The processes that shape our weather and climate

Q: What is the difference between incoming solar radiation and outgoing terrestrial radiation?

Incoming solar radiation (insolation) is high-energy short-wave radiation emitted by the Sun, whereas outgoing terrestrial radiation is long-wave infrared radiation re-emitted by the warmed Earth.

Q: Where are the global net solar surplus and deficit zones located, and at what latitude do they balance?

A net solar surplus exists between \(40^\circ\text{ N}\) and \(40^\circ\text{ S}\), while a net solar deficit exists from \(40^\circ\text{ to }90^\circ\text{ N and S}\), balancing at \(40^\circ\text{ N and S}\) .

Q: What are the key components of the daytime and night-time diurnal energy budgets?

The daytime budget consists of 6 components (insolation, reflection, absorption, sensible heat, latent heat, and long-wave radiation), while the night-time budget consists of 4 components (long-wave radiation, sub-surface heat supply, sensible heat, and latent heat).

Q: How do the Polar and Ferrel cells operate within the Tri-Cellular Model?

The Polar cell is thermally direct with sinking cold air at the poles and rising air at \(60^\circ\text{ N/S}\); the Ferrel cell is a thermally indirect mechanical cell driven between \(30^\circ\) and \(60^\circ\text{ N/S}\) producing prevailing westerlies.

Geography 3910 · CCEA A Level · 10 min read · Updated: 1 Sept 2026

Welcome to Atmospheric Processes!

Welcome to one of the most exciting and dynamic topics in your CCEA AS Level Geography course: Atmosphere (Sub-theme 3A: The processes that shape our weather and climate). Have you ever wondered why the tropics are consistently warm while the poles are frozen, or why coastal areas have milder winters than places far inland? In this chapter, we will break down the invisible energy transfers and global wind patterns that drive our planet's climate system.

Don't worry if atmospheric physics sounds intimidating at first. We will explore every concept step-by-step using clear analogies, memory tricks, and direct links to what CCEA examiners look for in your AS 1: Physical Geography exam.

1. The Global Energy Balance & Radiation Budgets

A. Incoming vs. Outgoing Radiation

The driving engine of all weather on Earth is energy from the Sun. However, not all radiation is the same:

1. Incoming Solar Radiation (Insolation): This is short-wave ultraviolet and visible light radiation emitted by the hot surface of the Sun. Because short-wave radiation has high energy, it passes relatively easily through atmospheric gases without directly heating the air.

2. Outgoing Terrestrial Radiation: The Earth absorbs incoming solar energy, heats up, and re-emits this energy back towards space as long-wave infrared radiation. The atmosphere absorbs long-wave radiation much more readily than short-wave radiation.

Top Examiner Tip: Never write that the atmosphere is heated directly from above by the Sun! The atmosphere is primarily heated from below by outgoing long-wave terrestrial radiation.

B. Planetary Energy Imbalance: Surplus and Deficit

Earth's global radiation budget is not balanced equally everywhere across the globe:

Net Solar Surplus (\(40^\circ\text{ N}\) to \(40^\circ\text{ S}\)): In equatorial and tropical regions, the amount of incoming short-wave insolation exceeds the amount of outgoing terrestrial radiation. This creates an energy surplus.

Net Solar Deficit (\(40^\circ\text{ to }90^\circ\text{ N}\) and \(40^\circ\text{ to }90^\circ\text{ S}\)): In high-latitude and polar regions, outgoing long-wave radiation exceeds incoming insolation. This creates an energy deficit.

The Critical Threshold: CCEA mark schemes specifically look for the \(40^\circ\text{ N}\) and \(40^\circ\text{ S}\) boundary lines where incoming and outgoing radiation balance out.

Why don't the tropics get hotter every year and the poles freeze solid? Because the planet continuously redistributes heat! Approximately \(80\%\) of excess tropical heat is transferred polewards by atmospheric circulation (winds), and the remaining \(20\%\) is transferred by ocean currents to maintain global thermal equilibrium.

C. Diurnal Energy Budgets (Day vs. Night)

On a local scale, we study the 24-hour cycle of energy exchanges at the Earth's surface (the diurnal budget).

