Overview
This lecture explains the global atmospheric circulation system, focusing on how the Sun's energy and Earth's rotation create movement and patterns in the atmosphere.
Uneven Heating of the Earth's Surface
- The Sun's rays are more concentrated near the equator, leading to higher temperatures.
- Energy is spread over a larger area at higher latitudes, resulting in cooler temperatures toward the poles.
- Warm surfaces heat the air above, causing it to rise; cold surfaces cool the air, causing it to sink.
Basic Atmospheric Circulation (Non-Rotating Earth)
- On a non-rotating Earth, hot air would rise at the equator and move poleward.
- Cold air would sink at the poles and move toward the equator, creating a simple single circulation cell in each hemisphere.
The Effect of Earth's Rotation (Coriolis Effect)
- The Earth's rotation causes the Coriolis effect, deflecting moving air to the right in the Northern Hemisphere.
- Air at the equator moves faster than at the poles due to larger rotational distance.
- Air moving poleward appears to move west to east; air moving equatorward appears to move east to west.
Three-Cell Model of Atmospheric Circulation
- Hadley Cell: Warm air rises at the tropics, moves poleward, cools, sinks in the subtropics, and returns to the equator.
- Polar Cell: Cold air sinks at the poles, moves toward the mid-latitudes, then rises and returns poleward.
- Ferrel Cell: Located between the Hadley and Polar cells, driven by mid-latitude storms; air rises at 50-70° N/S and sinks in the subtropics.
Pressure Systems and Weather Patterns
- Rising air (low pressure) leads to cloud formation and precipitation, common in the tropics and mid-latitudes.
- Sinking air (high pressure) leads to clear skies, typical in the subtropics and polar regions.
Surface Winds and Seasonal Shifts
- Trade winds (east to west) dominate the tropics due to the Hadley cell.
- Westerlies (west to east) dominate mid-latitudes, influencing weather in regions like the UK.
- The intertropical convergence zone (ITCZ), the area of lowest pressure, shifts north and south with the seasons.
Key Terms & Definitions
- Coriolis Effect — The deflection of moving air due to Earth's rotation, to the right in the Northern Hemisphere.
- Hadley Cell — Atmospheric circulation cell with rising air at the equator and sinking air in the subtropics.
- Polar Cell — Circulation cell with sinking air at the poles and rising air in the subpolar regions.
- Ferrel Cell — Mid-latitude cell driven by storms, situated between the Hadley and Polar cells.
- Trade Winds — Surface winds blowing from east to west in the tropics.
- Westerlies — Surface winds blowing from west to east in the mid-latitudes.
- Intertropical Convergence Zone (ITCZ) — The region near the equator where trade winds meet and air rises.
Action Items / Next Steps
- Review further resources and educational content on the Royal Meteorological Society's MetLink website.