GS Notes

1.5 - Geography Notes 26

14 min read

Elaborate Notes

Precipitation

Precipitation is any form of water, liquid or solid, that falls from the atmosphere and reaches the Earth’s surface. Its formation requires three fundamental conditions:

  1. Upliftment Mechanism: Moist air must be lifted to higher altitudes. As the air parcel rises, it expands due to lower atmospheric pressure and cools adiabatically (cooling without loss of heat to the surroundings). This cooling is the primary trigger for condensation. Major upliftment mechanisms include:

    • Convection: Intense heating of the ground surface.
    • Orographic Lifting: Forced ascent over a topographic barrier like a mountain.
    • Frontal Convergence: The meeting of two different air masses (warm and cold).
    • Cyclonic Convergence: Convergence of air towards a low-pressure center.

  2. Saturation and Cooling: The rising and cooling air must reach its dew point temperature—the temperature at which it becomes saturated with water vapor (100% relative humidity). Further cooling below the dew point causes the invisible water vapor to condense into visible water droplets or ice crystals.

  3. Presence of Hygroscopic Nuclei: Condensation requires a surface. In the atmosphere, microscopic particles such as dust, salt crystals from sea spray, pollen, and smoke act as condensation nuclei. Water vapor condenses around these particles to form cloud droplets. For precipitation to occur, these tiny droplets must grow large and heavy enough to overcome air resistance and fall. This growth happens through processes like collision-coalescence in warm clouds and the Bergeron-Findeisen process (involving ice crystals and supercooled water) in cold clouds.

Types of Precipitation (Rainfall)

a) Convectional Rainfall

  • Mechanism: It is caused by the thermal convection of air currents. When the Earth’s surface is intensely heated, the air in contact with it becomes warm, expands, and rises rapidly. This creates strong vertical updrafts. As the air ascends, it cools adiabatically, leading to condensation and the formation of tall, vertically developed cumulonimbus clouds.
  • Characteristics: This type of rainfall is typically heavy, localized, and of short duration. It is often accompanied by thunderstorms, lightning, and sometimes hail, due to the violent atmospheric instability and strong updrafts and downdrafts within the cumulonimbus clouds.
  • Distribution & Examples:
    • It is a characteristic feature of equatorial regions (e.g., Amazon Basin, Congo Basin) where it occurs almost daily in the afternoons, throughout the year, due to consistent high insolation.
    • In tropical and continental interiors, it is common during the summer months when ground heating is most intense.

b) Orographic Rainfall

  • Mechanism: This type of rainfall occurs when a moisture-laden air mass is forced to ascend a topographic barrier, such as a mountain range. The side of the mountain facing the wind is called the windward side, and the opposite side is the leeward side.
    • Windward Side: As the air is forced to rise, it cools adiabatically, leading to saturation, condensation, and significant precipitation.
    • Leeward Side: After crossing the mountain crest, the air descends on the leeward side. During descent, it gets compressed and warms adiabatically. This warming increases the air’s moisture-holding capacity, leading to evaporation of clouds and resulting in minimal or no rainfall. This dry region on the leeward side is known as a rain shadow area.
  • Examples:
    • The Western Ghats in India: The windward side (e.g., Mumbai) receives heavy rainfall from the Southwest Monsoon, while the leeward side (e.g., Pune) lies in a rain shadow region and receives much less.
    • The Andes Mountains in South America create a rain shadow over Patagonia.

c) Frontal Rainfall (Cyclonic Rainfall)

  • Mechanism: This form of precipitation is associated with the meeting of two large air masses with different temperatures and densities. The boundary zone between them is called a front. Since warm air is lighter and less dense than cold air, it is forced to rise over the colder, denser air mass. This uplift is gradual and occurs over a large area.
  • Characteristics: The gradual cooling of the rising warm air results in the formation of layered clouds (like nimbostratus) and produces steady, moderate to light precipitation over a wide area for a prolonged period.
  • Distribution & Context:
    • Frontal rainfall is the dominant type of precipitation in the temperate or mid-latitude regions, where polar and tropical air masses frequently converge.
    • The modern theory of fronts and associated cyclones was developed by the Bergen School of Meteorology in Norway, led by Vilhelm Bjerknes in the early 20th century.