The Daytime Energy Budget (6 Key Components)

During the day, net radiation is generally positive because the Sun is shining:

1. Incoming Solar Radiation (Insolation): Short-wave energy arriving directly or diffusely from the Sun.

2. Reflected Solar Radiation: A proportion of insolation is reflected back into the atmosphere by the ground surface depending on its reflectivity (albedo).

3. Surface Absorption: The remaining energy absorbed by soil, vegetation, and water, which warms the surface.

4. Sensible Heat Transfer: Heat energy transferred directly between the ground surface and the overlying air via conduction (direct contact) and convection (rising warm air). You can sense this transfer as a change in air temperature.

5. Latent Heat Transfer: Energy used up to change the physical state of water (e.g., liquid water evaporating from soil or plants into water vapour) without changing its temperature. This heat is stored as "hidden" energy in the vapour.

6. Outgoing Long-wave Terrestrial Radiation: The warmed surface radiates infrared energy back upwards into the atmosphere.

The Night-time Energy Budget (4 Key Components)

At night, with no incoming solar insolation, the net budget becomes negative:

1. Long-wave Terrestrial Radiation: The surface continues to radiate long-wave heat upwards to space and clouds, cooling the ground rapidly.

2. Sub-surface Heat Supply: Heat stored deep in the soil and rocks during the day flows back upwards to the cooler surface.

3. Sensible Heat Transfer: Air that remains warmer than the ground transfers heat downwards to the cold surface by conduction.

4. Latent Heat Transfer: Water vapour in the air cools and condenses onto cold surfaces (forming dew, frost, or mist), releasing its stored latent heat back to the immediate environment.

Section Key Takeaway: The planet has an energy surplus between \(40^\circ\text{ N}\) and \(40^\circ\text{ S}\) and a deficit beyond \(40^\circ\text{ N/S}\). The daytime budget has 6 components, while the night-time budget has 4.

2. Factors Controlling Air Temperature & Insolation Receipts

Why do temperatures vary so dramatically around the world? Four major geographic factors control how much insolation a location receives and absorbs:

1. Latitude and the Angle of the Sun

Equator: The Sun strikes the Earth at a high angle (close to \(90^\circ\)). Insolation is concentrated over a smaller surface area, and rays pass through a shorter, thinner layer of atmosphere, reducing losses from scattering and absorption.

Poles: The Sun strikes at a low, oblique angle. The same amount of solar energy is spread over a much larger surface area, and rays travel through a much longer atmospheric path, leading to significant reflection and scattering.

2. Surface Albedo (Reflectivity)

Albedo refers to the proportion of incoming solar radiation reflected by a surface, expressed as a percentage or fraction.

High Albedo: Light-coloured surfaces like fresh snow and ice reflect up to \(80\%\text{ to }90\%\) of incoming insolation, absorbing very little heat.

Low Albedo: Dark surfaces like open ocean water, asphalt, and dense dark forests reflect only \(5\%\text{ to }15\%\), absorbing the vast majority of incoming solar energy.

3. Distance from the Sea (Continentality vs. Maritime Effect)

Water and land heat up and cool down at completely different rates because water has a significantly higher specific heat capacity than rock and soil:

Maritime Climates (Coastal areas): The sea warms slowly in summer and cools slowly in winter. Ocean winds moderate coastal temperatures, creating mild winters, cool summers, and a low annual temperature range.

Continental Climates (Inland areas): Land heats up rapidly in summer and loses heat quickly in winter. Far from the moderating sea, continental interiors experience hot summers, freezing winters, and an extreme annual temperature range.

4. Altitude & The Environmental Lapse Rate

Because the atmosphere is heated from below by terrestrial radiation, air temperature decreases with increasing altitude in the troposphere.

On average, in still, stable conditions, air temperature drops at the Normal (Environmental) Lapse Rate of approximately \(6.5^\circ\text{C}\) per \(1,000\text{ m}\) (or \(0.65^\circ\text{C}\) per \(100\text{ m}\)). Higher elevations also have thinner, less dense air with fewer greenhouse gas molecules to absorb outgoing long-wave radiation.

Section Key Takeaway: Temperature patterns are determined by solar angle (latitude), surface reflectivity (albedo), land-sea thermal differences (continentality), and elevation (lapse rate of \(6.5^\circ\text{C}\text{ per }1,000\text{ m}\)).

3. General Atmospheric Circulation (The Tri-Cellular Model)

To transfer excess heat from the equator to the poles, Earth relies on a global wind system known as the Tri-Cellular Model.

