Weather hazards — AQA GCSE Geography

Global atmospheric circulation is the large-scale movement of air that distributes heat across the Earth, determining weather and climate patterns. The system consists of three convection cells per hemisphere: Hadley, Ferrel, and Polar. At the equator, solar radiation causes air to rise, creating a low-pressure zone with heavy rainfall and tropical climates. This air cools, moves poleward, and sinks at 30° north and south, forming high-pressure zones associated with dry, desert climates. The interaction of these cells, combined with the Coriolis effect deflecting winds, drives global wind patterns like the trade winds. By transferring heat from the equator to the poles, this circulation dictates regional temperature and precipitation.

The general atmospheric circulation model explains how thermal energy is redistributed across the Earth through three distinct convection cells in each hemisphere: the Hadley, Ferrel, and Polar cells. At the Equator, intense solar radiation causes warm air to rise, creating a global band of low pressure. As this air cools and spreads towards the poles, it sinks at approximately 30° north and south, forming high-pressure belts characterised by dry, settled conditions. Surface winds blow from these high-pressure areas towards low-pressure zones to balance the atmosphere. The Earth's rotation deflects these winds due to the Coriolis effect, creating prevailing surface winds such as the Trade Winds, which blow towards the Equator.

Tropical storms, known regionally as hurricanes (Atlantic), cyclones (Indian Ocean), and typhoons (Pacific), require specific physical conditions to develop. They form over oceans with surface temperatures of at least 27°C and a water depth of at least 60 metres, providing abundant heat and moisture. Warm, moist air rises rapidly, creating intense low pressure. As this air rises, it cools and condenses, releasing latent heat that powers the system. They only form between 5° and 30° north and south of the Equator; at the Equator itself, the Coriolis effect is too weak to cause the rising air to spin. The combination of deep, warm water and sufficient planetary rotation allows these powerful weather systems to organise and intensify.

Tropical storms are intense low-pressure systems developing over warm oceans (above 27°C). Their global distribution is restricted to latitudes between 5° and 30° north and south of the equator. Here, the Coriolis effect is strong enough to cause the storm to spin, and ocean temperatures are sufficiently high. They are known by different regional names. In the Atlantic and Eastern Pacific, they are called hurricanes. In the Western North Pacific, affecting countries like Japan, they are known as typhoons. In the Indian Ocean and South Pacific, affecting regions such as Australia, they are referred to as cyclones. They do not form exactly on the equator because the Coriolis effect is too weak there to generate the necessary rotation.

The general atmospheric circulation model dictates where tropical storms form. They develop in areas of intense low pressure, corresponding to the rising limbs of the Hadley cells. As warm, moist air rises rapidly, it creates the instability required for storm formation. However, they only form between 5° and 30° latitude, where the Coriolis effect can initiate rotation. The global circulation system also drives the movement of these storms. Surface winds within the Hadley cells, known as the trade winds, blow from east to west, steering tropical storms across oceans. For example, Atlantic hurricanes are pushed westward towards the Caribbean and North America by these prevailing easterly trade winds.

Tropical storms (hurricanes, cyclones, typhoons) form over oceans with temperatures of at least 27°C, typically between 5° and 30° north and south of the equator. The sequence begins when warm, moist air rises rapidly, creating an area of low pressure at the surface. This draws in more air, which also warms and rises. As the air rises, it cools and condenses to form towering cumulonimbus clouds, releasing latent heat that powers the storm. The Coriolis effect causes the rising air to spin. As the system develops, an eye forms in the centre where air descends. The storm moves westward, driven by trade winds, and intensifies over warm water but loses energy and dissipates upon reaching land or cooler waters.

A tropical storm has a distinct, symmetrical structure spanning hundreds of kilometres. At its centre is the 'eye', a roughly circular area of descending air characterised by calm conditions, clear skies, and extremely low pressure. Surrounding the eye is the 'eyewall', a towering ring of cumulonimbus clouds where the most intense, destructive winds and torrential rainfall occur. Beyond the eyewall, spiral rainbands radiate outwards, bringing heavy rain and strong, gusty winds that gradually decrease in intensity towards the storm's outer edges. The entire system rotates cyclonically (anticlockwise in the Northern Hemisphere) and features a canopy of cirrus clouds at the top where air flows outwards.

