Heat Budget and Circulation

The earth receives an enormous amount of energy from the sun in the form of solar radiation, commonly termed as insolation. This incoming solar radiation interacts with the atmosphere, land, and oceans, setting in motion a complex system of energy balance, temperature variation, pressure distribution, and atmospheric movement—together forming the global heat budget and circulation system. It is this mechanism that drives winds, ocean currents, weather patterns, and influences life systems across latitudes.

HEAT AND TEMPERATURE

Heat is the total kinetic energy of molecules, while temperature is a measure of this energy, indicating how hot or cold a substance or place is. The distribution of temperature on Earth—both horizontally (across latitudes) and vertically (through atmospheric layers)—is uneven due to various geographical, atmospheric, and terrestrial factors.

FACTORS INFLUENCING TEMPERATURE DISTRIBUTION

1. Latitude – Areas near the equator receive vertical rays of the sun, hence more energy, while poles get slanted rays, receiving less heat.
2. Altitude – Temperature decreases with height due to the thinning of the atmosphere. Hill stations are cooler than nearby plains.
3. Distance from Sea – Coastal areas have moderate climate due to the influence of land and sea breezes; interiors experience extremes.
4. Ocean Currents – Warm currents (e.g., Gulf Stream) raise temperatures, while cold currents (e.g., Peru Current) lower coastal temperatures.
5. Vegetation Cover – Forests cool the atmosphere through evapotranspiration, while barren areas heat up rapidly.
6. Topography and Local Conditions – Slopes facing the sun are warmer; valleys trap cold air causing temperature inversion.

GLOBAL TEMPERATURE DISTRIBUTION

1. Horizontal Distribution

• It refers to the spread of temperature across the earth’s surface, visualized through isotherms (lines joining places with equal temperature).
• The equatorial belt remains uniformly warm throughout the year.
• As one moves poleward, temperature decreases steadily.
• Landmasses heat and cool more rapidly than oceans—this creates seasonal temperature contrasts, especially noticeable in January and July isotherm maps.

2. Vertical Distribution

• The general decrease of temperature with altitude is called the Normal Lapse Rate, averaging 6.5°C per 1000 metres.
• However, this lapse rate is not constant—it varies with time, moisture, latitude, and atmospheric conditions.
• At times, the temperature increases with height—a phenomenon called Temperature Inversion, seen in valleys or during winter nights under clear skies.

HEAT BUDGET OF THE EARTH

The Earth’s heat budget is a balance sheet of incoming and outgoing energy. About 51% of solar radiation is absorbed by the Earth’s surface, 19% by clouds and atmosphere, and the rest is reflected back (albedo effect).

To maintain thermal equilibrium:

• Shortwave solar radiation is absorbed.
• Earth radiates longwave infrared radiation back.
• Some of this is trapped by greenhouse gases, maintaining a habitable climate.

Any imbalance in this budget due to human activities (e.g., excess greenhouse gases) leads to global warming and climate disturbances.

ATMOSPHERIC PRESSURE AND ITS CIRCULATION

Air pressure is the weight of the atmosphere exerted per unit area on the Earth’s surface. It is measured using a barometer and is expressed in millibars (mb) or hectopascals (hPa).

VERTICAL DISTRIBUTION OF PRESSURE

• Atmospheric pressure is highest at sea level and decreases with altitude.
• This is due to the compressibility of air—the lower layers are denser, while upper layers are lighter.
• The rate of pressure decrease is not uniform, as it depends on temperature, moisture content, and gravity.

HORIZONTAL DISTRIBUTION: PRESSURE BELTS OF THE EARTH

The global atmosphere is divided into seven pressure belts, shaped largely due to the thermal effects of insolation and Earth’s rotation:

• Equatorial Low Pressure Belt (0°–5° N/S)

  • Heated intensely → warm air rises → creates low pressure.
  • Zone of calm, rising air = Doldrums.

• Subtropical High Pressure Belts (30° N/S)

  • Descending limb of Hadley cell → dry, cool air → high pressure.
  • Region of clear skies and deserts (e.g., Sahara, Thar).
  • Winds blow outwards as Trade Winds (towards equator) and Westerlies (towards poles).

• Subpolar Low Pressure Belts (60° N/S)

  • Warm Westerlies meet cold Polar Easterlies → rising air → low pressure.
  • Characterised by cyclonic storms, especially in winter.

• Polar High Pressure Belts (90° N/S)

  • Extreme cold causes air to contract and descend, creating high pressure zones.
  • Winds from here blow towards subpolar regions as Polar Easterlies.

ATMOSPHERIC CIRCULATION CELLS

1. Hadley Cell (0°–30°)

• Warm equatorial air rises, moves poleward at high altitude, cools and descends at 30°, forming a convection loop.
• Descending air forms subtropical high, generating Trade Winds.

2. Ferrel Cell (30°–60°)

• A secondary cell driven by movement of Hadley and Polar cells.
• Air at 30° moves poleward as Westerlies, meets cold Polar Easterlies at 60°, rises, forming cyclones.
• It is a zone of unstable weather and mid-latitude depressions.

3. Polar Cell (60°–90°)

• Cold air sinks at poles, flows outward as Easterlies, meets warmer air from Ferrel Cell → rises at 60°.
• Poles act as thermal deserts, despite having snow cover.

JET STREAMS AND THEIR ROLE

At the boundaries between these cells (Hadley–Ferrel and Ferrel–Polar), strong high-altitude westerly winds called Jet Streams blow. These include:

• Subtropical Jet Stream (STJ)
• Polar Front Jet Stream (PFJ)

Jet streams drive weather systems, influence monsoon onset, and affect aviation routes.

WALKER CIRCULATION AND EL NIÑO PHENOMENON

• The Walker circulation is an east-west tropical air cell over the Pacific Ocean.
• Under normal conditions, trade winds push warm water westward (towards Indonesia/Australia), causing upwelling of cold nutrient-rich water off the Peruvian coast.
• This supports rich fisheries and marine ecosystems.
• However, when these Trade Winds weaken, the warm waters drift back eastward, disrupting the circulation. This anomaly is known as El Niño:
• Causes droughts in Australia/Indonesia and floods in South America.
• Suppresses upwelling → collapse of fish population → economic losses.
• The opposite phenomenon, La Niña, intensifies upwelling and rainfall in western Pacific.

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Subject: Geography

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