A. The Forces Driving Wind

Pressure Gradient Force (PGF): Air naturally moves from areas of High Pressure to Low Pressure to equalize atmospheric weight. The steeper the pressure difference, the stronger the wind.

Coriolis Force: Because the Earth rotates on its axis from west to east, moving air is deflected sideways rather than traveling in a straight line:

Winds are deflected to the RIGHT in the Northern Hemisphere.

Winds are deflected to the LEFT in the Southern Hemisphere.

Memory Trick: North = turn Right (think Nice & Right); South = turn Left.

B. The Three Circulation Cells

In each hemisphere, atmospheric circulation is divided into three distinct meridional cells:

1. The Hadley Cell (Thermally Direct)

At the Equator (\(0^\circ\)): Intense solar heating warms surface air, causing it to become less dense and rise rapidly at the Inter-Tropical Convergence Zone (ITCZ). This rising air creates a permanent belt of equatorial Low Pressure, forming heavy convective rainfall and thunderstorms.

Aloft: The rising air reaches the tropopause and diverges polewards towards the north and south.

At \(30^\circ\text{ N and }30^\circ\text{ S}\): As the air moves polewards, it cools and becomes denser. The air sinks back towards the surface, creating Subtropical High Pressure belts. Sinking air warms adiabatically, preventing cloud formation, which explains why the world's major hot deserts are located along these latitudes.

At the Surface: Air flows back towards the equatorial low pressure. Deflected by the Coriolis force, these return winds form the reliable Trade Winds (North-East Trades in the Northern Hemisphere and South-East Trades in the Southern Hemisphere).

2. The Polar Cell (Thermally Direct)

At the Poles (\(90^\circ\text{ N and }90^\circ\text{ S}\)): Extreme cold causes air to become dense and sink, forming the Polar High Pressure systems.

At the Surface: The cold, dense air diverges outwards, flowing towards lower latitudes. Coriolis deflection turns these into the Polar Easterlies.

At \(60^\circ\text{ N and }60^\circ\text{ S}\): The cold polar air meets warmer air from mid-latitudes, where it is forced to rise at the Subpolar Low Pressure belt (the Polar Front).

3. The Ferrel Cell (Thermally Indirect)

Located between \(30^\circ\) and \(60^\circ\text{ N/S}\), the Ferrel cell is not driven directly by heating or cooling. Instead, it is an indirect mechanical cell driven by friction and drag from the adjacent Hadley and Polar cells.

Surface air flows polewards from the subtropical high towards the subpolar low. Coriolis deflection turns this flow eastward, creating the prevailing Mid-latitude South-Westerlies in the Northern Hemisphere (and North-Westerlies in the Southern Hemisphere).

C. Upper-Atmosphere Wind Dynamics

High above the surface (in the upper troposphere), the balance between the Pressure Gradient Force and the Coriolis Force produces Geostrophic Winds that blow parallel to upper-level isobars.

Rossby Waves: Giant meandering horizontal waves of air that circle the globe in the mid-latitudes.

Polar Front Jet Stream: A narrow, high-velocity ribbon of fast-moving westerly wind found near the tropopause at around \(60^\circ\text{ N/S}\). It forms along the steep temperature and pressure boundary where cold polar air meets warm tropical air, heavily steering mid-latitude weather systems (depressions and anticyclones) towards regions like the British Isles.

Section Key Takeaway: The Tri-Cellular model consists of the thermally direct Hadley and Polar cells, and the thermally indirect Ferrel cell. Wind is driven by the Pressure Gradient Force and deflected by the Coriolis Force (right in the North, left in the South).

4. Common Exam Pitfalls & How to Avoid Them

Review these examiner-reported mistakes before sitting your AS 1 exam:

Pitfall 1: Confusing Short-Wave and Long-Wave Radiation.
Correction: Incoming insolation from the Sun is short-wave. Outgoing radiation emitted from Earth's surface is long-wave. The atmosphere is heated from below by long-wave radiation.

Pitfall 2: Forgetting Exact Balance Latitudes.
Correction: Always quote \(40^\circ\text{ N}\) and \(40^\circ\text{ S}\) as the boundary lines between net energy surplus (equator to \(40^\circ\)) and net energy deficit (\(40^\circ\) to \(90^\circ\)).

