6.1B Tricellular model of atmospheric circulation Notes

1. The Earth receives uneven heating from the Sun because sunlight strikes different latitudes at different angles.
2. The equator receives direct, concentrated solar radiation, which warms the surface and the air above it more intensely.
3. The poles receive sunlight at a much lower angle, spreading the same energy over a larger area, which results in much cooler temperatures.
4. This unequal heating creates pressure differences and sets the atmosphere in motion, forming a global circulation system known as the tricellular model.
5. The tricellular model redistributes heat from the equator toward the poles, helping to balance global temperatures and maintain stable climate patterns.

Why Differential Heating Occurs

1. The Earth is spherical, so sunlight strikes the equator more directly than the poles.
2. Tropical regions absorb significantly more solar radiation than polar regions.
3. Snow and ice in polar regions have higher albedo, meaning they reflect much of the incoming radiation.
4. Heat input exceeds heat loss between about 38°N and 38°S, creating a surplus of energy at low latitudes.
5. At higher latitudes, heat loss exceeds heat input, producing an energy deficit.
6. Atmospheric circulation develops to redistribute this energy and reduce extreme temperature contrasts.

The Tricellular Model

1. The tricellular model divides each hemisphere into three large atmospheric circulation cells:
   1. The Hadley Cell
   2. The Ferrel Cell
   3. The Polar Cell
2. Together, these cells create a global pattern of rising and sinking air, wind belts, and pressure zones.

1. The Hadley Cell (0° to 30° latitude)

1. Strong heating at the equator causes warm, moist air to rise, creating a zone of low pressure known as the Intertropical Convergence Zone (ITCZ).
2. As this air rises, it cools and loses moisture, producing heavy equatorial rainfall and tropical rainforests.
3. The rising air spreads toward the subtropics and sinks at around 20°–30° latitude, creating regions of high pressure, dry conditions, and major deserts.
4. The sinking air then flows back toward the equator at the surface, forming the trade winds.

2. The Ferrel Cell (30° to 60° latitude)

1. The Ferrel cell lies between the Hadley and Polar cells.
2. It is indirectly driven by the movements of the other two cells rather than by temperature differences.
3. Air at the surface flows from subtropical high pressure toward subpolar low pressure, forming the westerly winds.
4. Rising air at around 60° latitude leads to cloud formation and moderate precipitation.
5. This zone supports temperate forests and grasslands.

3. The Polar Cell (60° to 90° latitude)

1. Very cold, dense air sinks at the poles, creating high-pressure systems.
2. Surface air flows outward from the poles toward lower latitudes.
3. At around 60° latitude, it meets warmer air from the Ferrel cell and rises, creating low pressure and frontal weather systems.