Distribution of Precipitation

  • Equatorial Regions (approx. 10°N - 10°S): Experience the highest precipitation, generally exceeding 200 cm annually, due to high humidity and intense convection associated with the Inter-Tropical Convergence Zone (ITCZ).
  • Subtropical Regions (approx. 25°-35° N/S): These are regions of minimum rainfall, often less than 25 cm per year. This is due to the presence of subtropical high-pressure belts where air subsides, warms, and becomes stable, inhibiting cloud formation (e.g., Sahara, Atacama deserts).
  • Mid-latitude Regions (approx. 40°-60° N/S): Receive moderate and more uniform precipitation throughout the year due to the frequent passage of temperate cyclones and associated frontal rainfall.
  • Oceans vs. Continents: Oceans generally receive more precipitation than continents as they are the primary source of atmospheric moisture.
  • Coastal vs. Interior: Coastal regions receive significantly higher precipitation than continental interiors, which are far from moisture sources.

Thunderstorm and Lightning

  • Formation: Thunderstorms are intense, localized storms associated with Cumulonimbus clouds. They form under conditions of great atmospheric instability, characterized by intense surface heating, high humidity, and strong vertical convection (updrafts).
  • Lightning: Inside the towering cumulonimbus cloud, strong updrafts carry water droplets upwards, while hail and ice fall downwards. Collisions between these particles cause a separation of electric charge. Typically, positive charges accumulate at the top of the cloud and negative charges at the bottom.
    • When the potential difference between the positive and negative charge centers becomes sufficiently large, it overcomes the insulating properties of air, resulting in a massive electrical discharge. This flash of light is lightning. It can occur within a cloud (intra-cloud), between clouds, or between the cloud and the ground.
  • Thunder: Lightning heats the air in its path to temperatures as high as 30,000°C in a fraction of a second. This causes the air to expand explosively, creating a powerful acoustic shockwave that we hear as thunder. Because light travels much faster than sound, we see the lightning flash before we hear the thunder.

Cloud Burst

  • Definition: The India Meteorological Department (IMD) defines a cloudburst as an extreme rainfall event where precipitation of 10 cm (100 mm) or more occurs in one hour over a highly localized area (a few square kilometers).
  • Formation of Cloud Burst:
    1. Favourable Conditions: High temperature and high humidity provide abundant moisture and energy for strong, rapid convection.
    2. Topographic Influence: Steep mountain slopes can cause an orographic lift and a “funnelling effect,” forcing moist air to converge and rise rapidly over a small area. This leads to a massive and rapid buildup of clouds.
    3. Intense Updrafts: The extremely strong convective updrafts within the cloud are powerful enough to hold up a very large volume of water droplets, preventing them from falling as normal rain.
    4. The Event: Eventually, the updrafts weaken or the accumulated water load becomes too heavy for the air currents to support. This triggers a sudden release of the entire water mass, which falls to the ground in a quick, devastating torrent, resulting in a cloudburst.
  • Locations: In India, cloudbursts are most common in the Himalayas and the Western Ghats during the monsoon season due to the combination of steep topography and moisture-laden winds. They have also been reported in desert regions like Rajasthan. The Kedarnath tragedy in 2013 was exacerbated by such an extreme rainfall event.

Tornado

  • Definition: A tornado is a violently rotating column of air that is in contact with both the Earth’s surface and a cumulonimbus cloud. It is often, but not always, visible as a funnel cloud.
  • Characteristics: Tornadoes are characterized by extremely high wind speeds, which can exceed 400 km/h, and a very deep low-pressure center. They are relatively small in diameter but are the most violent and destructive of all atmospheric storms.
  • Formation: They typically form within severe thunderstorms known as supercells. The process begins with wind shear—a change in wind speed and direction with height. This creates a horizontal rolling effect in the lower atmosphere. The powerful updraft within the thunderstorm can then tilt this horizontal tube of spinning air into a vertical position, forming a rotating column of air known as a mesocyclone, from which a tornado can form and descend.
  • Waterspout: A waterspout is essentially a tornado that forms over a body of water.

