). This compresses the air at the surface, increasing its density and creating high pressure.
      • Divergence and Ascent: When air diverges at high altitudes, it promotes the rising (ascent) of air from the surface. This reduces the weight of the air column, creating low pressure at the surface.
    • Rotation of Earth (Dynamic Factor): The Coriolis force, an effect of Earth’s rotation, deflects moving air masses. This deflection plays a crucial role in the formation of dynamically-induced pressure belts, particularly the subtropical highs and sub-polar lows.

Formation of Pressure Belts

The global pattern of pressure is organized into distinct belts. Their formation is a result of the interplay between thermal factors (temperature) and dynamic factors (Earth’s rotation and air circulation). This is explained by the Tri-cellular Model of atmospheric circulation.

  1. Equatorial Low-Pressure Belt (Thermally Induced):

    • Located between 5°N and 5°S.
    • Characterized by intense solar heating throughout the year. The hot air expands, becomes light, and rises, creating a zone of low pressure.
    • This zone of converging winds and convection is also known as the Inter-Tropical Convergence Zone (ITCZ) or the Doldrums.

  2. Sub-tropical High-Pressure Belts (Dynamically Induced):

    • Located around 30°N and 30°S. Also known as the Horse Latitudes.
    • Air that rises at the equator moves poleward at the tropopause. As it cools and is acted upon by the Coriolis force, it begins to descend around 30° latitude. This large-scale subsidence of air compresses the air at the surface, creating a zone of high pressure.

  3. Sub-polar Low-Pressure Belts (Dynamically Induced):

    • Located around 60°N and 60°S.
    • Cold, dense air flowing from the poles (Polar Easterlies) meets warmer, lighter air moving from the subtropics (Westerlies). This convergence of contrasting air masses forces the warmer air to rise, creating a zone of low pressure. This boundary is known as the Polar Front.

  4. Polar High-Pressure Belts (Thermally Induced):

    • Located over the poles (around 90°N and 90°S).
    • Extreme cold temperatures cause the air to be very dense and to sink, creating persistent high pressure at the surface.
  • Atmospheric Circulation Cells: This global pressure system drives a three-cell circulation pattern in each hemisphere.
    • Hadley Cell (0°-30°): Named after George Hadley who proposed the model in 1735. Air rises at the ITCZ, flows poleward at the tropopause, sinks in the subtropics, and returns to the equator as surface Trade Winds.
    • Ferrel Cell (30°-60°): Named after William Ferrel who detailed the model in the mid-19th century. This is a thermally indirect cell. Air sinks in the subtropics, moves poleward as surface Westerlies, rises at the polar front, and returns equatorward at high altitudes.
    • Polar Cell (60°-90°): Air sinks over the poles, flows equatorward as surface Polar Easterlies, rises at the polar front, and returns to the poles at high altitudes.

Prelims Pointers

  • Temperature Measurement: The instrument used is a thermometer.
  • Diurnal Range: Difference between the daily maximum and minimum temperature.
  • Annual Range: Difference between the highest and lowest mean monthly temperature.
  • Highest Official Temperature: 56.7°C in Death Valley, California, USA (WMO).
  • Lowest Official Temperature: -89.2°C at Vostok Station, Antarctica (WMO).
  • Isotherm: An imaginary line connecting places having the same temperature.
  • Isotherm Bending (N. Hemisphere):
    • In January (winter), isotherms bend poleward over oceans.
    • In July (summer), isotherms bend equatorward over oceans.
  • Normal Lapse Rate: The average rate of temperature decrease with altitude in the troposphere, approximately 6.5°C per 1000 meters.
  • Temperature Inversion: A condition where temperature increases with altitude.
  • Types of Inversion:
    1. Radiation Inversion: Caused by rapid cooling of the ground at night.
    2. Air Drainage/Valley Inversion: Cold, dense air sinks into valleys. Associated with katabatic winds.
    3. Advection Inversion: Warm air moves over a cold surface.
    4. Frontal Inversion: Warm air is forced to rise over cold air at a weather front.
  • Atmospheric Pressure: Measured by a barometer. Standard sea-level pressure is 1013.25 mb.
  • Relationship: Generally, High Temperature → Low Pressure; Low Temperature → High Pressure.
  • Global Pressure Belts:
    1. Equatorial Low (Doldrums / ITCZ)
    2. Subtropical Highs (Horse Latitudes)
    3. Sub-polar Lows
    4. Polar Highs
  • Origin of Pressure Belts:
    • Thermally Induced: Equatorial Low, Polar Highs.
    • Dynamically Induced: Subtropical Highs, Sub-polar Lows.
  • Tri-cellular Model of Circulation:
    • Hadley Cell: 0° - 30° latitude.
    • Ferrel Cell: 30° - 60° latitude.
    • Polar Cell: 60° - 90° latitude.
  • Planetary Winds:
    • Trade Winds: Flow from Subtropical Highs to Equatorial Low (0°-30°).
    • Westerlies: Flow from Subtropical Highs to Sub-polar Lows (30°-60°).
    • Polar Easterlies: Flow from Polar Highs to Sub-polar Lows (60°-90°).

