Global Hazards Revision Notes

The Global Atmospheric Circulation Model

The global atmospheric circulation model consists of three main cells in each hemisphere: the Hadley, Ferrel, and Polar cells. The Hadley cell starts at the equator, where intense solar radiation causes air to heat, expand, and rise, creating a low-pressure zone. This rising air cools, releases moisture, forming clouds and rainfall (explaining equatorial rainforests), then moves polewards at high altitudes. Around 30 degrees latitude, this cool, dry air sinks, creating high-pressure zones and leading to arid conditions (explaining many deserts). The Ferrel cell is an indirect cell between 30 and 60 degrees latitude, driven by the other two cells. Air rises at 60 degrees, creating low pressure and frontal rainfall, and sinks at 30 degrees. The Polar cell is at the poles, where cold, dense air sinks, creating high pressure, then flows towards 60 degrees latitude, where it meets warmer air from the Ferrel cell and rises. This system transfers heat from the equator to the poles.

Impact of the Coriolis Effect

The Coriolis effect is a force resulting from the Earth's rotation that deflects moving objects, including air and ocean currents. In the Northern Hemisphere, it deflects objects to the right, and in the Southern Hemisphere, to the left. This deflection is crucial for the formation of global wind patterns. Without the Coriolis effect, winds would blow directly from high to low pressure. Instead, it causes winds to curve, leading to predictable patterns like the Easterly Trade Winds (0-30 degrees latitude), Westerlies (30-60 degrees latitude), and Polar Easterlies (60-90 degrees latitude). This effect also plays a significant role in the rotation of tropical cyclones and mid-latitude depressions, influencing their direction and intensity.

Causes of Extreme Weather Events

Extreme weather events like tropical cyclones, droughts, and heatwaves are often linked to anomalies or intensifications within the global atmospheric circulation system. Tropical cyclones, for example, require specific conditions: warm ocean waters (above 26.5°C), high humidity, low wind shear, and sufficient Coriolis force for rotation. They form over tropical oceans where rising air creates low pressure. Droughts can be caused by persistent high-pressure systems, which lead to sinking air and prevent cloud formation and precipitation. Heatwaves are also associated with stable, high-pressure systems that trap warm air and lead to prolonged periods of excessively hot weather. Changes in global circulation patterns, potentially influenced by climate change, can alter the frequency, intensity, and distribution of these extreme events.

Predicting and Mitigating Extreme Weather

Understanding global atmospheric circulation is fundamental to predicting weather and extreme events. Meteorologists use atmospheric models, satellite data, and ground observations to forecast weather patterns, including the genesis and track of tropical cyclones or the likelihood of prolonged droughts. This enables early warning systems, allowing communities to prepare and mitigate impacts. Mitigation strategies for extreme weather vary: for cyclones, this includes evacuation plans and sturdy infrastructure; for droughts, water conservation and efficient irrigation; and for heatwaves, public health advisories and cooling centres. Long-term strategies involve addressing climate change, which is influencing the frequency and intensity of these events by altering global energy balances and atmospheric circulation.