Climate change is altering the behaviour of tropical storms by increasing global ocean temperatures. For a tropical storm to form, sea surface temperatures must be at least 27°C. As global warming causes more of the world's oceans to reach this threshold, the distribution of tropical storms may expand further north and south of the equator into regions previously unaffected, such as the South Atlantic. The frequency of these storms might not necessarily increase overall, but the proportion of high-intensity storms (Categories 4 and 5 on the Saffir-Simpson scale) is expected to rise. This increased intensity is driven by warmer oceans providing more latent heat energy, leading to stronger winds and heavier rainfall during the storms.

Tropical storms cause devastating primary and secondary effects on both human populations and the natural environment. Effects on people include primary impacts like injury and death from high winds, and secondary impacts such as homelessness, waterborne diseases, and loss of livelihood due to destroyed agricultural land. Environmental effects involve the destruction of coastal habitats, such as mangrove forests, by storm surges. Heavy rainfall causes severe inland flooding, leading to soil erosion and the contamination of freshwater ecosystems with saltwater and sewage. For example, Typhoon Haiyan in 2013 caused over 6,000 fatalities and destroyed vast areas of coconut plantations, demonstrating both human and environmental devastation.

Tropical storms cause devastating impacts categorised into primary and secondary effects. Primary effects are the direct, immediate results of the storm's physical forces, such as high winds destroying buildings, storm surges flooding coasts, and torrential rain damaging infrastructure. Secondary effects are the indirect consequences occurring as a result of the primary effects. For example, a primary effect of flooding can lead to the secondary effect of waterborne diseases spreading due to contaminated water. Other secondary effects include economic recession from destroyed businesses, food shortages due to ruined crops, and long-term homelessness. Understanding this distinction is crucial for evaluating the overall impact of a tropical storm.

Responses to tropical storms are categorised into immediate and long-term actions. Immediate responses occur during or just after the storm to save lives and reduce harm. These include search and rescue operations, evacuating people, providing emergency medical care, and distributing food, water, and temporary shelter. Long-term responses take place weeks, months, or years later to rebuild communities and improve future resilience. Examples include repairing infrastructure, rebuilding homes to higher safety standards, restoring local economies, and implementing new warning systems or flood defences. Evaluating the effectiveness of both types of response is key to understanding how countries manage and recover from severe weather hazards.

Studying a named example of a tropical storm, such as Typhoon Haiyan (Philippines, 2013), requires categorising its impacts and the subsequent management strategies. Effects must be divided into primary impacts, like the immediate destruction of homes by storm surges, and secondary impacts, such as waterborne diseases or economic losses from ruined crops. Responses are similarly split into immediate actions, including search and rescue or providing emergency shelter, and long-term strategies, such as rebuilding infrastructure to higher standards and restoring livelihoods. By linking the specific effects of your chosen storm to the exact responses implemented, you can demonstrate how communities recover from and adapt to severe weather hazards.

Reducing the effects of tropical storms relies on monitoring, prediction, protection, and planning. Monitoring uses satellites and aircraft to track storm movement. Prediction feeds this data into computer models to forecast the storm's path, enabling authorities to issue timely warnings. Protection focuses on physical mitigation, such as constructing sea walls against storm surges, reinforcing buildings, and installing storm drains. Planning prepares communities through education, establishing evacuation routes, and stockpiling emergency supplies. Combining these four strategies significantly lowers population vulnerability, minimising both casualties and long-term economic damage when a severe weather event makes landfall.

The UK experiences a variety of extreme weather events that can become hazards when they threaten human life, property, or economic activity. Due to its geographical location at the boundary of different air masses, the UK is vulnerable to severe weather conditions. These include prolonged rainfall leading to flooding, heavy snow and extreme cold causing transport disruption, strong winds and gales from Atlantic depressions, and occasional heatwaves or droughts during summer high-pressure systems. Understanding that the UK is not immune to weather hazards is crucial, as these events can have significant social, economic, and environmental impacts across different regions of the country.

