2.1.1 global atmospheric circulation

Why do the tropics receive more solar energy than higher latitudes?

• High angle of incidence → energy concentrated over smaller area.

• Shorter atmospheric path length → less energy lost before reaching Earth's surface.

• Relatively consistent daylength and insolation throughout the year.

What is angle of incidence?

The angle at which the Sun's rays strike Earth's surface.

Why does atmospheric path length matter?

Longer path → more solar radiation is absorbed, reflected or scattered → less reaches Earth's surface.

What is albedo?

The overall reflectivity of an object or surface.

What causes seasonal variations in insolation?

Earth's 23.5° axial tilt + revolution around the Sun.

What happens during the June solstice?

• 20–21 June

• Sun overhead at Tropic of Cancer

• Northern Hemisphere receives more solar energy.

What happens during the December solstice?

• 21–22 December

• Sun overhead at Tropic of Capricorn

• Southern Hemisphere receives more solar energy.

What happens during the equinoxes?

• 21 March & 21 September

• Sun overhead at Equator.

• Everywhere receives approximately 12 hours daylight.

• Both hemispheres receive approximately equal insolation.

What is air pressure?

Force exerted by the weight of air molecules on a unit area of Earth's surface.

What is specific heat capacity?

Energy required to increase the temperature of 1 g of a substance by 1°C.

How does the specific heat capacity of water compare with land?

Water is about 5× greater. Sea Slow.

What is the main driving force of wind?

Pressure Gradient Force (PGF), high pressure to low pressure.

What is the Coriolis Effect?

Deflection of moving objects caused by Earth's rotation.

Which way are winds deflected?

Northern Hemisphere → RIGHT

Southern Hemisphere → LEFT

Equator → 0

How does Coriolis strength change with latitude?

Equator → 0
Moving poleward → increases

How does friction affect wind?

Earth's surface → slows wind → reduces Coriolis deflection.

How does friction change with altitude?

It decreases with altitude, so upper-level winds are generally faster and smoother.

What is the main purpose of global atmospheric circulation?

Redistributes thermal energy from the Equator towards the poles.

What drives global atmospheric circulation?

Unequal heating of Earth's surface, producing pressure differences and wind patterns.

Where does the Hadley Cell operate?

Approximately 0–30° N/S.

Explain the Hadley Cell.

Equatorial heating
→ air rises at ITCZ
→ upper air moves poleward
→ air descends around 30° N/S
→ forms STHP
→ surface air returns towards Equator
→ forms trade winds
→ converges again at ITCZ.

What are trade winds?

Predominantly easterly surface winds of the Hadley Cell.
Northern Hemisphere → Northeast trades
Southern Hemisphere → Southeast trades

Why are trade winds important for tropical rainfall?

They travel across warm tropical oceans → gain large amounts of moisture → transport water vapour towards ITCZ.

What is the Intertropical Convergence Zone (ITCZ)?

Belt of convergent airflow and low pressure near the Equator where tropical trade winds from both hemispheres meet.

Why does air rise at the ITCZ?

• Strong solar heating warms air.

• Warm air expands → becomes less dense → rises.

• Converging trade winds further force air upward.

Explain why the ITCZ produces heavy rainfall.

Moist trade winds converge
→ air rises
→ expands
→ adiabatically cools
→ condensation
→ cumulonimbus clouds
→ heavy convectional rainfall

How does latent heat reinforce uplift at the ITCZ?

Condensation releases latent heat → warms surrounding air → encourages further uplift.

What is the STHP?

Subtropical High-Pressure belt of hot, dry descending air around 20–35° N/S.

Why does the STHP produce dry conditions?

Air descends
→ compresses
→ adiabatically warms
→ relative humidity falls
→ condensation/cloud formation suppressed
→ dry, clear conditions

Why does the ITCZ migrate seasonally?

It broadly follows the seasonal movement of the overhead Sun, with a time lag.

Why does the ITCZ migrate further over land than oceans?

Land has lower specific heat capacity → heats/cools faster → greater seasonal temperature and pressure changes.