6.5 - Global Circulation of the Atmosphere
The structure and function of atmospheric circulatory cells
The atmosphere features a complex system of air movement known as global circulation, which involves large-scale circular patterns of air in each hemisphere of the Earth. These patterns, or cells, play a crucial role in distributing heat and moisture across the planet, influencing weather and climate.
Overview of the three circulatory cells
There are three major circulatory cells in each hemisphere—Hadley, Ferrel, and Polar cells. Each operates within specific latitudinal zones and contributes to the overall movement of air from the Equator towards the poles.
| Cell | Location (Latitude) | Key Air Movement |
|---|---|---|
| Hadley Cell | 0° (Equator) to 30° N and S | Warm air rises at the Equator, creating thunderstorms, then flows outwards and sinks at 30°, forming dry subtropical regions. |
| Ferrel Cell | 30° to 60° N and S | Air sinks at 30° alongside the Hadley Cell, moves across mid-latitudes, and rises again at 60° where it meets the Polar Cell. |
| Polar Cell | 60° to 90° N and S (Poles) | Cold air sinks at the poles, flows towards mid-latitudes, and rises at 60° upon meeting the Ferrel Cell. |
Mechanism of air circulation
- Equatorial heating - The Sun's intense heat at the Equator warms the air, causing it to rise and create areas of low pressure.
- Poleward movement - As warm air rises, it moves towards higher latitudes, cooling and sinking at specific points, which creates high pressure zones.
- Interaction between cells - The cells work together to redistribute heat, with warm air moving poleward and cold air moving towards the Equator, balancing global temperatures.
The role of circulatory cells in creating climate zones
The global circulation system is fundamental in shaping the Earth's climate zones, which are broad regions defined by average temperatures and rainfall patterns. These zones include tropical, temperate, and polar regions, each influenced by the behaviour of the circulatory cells.
Influence of cells on climate zones:
- Tropical zone (near Equator, 0°) - Dominated by the Hadley Cell, where rising warm, moist air leads to heavy rainfall and a hot, wet climate.
- Subtropical zone (around 30° N and S) - High pressure from sinking air in the Hadley Cell results in dry, hot conditions typical of desert regions.
- Temperate zone (around 30° to 60° N and S) - Influenced by the Ferrel Cell, with rising and sinking air creating variable weather, often with distinct seasons.
- Polar zone (around 60° to 90° N and S) - Dominated by the Polar Cell, where sinking cold air leads to cold, dry conditions with minimal precipitation.
The characteristics of high and low pressure systems
Atmospheric pressure, measured in millibars, varies across the Earth's surface due to the movement of air in the circulatory cells. Pressure systems are categorised as high or low, each with distinct characteristics that influence local weather.
High pressure systems
- Cause of high pressure - Occurs when cold air sinks, becoming denser and pressing down on the Earth's surface.
- Location in circulation - Found where air descends, such as at 30° N and S (subtropics) between the Hadley and Ferrel Cells, and at the poles in the Polar Cell.
- Air behaviour - As cool air sinks, it warms upon reaching the surface, causing any moisture to evaporate, leading to minimal cloud formation.
Low pressure systems
- Cause of low pressure - Occurs when warm air rises, reducing the weight of air at the surface.
- Location in circulation - Found where air ascends, such as at the Equator between the two Hadley Cells, and at 60° N and S between the Ferrel and Polar Cells.
- Air behaviour - Rising warm air cools, allowing water vapour to condense into clouds, often resulting in precipitation like rain, sleet, snow, or hail.
The impact of pressure systems on weather patterns
High and low pressure systems directly affect weather conditions, creating distinct patterns that vary based on whether air is rising or sinking. These systems are integral to understanding daily and seasonal weather variations.
Weather associated with high pressure
- Clear skies - Sinking air prevents cloud formation, leading to sunny, settled conditions.
- Dry conditions - With moisture evaporating as air warms near the surface, rainfall is unlikely, often seen in subtropical deserts.
- Light winds - The stable air mass results in calm weather with minimal wind activity.
Weather associated with low pressure
- Cloudy and stormy conditions - Rising air cools and condenses, forming clouds and often leading to heavy rainfall or other forms of precipitation.
- Consistent temperatures - Cloud cover reflects sunlight during the day and traps heat at night, reducing temperature fluctuations between day and night.
- Variable weather - Low pressure areas, especially at mid-latitudes, can bring unsettled weather with frequent storms or changing conditions.