An overview of UK weather hazards encompasses several distinct types of extreme conditions. Prolonged or heavy rainfall can lead to river and surface water flooding, damaging homes and businesses. Strong winds and gales, often associated with deep Atlantic depressions, can bring down trees and disrupt power supplies. Extreme cold and heavy snowfall cause severe travel disruption and pose health risks to vulnerable people. Conversely, prolonged high pressure in summer can result in heatwaves and droughts, leading to water shortages, crop failures, and health emergencies. Students must be able to categorise these hazards and understand the specific conditions that cause them in the UK.

Extreme weather events in the UK, such as severe gales, heavy snowfall, prolonged heatwaves, and intense rainfall leading to flooding, significantly disrupt human activity. These events impact various sectors of society and the economy. For instance, heavy snow can cause severe transport disruption, closing roads and grounding flights, which prevents people from commuting. Heatwaves can lead to health emergencies, such as heat exhaustion, while also causing droughts that reduce agricultural crop yields. Flooding from intense rainfall damages homes and businesses, resulting in high insurance claims and costly repairs. Understanding these diverse impacts is crucial for evaluating how vulnerable UK communities are to extreme weather.

You must study a specific example of a recent extreme weather event in the UK to illustrate its causes, impacts, and management. A popular case study is the Somerset Levels floods or Storm Desmond. You need to illustrate the specific causes of the event, such as prolonged rainfall on saturated ground. Furthermore, you must illustrate the social, economic, and environmental impacts, such as homes evacuated, businesses losing income, and habitats destroyed. Finally, use your chosen example to illustrate how management strategies, including immediate emergency responses and long-term planning like dredging rivers or building flood defences, can reduce the risk of future hazards.

Understanding the origins of a recent UK extreme weather event requires examining physical and human factors. Using the 2014 Somerset Levels floods as an example, physical drivers included a prolonged sequence of Atlantic depressions driven by a powerful jet stream, delivering record-breaking rainfall over several weeks. High tides and storm surges simultaneously prevented fresh water from draining into the Bristol Channel, forcing rivers to back up. Human actions exacerbated the situation; rivers such as the Tone and Parrett had not been dredged for 20 years, significantly reducing their capacity to hold water. Changing land-use practices also increased surface runoff, accelerating the speed at which water reached the river channels.

When studying a recent UK extreme weather event, you must evaluate how it affected people, the local economy, and the natural surroundings. Using the 2014 Somerset Levels floods, human consequences included over 600 houses being flooded and 16 farms evacuated, causing immense distress and disrupting daily life as villages like Moorland were cut off. Financial effects were severe, with estimated costs exceeding £80 million; local businesses lost trade, and agricultural land was submerged for months, destroying crops and pasture. Ecological damage involved floodwaters heavily contaminated with sewage and chemicals, which destroyed habitats and required extensive pumping operations that disrupted local ecosystems and river banks.

Management strategies reduce the risk of UK extreme weather events, such as severe flooding, through monitoring, prediction, protection, and planning. The Environment Agency monitors river levels and weather patterns to predict flood risks. This allows for early warnings, giving people time to evacuate or protect property. Protection involves physical defences, like building embankments, dredging rivers, or deploying temporary flood barriers to prevent water from reaching homes. Planning includes restricting new housing developments in high-risk floodplains and educating communities on emergency procedures. Together, these strategies mitigate impacts, reducing economic loss and protecting lives.

Evidence indicates that UK weather is becoming more extreme, characterised by broken historical records and an increased frequency of severe events. Temperature records show a clear warming trend, with the UK's ten warmest years all occurring since 2002, and unprecedented heatwaves, such as exceeding 40 °C in 2022. Rainfall patterns are also intensifying; the UK has experienced its wettest winters on record recently, leading to severe flooding events like those caused by Storm Desmond in 2015, which brought record-breaking 24-hour rainfall. Furthermore, there is an observed increase in the intensity of winter storms and coastal storm surges. These documented anomalies provide robust evidence of a shift towards more extreme weather.