Jet Streams

  • Definition: The World Meteorological Organization (WMO) defines a jet stream as “a strong narrow current, concentrated along a quasi-horizontal axis in the upper troposphere or the stratosphere, characterized by strong vertical and lateral wind shear and featuring one or more velocity maxima.”

  • Characteristics:

    • They are like rivers of air flowing at high altitudes (9-12 km).
    • They are thousands of kilometers long, hundreds of kilometers wide, but only a few kilometers thick.
    • They typically blow from west to east due to the Earth’s rotation (Coriolis effect).
    • Winds are stronger in winter when the temperature difference between the poles and the equator is greatest, and they shift equatorward. In summer, they are weaker and shift poleward.
    • They follow a meandering, wavy path known as Rossby waves, a concept developed by meteorologist Carl-Gustaf Rossby in the 1930s.

  • Causes: Jet streams are a result of the strong temperature and pressure gradients that exist between large, contrasting air masses (e.g., between polar and temperate air). According to the thermal wind relationship, a strong horizontal temperature gradient at the surface leads to a strong vertical wind shear, creating very high wind speeds at altitude.

Types of Jet Streams

  • Polar Front Jet Stream: Forms around 60° N/S latitudes at the boundary of the cold polar air (Polar cell) and warmer temperate air (Ferrel cell). It is highly irregular and plays a crucial role in steering temperate cyclones.
  • Subtropical Westerly Jet Stream (STWJ): Forms around 30° N/S latitudes at the boundary of the warm tropical air (Hadley cell) and cooler temperate air (Ferrel cell). It is more persistent and stronger than the Polar Jet.
  • Tropical Easterly Jet Stream (TEJ): A unique, seasonal jet stream that forms only in the Northern Hemisphere summer over India and Africa. It blows from east to west. Its formation is a thermal effect caused by the intense heating of the Tibetan Plateau, which creates a high-pressure system in the upper troposphere. Meteorologist P. Koteswaram made significant contributions to understanding its role in the Indian Monsoon.
  • Polar Night Jet Stream: A stratospheric jet stream that forms over the poles during their respective winters. It encircles the cold polar air, leading to the formation of the polar vortex and Polar Stratospheric Clouds, which are critical for the chemical reactions that cause the depletion of the ozone layer.
  • Local Jet Streams: Smaller, more localized jet streams, often at lower levels. An example is the Somali Jet, a low-level jet that is vital for transporting moisture across the equator towards the Indian subcontinent during the monsoon.

Significance of the Jet Streams

  • Weather Influence: The meandering of jet streams creates areas of upper-level convergence and divergence. Upper-level divergence (air spreading out) promotes rising air and the formation of low-pressure systems (cyclones) at the surface, while upper convergence forces air to sink, creating high-pressure systems (anticyclones).
  • Aviation: Airlines use jet streams to their advantage, flying with them (tailwind) to save time and fuel, and avoiding flying against them (headwind).
  • Indian Monsoon: The monsoon is heavily influenced by jet streams:
    • STWJ: Its southward shift in winter brings Western Disturbances to Northwest India. Its position in summer influences the monsoon’s northward progress.
    • TEJ: Its appearance signals the onset and strengthening of the summer monsoon.
    • Somali Jet: Acts as a powerful cross-equatorial moisture feeder for the monsoon.
  • Extreme Weather:
    • Heat Domes: A stalled, high-amplitude Rossby wave can create a blocking high-pressure system, trapping hot air and causing prolonged heatwaves.
    • Polar Vortex Outbreaks: A weak and meandering Polar Jet allows frigid arctic air to spill south into the mid-latitudes, causing severe cold waves.
  • Pollutant Transport: They can rapidly transport pollutants and volcanic ash over long distances in the upper atmosphere.

