Unit 3: Climatology

1. Heat Budget of the Earth

The heat budget is the equilibrium between the amount of solar radiation (insolation) received by the Earth and the amount of heat lost by the Earth through terrestrial radiation.

A. The Incoming Solar Radiation (Insolation)

Assume the total incoming radiation at the top of the atmosphere is 100 units.

• Reflected (Albedo): 35 units are reflected back into space before reaching the surface.
  • 27 units reflected by clouds.
  • 2 units reflected by snow and ice fields.
  • 6 units scattered by atmospheric particles.
  • Earth's Albedo is approx. 35%.
• Absorbed: 65 units enter the system.
  • 14 units absorbed by the atmosphere (water vapor, dust, ozone).
  • 51 units absorbed by the Earth's surface.

B. The Outgoing Terrestrial Radiation

The 51 units absorbed by the surface must be radiated back to maintain thermal equilibrium. This occurs via long-wave radiation.

• Radiation from Surface:
  • 17 units radiated directly to space.
  • 34 units absorbed by the atmosphere (greenhouse effect).
• Atmospheric Heat Balance:
  • Input: 14 units (from insolation) + 34 units (from terrestrial radiation) + latent heat/convection transfers = 48 units.
  • Output: The atmosphere radiates these 48 units back into space.

Net Balance: 17 (surface direct) + 48 (atmosphere) + 35 (reflected) = 100 units outgoing.

C. Latitudinal Heat Balance

• Surplus Zone: Between 40° N and 40° S, insolation exceeds outgoing radiation.
• Deficit Zone: Poleward of 40°, outgoing radiation exceeds insolation.
• Note: The atmosphere and ocean currents transfer heat from surplus to deficit zones, preventing the tropics from overheating and poles from freezing completely.

2. Temperature and Pressure Belts of the World

A. Horizontal Distribution of Temperature

• Isotherms: Lines connecting points of equal temperature. They generally run parallel to latitudes but deviate due to land-water contrasts.
• Factors Affecting Temperature: Latitude (angle of incidence), Altitude (lapse rate), Distance from sea (continentality), Ocean currents, Prevailing winds.

B. Pressure Belts

Atmospheric pressure is the weight of the air column. There are seven distinct pressure belts.

C. Shifting of Belts

The pressure belts shift north and south with the apparent movement of the sun (Solstices).

• June: Belts shift North.
• December: Belts shift South.
• This shift influences climatic regions, such as the Mediterranean climate (winter rain).

3. Atmospheric Circulation: Winds

A. Planetary (Permanent) Winds

These blow constantly throughout the year in a particular direction.

1. Trade Winds (Easterlies):
   • Blow from Sub-Tropical Highs to Equatorial Low.
   • NE Trades in Northern Hemisphere; SE Trades in Southern Hemisphere.
   • Steady and regular.
2. Westerlies:
   • Blow from Sub-Tropical Highs to Sub-Polar Lows.
   • Direction: SW to NE (Northern Hemisphere); NW to SE (Southern Hemisphere).
   • Stronger in the Southern Hemisphere (Roaring Forties, Furious Fifties) due to lack of landmass friction.
3. Polar Easterlies:
   • Blow from Polar Highs to Sub-Polar Lows.
   • Cold, dry, and variable.

B. Local Winds

Winds driven by local thermal or topographic differences.

• Land and Sea Breezes: Diurnal cycle due to differential heating of land and water.
  • Sea Breeze: Day (Sea to Land).
  • Land Breeze: Night (Land to Sea).
• Mountain and Valley Breezes:
  • Valley Breeze (Anabatic): Day. Warm air rises up slopes.
  • Mountain Breeze (Katabatic): Night. Cold dense air flows down slopes.
• Specific Local Winds:
  • Chinook/Foehn: Warm, dry wind flowing down the leeward side of mountains (Rockies/Alps); "Snow eater."
  • Mistral/Bora: Cold, violent winds blowing from highlands to coasts (France/Italy).
  • Loo: Hot, dry wind in N. India (Summer).
  • Harmattan: Dry, dusty wind in West Africa ("The Doctor").

C. Monsoons

Periodic seasonal winds that reverse direction.

• Mechanism:
  • Thermal Concept (Halley): Differential heating of land/sea. Summer: Low pressure over land = Sea to Land wind. Winter: High pressure over land = Land to Sea wind.
  • Dynamic Concept (Flohn): Seasonal shift of the ITCZ.
• Asian Monsoon:
  • SW Monsoon (Summer): ITCZ shifts north over the Gangetic plain. High pressure in the Indian Ocean. Winds blow SW to NE, bringing heavy rain.
  • NE Monsoon (Winter): High pressure over Siberia/Land. Winds blow NE to SW (dry, except for Coromandel Coast).

D. Jet Streams

Narrow bands of strong winds in the upper troposphere (High Altitude).

• Sub-Tropical Jet Stream (STJ): Located at 30° lat. Influences winter weather and western disturbances in India.
• Polar Front Jet Stream (PFJ): Located at 60° lat. Associated with the boundary between polar and tropical air (Polar Front).
• Significance: They steer cyclones and anticyclones; essential for aviation.

4. Air Masses and Frontogenesis

A. Air Masses

A large body of air having relatively uniform temperature and moisture characteristics horizontally.

• Source Regions: Must be extensive and uniform (e.g., deserts, oceans, ice caps).
• Classification:
  • Latitude: Tropical (T - warm) vs. Polar (P - cold).
  • Surface: Maritime (m - moist) vs. Continental (c - dry).
  • Major Types: cP (Continental Polar), mP (Maritime Polar), cT (Continental Tropical), mT (Maritime Tropical).