Mains Insights

GS Paper I (Geography)

  • Influence of Temperature Distribution on Climate and Human Activities: The differential heating of land and water, evident in the behavior of isotherms, is the primary driver of global wind and pressure systems, including the monsoons. The pronounced seasonal temperature variation in the Northern Hemisphere’s continental interiors creates strong high-pressure systems in winter (e.g., Siberian High) and low-pressure systems in summer, which are critical for the Indian Summer Monsoon.
  • Significance of Pressure Belts in Defining Climate Zones: The global pressure belts and the associated tri-cellular circulation are fundamental to understanding the distribution of the world’s major climate zones.
    • Equatorial Low: Associated with high rainfall and tropical rainforests (Af climate).
    • Subtropical Highs: Associated with subsiding, dry air, leading to the formation of the world’s major hot deserts (e.g., Sahara, Kalahari, Atacama) around 30° N/S.
    • Sub-polar Lows: Zone of convergence (Polar Front) leading to cyclonic activity and variable weather typical of temperate regions (e.g., Western Europe).
  • Temperature Inversion and Human Geography: Temperature inversion has a direct bearing on settlement patterns and agriculture, especially in topographically diverse regions. The concept of a ‘thermal belt’ on mountain slopes explains why settlements and horticulture are concentrated there, while valley floors are often avoided due to frost risk. This demonstrates the intricate relationship between micro-climatic phenomena and human adaptation strategies.

GS Paper III (Environment, Disaster Management)

  • Temperature Inversion and Air Pollution:
    • Cause-Effect: Temperature inversion acts as a catalyst for severe air pollution events. The stable atmospheric ‘lid’ traps pollutants, leading to dangerously high concentrations near the surface. This is a recurring environmental and health crisis in cities like Delhi during winter.
    • Policy Linkage: Understanding this phenomenon is crucial for effective policy-making. For instance, the Graded Response Action Plan (GRAP) in the NCR is designed to be implemented based on air quality, which is often worst during inversion conditions. Strategies like controlling stubble burning and vehicle emissions become more critical during the inversion-prone winter months.
    • Disaster Management: The formation of dense fog and smog due to inversion is a recurring disaster, causing transportation accidents and public health emergencies. Early warning systems and public advisories based on meteorological forecasts of inversion are essential components of disaster risk reduction.
  • Climate Change and Atmospheric Circulation:
    • Global warming is altering these fundamental patterns. Evidence suggests that the Hadley Cell is expanding poleward.
    • Consequences: This expansion is shifting the subtropical dry zones further towards the poles, potentially increasing desertification and water stress in regions like the Mediterranean, Southwestern USA, and Southern Australia. It also alters the tracks of storms and the patterns of the Westerlies, with wide-ranging impacts on global weather.

GS Paper IV (Ethics, Integrity, and Aptitude)

  • Environmental Ethics and Policy Paralysis: The predictable nature of winter smog in North India, driven by temperature inversions trapping pollutants from agricultural and industrial sources, raises ethical questions about governance. The inability to implement long-term solutions despite scientific understanding points to issues of political will, inter-state coordination, and the conflict between economic activities and the public’s right to a healthy environment (Article 21). An administrator must navigate these complex ethical dilemmas to enforce environmental regulations effectively.

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