Pitfall 3: Coriolis Deflection Directions.
Correction: Always specify the hemisphere! Winds deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Pitfall 4: Confusing Sensible and Latent Heat.
Correction: Sensible heat causes a measurable temperature change you can feel via conduction and convection. Latent heat is energy absorbed or released during a state change (like evaporation or condensation) without changing temperature during the transition.

5. Quick Summary Revision Checklist

Can you explain each of these concepts to a classmate?

Global Balance: Solar surplus (\(0^\circ\text{ to }40^\circ\text{ N/S}\)) vs. Solar deficit (\(40^\circ\text{ to }90^\circ\text{ N/S}\)).

Diurnal Components: 6 daytime factors vs. 4 night-time factors.

Temperature Controls: Solar angle, albedo (\(80\%\text{ to }90\%\) for snow), continentality, and the Normal Lapse Rate (\(6.5^\circ\text{C}\text{ per }1,000\text{ m}\)).

Circulation Cells: Hadley (\(0^\circ\text{ to }30^\circ\)), Ferrel (\(30^\circ\text{ to }60^\circ\)), Polar (\(60^\circ\text{ to }90^\circ\)).

Upper Air Flow: Geostrophic balance, Rossby waves, and the Polar Front Jet Stream.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the difference between incoming solar radiation and outgoing terrestrial radiation?

Incoming solar radiation (insolation) is high-energy short-wave radiation emitted by the Sun, whereas outgoing terrestrial radiation is long-wave infrared radiation re-emitted by the warmed Earth.

Where are the global net solar surplus and deficit zones located, and at what latitude do they balance?

A net solar surplus exists between \(40^\circ\text{ N}\) and \(40^\circ\text{ S}\), while a net solar deficit exists from \(40^\circ\text{ to }90^\circ\text{ N and S}\), balancing at \(40^\circ\text{ N and S}\).

What are the key components of the daytime and night-time diurnal energy budgets?

The daytime budget consists of 6 components (insolation, reflection, absorption, sensible heat, latent heat, and long-wave radiation), while the night-time budget consists of 4 components (long-wave radiation, sub-surface heat supply, sensible heat, and latent heat).

How do latitude and solar angle influence the receipt of insolation at the Earth's surface?

At the equator, the Sun strikes at a high angle, concentrating energy over a small area and passing through a thin atmosphere; at the poles, low-angle rays spread over a large area and pass through a thicker atmospheric layer.

What is albedo and how does it affect surface temperature?

Albedo is the proportion of incoming solar radiation reflected by a surface, where high-albedo surfaces reflect most energy (remaining cooler) and low-albedo surfaces absorb most energy (warming up).

How does distance from the sea (continentality) affect a region's annual temperature range?

Maritime climates have mild winters, cool summers, and a low annual temperature range, whereas continental climates have hot summers, freezing winters, and an extreme temperature range.

What is the Environmental Lapse Rate and why does air temperature decrease with altitude?

The Environmental Lapse Rate is the average rate at which still air cools with height, dropping by \(6.5^\circ\text{C}\text{ per }1,000\text{ m}\) (\(0.65^\circ\text{C}\text{ per }100\text{ m}\)).

How do the Pressure Gradient Force and the Coriolis Force combine to drive surface winds?

The Pressure Gradient Force drives air from high to low pressure, while the Coriolis Force deflects this moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

What is the structure of the Hadley Cell and why does it create subtropical deserts?

Air rises at the equatorial ITCZ forming low pressure and rain, moves poleward aloft, and sinks at \(30^\circ\text{ N/S}\) creating subtropical high pressure, clear skies, and deserts.

How do the Polar and Ferrel cells operate within the Tri-Cellular Model?

The Polar cell is thermally direct with sinking cold air at the poles and rising air at \(60^\circ\text{ N/S}\); the Ferrel cell is a thermally indirect mechanical cell driven between \(30^\circ\) and \(60^\circ\text{ N/S}\) producing prevailing westerlies.

What is the Polar Front Jet Stream and what role does it play in mid-latitude weather?

The Polar Front Jet Stream is a narrow, fast-moving ribbon of upper-troposphere westerly wind at around \(60^\circ\text{ N/S}\) that steers mid-latitude depressions and anticyclones.

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