B. Frontogenesis

The process of formation of a front (the boundary between two contrasting air masses).

• Cold Front: Cold air mass advances against warm air. Steep slope. Causes rapid uplift, cumulonimbus clouds, and thunderstorms. Represented by blue triangles on maps.
• Warm Front: Warm air mass moves over cold air. Gentle slope. Causes stratus clouds and steady, continuous rain. Represented by red semicircles.
• Occluded Front: When a cold front overtakes a warm front, lifting the warm sector completely off the ground.
• Stationary Front: Two air masses stall; neither is strong enough to replace the other.

5. Cyclones: Temperate and Tropical

A. Temperate Cyclones (Extratropical / Wave Cyclones)

• Origin: Form along the Polar Front (60° latitude) due to the interaction of warm westerlies and cold polar easterlies.
• Mechanism: Polar Front Theory (Bjerknes).
  1. Stationary front stage.
  2. Wave formation (kink).
  3. Mature stage (distinct warm and cold sectors).
  4. Occlusion (dissipation).
• Characteristics:
  • Move West to East (steered by Westerlies).
  • Covers large areas (thousands of km).
  • Precipitation is gradual and long-lasting.
  • Occurs in both winter and summer.

B. Tropical Cyclones

• Origin: Thermal origin over warm tropical oceans (SST > 27°C).
• Location: 5° to 20° N/S latitudes (Coriolis force is zero at the equator, so they do not form there).
• Mechanism: Intense low pressure induces rapid inflow of air. Air rises spirally, cools, condenses, releasing Latent Heat of Condensation (the energy source).
• Structure:
  • Eye: Center. Calm, subsiding air, clear skies.
  • Eye Wall: Ring of tallest cumulonimbus clouds. Highest wind speed and rainfall.
  • Spiral Bands: Rainbands extending outward.
• Characteristics:
  • Move East to West (steered by Trade Winds).
  • Smaller area but much higher intensity/destruction.
  • Dissipate rapidly upon landfall (cutoff from moisture source).

6. Types and Distribution of Precipitation

A. Types of Precipitation (Based on Lifting Mechanism)

1. Convectional Rainfall:
   • Caused by intense surface heating.
   • Air expands, rises, cools adiabatically to dew point.
   • Associated with Cumulonimbus clouds, thunder, and lightning.
   • Region: Equatorial regions (daily afternoon showers), interior of continents in summer.
2. Orographic (Relief) Rainfall:
   • Moist air is forced to ascend a mountain barrier.
   • Windward Side: Heavy rainfall.
   • Leeward Side (Rain Shadow): Air descends, warms, and humidity drops. Minimal rain (e.g., Patagonian Desert, Deccan Plateau).
3. Cyclonic (Frontal) Rainfall:
   • Caused by the convergence of air masses (Temperate) or intense convection in low pressure (Tropical).
   • Warm air rises over cold air (frontal wedging).

B. Global Distribution of Precipitation

• Heavy Rainfall (>200 cm): Equatorial belt, windward slopes of coastal mountains, monsoon regions (SE Asia).
• Moderate Rainfall (100–200 cm): Coastal regions of Trade winds, coastal areas in Westerlies zone.
• Low Rainfall (<50 cm):
  • Sub-tropical high-pressure belts (Deserts like Sahara, Atacama, Australian).
  • Interiors of large continents (distance from sea).
  • Polar regions (air is too cold to hold moisture).

7. Global Climatic Change

Climate change refers to significant and lasting changes in the statistical distribution of weather patterns.

A. Evidence of Climate Change

• Geological records: Ice cores, tree rings (dendrochronology), pollen analysis.
• Modern metrics: Rising global average temperatures, retreating glaciers, rising sea levels, ocean acidification, shrinking ice sheets.

B. Causes of Climate Change

1. Natural Causes:

• Milankovitch Cycles: Variations in Earth's orbit (eccentricity), tilt (obliquity), and wobble (precession) affecting insolation.
• Solar Output: Sunspot cycles (11-year cycle) and solar irradiance variations.
• Volcanism: Ash and sulfur dioxide block sunlight, causing short-term cooling (e.g., Mt. Pinatubo).

2. Anthropogenic (Human) Causes:

• Greenhouse Gases (GHGs): Burning fossil fuels (CO2), agriculture/livestock (Methane - CH4), industrial processes (Nitrous Oxide).
• Greenhouse Effect: GHGs trap outgoing long-wave terrestrial radiation, warming the lower atmosphere.
• Deforestation: Reduces carbon sinks.
• Aerosols: Industrial smog can cause localized cooling (global dimming) but alters rain patterns.

C. Consequences

• Weather Extremes: Increased frequency of heatwaves, intense hurricanes, droughts, and floods.
• Cryosphere: Melting permafrost (releasing methane), loss of Arctic sea ice.
• Hydrosphere: Sea level rise (thermal expansion + meltwater), threatening coastal cities and islands (e.g., Maldives).
• Biosphere: Shifts in biome boundaries, species extinction, coral bleaching.
• Agriculture: Changing growing seasons, crop failure risks.

D. Urban Heat Island (UHI)

• A micro-climatic phenomenon where urban areas are significantly warmer than surrounding rural areas.
• Causes: Concrete/asphalt absorb heat (high thermal mass), lack of vegetation (evapotranspiration), waste heat from vehicles/ACs.