Prelims Pointers

  • Three conditions for precipitation: Upliftment mechanism, Cooling below dew point, and Presence of hygroscopic nuclei.
  • Cumulonimbus clouds are associated with convectional rainfall and thunderstorms.
  • Orographic rainfall occurs on the windward side of mountains.
  • The dry area on the leeward side of a mountain is called a rain shadow region.
  • Frontal rainfall is common in temperate (mid-latitude) regions.
  • Highest annual precipitation (>200 cm) is found in equatorial regions.
  • Lowest annual precipitation (<25 cm) is found in subtropical high-pressure belts.
  • Lightning is an electrical discharge caused by charge separation in cumulonimbus clouds.
  • Thunder is the sound wave created by the rapid expansion of air heated by lightning.
  • IMD Definition of Cloudburst: Rainfall over 10 cm (100 mm) per hour in a localized area.
  • A tornado is a violently rotating column of air extending from a thunderstorm to the ground.
  • A waterspout is a tornado that forms over a water body.
  • Wind shear is a key ingredient for tornado formation.
  • Jet streams are strong, narrow air currents in the upper troposphere.
  • Jet streams generally blow from west to east.
  • The meandering path of a jet stream is known as a Rossby wave.
  • The Tropical Easterly Jet (TEJ) forms only in summer over India and Africa and blows from east to west.
  • The Subtropical Westerly Jet (STWJ) is responsible for bringing Western Disturbances to India in winter.
  • The Polar Night Jet stream is associated with the formation of the ozone hole.
  • The Somali Jet is a low-level jet stream important for the Indian Monsoon.

Mains Insights

  1. Topography and Climate Interplay (GS Paper I): The mechanism of orographic rainfall is a prime example of how geography shapes climate, ecosystems, and human activity. The stark contrast between the lush windward slopes (e.g., Western Ghats coast) and the arid rain shadow regions (e.g., Vidarbha, interior Karnataka) directly influences agriculture, water resources, biodiversity, and socio-economic development, often leading to regional disparities.
  2. Climate Change and Extreme Events (GS Paper I & III): There is growing evidence linking climate change to an increase in the frequency and intensity of extreme weather events.
    • Cloudbursts: A warmer atmosphere can hold more moisture, potentially leading to more intense rainfall events like cloudbursts, especially in ecologically fragile regions like the Himalayas, increasing disaster risk.
    • Jet Stream Behavior: The weakening of the pole-to-equator temperature gradient (due to Arctic amplification) may be making the Polar Jet Stream more wavy or “meandering.” This leads to more persistent weather patterns, causing prolonged heatwaves (Heat Domes), cold snaps (Polar Vortex outbreaks), and floods.
  3. Monsoon Dynamics and Jet Streams (GS Paper I): The Indian Monsoon is not a simple phenomenon but a complex interplay of various global and regional factors, with jet streams playing a pivotal role. The timely arrival of the TEJ, the northward retreat of the STWJ, and the strength of the Somali Jet are all crucial for a normal monsoon. Understanding this complex mechanism is vital for accurate monsoon forecasting, which is the backbone of India’s agrarian economy and water security.
  4. Disaster Management and Forecasting (GS Paper III): Events like thunderstorms, cloudbursts, and tornadoes pose significant natural hazards.
    • Challenges: While large-scale systems like cyclones are predictable, localized and rapidly developing events like cloudbursts and tornadoes are extremely difficult to forecast with precision.
    • Strategy: This necessitates a shift in disaster management strategy from purely forecast-based responses to building community resilience, implementing effective land-use planning in vulnerable zones (especially mountainous areas), and developing robust early warning dissemination systems (e.g., Doppler radar networks for nowcasting).
  5. Upper Atmosphere-Surface Linkages (GS Paper I): Jet streams demonstrate the strong coupling between the upper atmosphere and surface weather. The patterns of convergence and divergence in the jet stream directly create and steer surface-level pressure systems (cyclones and anticyclones). This highlights that surface weather is not an isolated phenomenon but is driven by large-scale dynamics occurring kilometers above the Earth.

Graph View