Weather Hazards

What is global atmospheric circulation?

The large-scale movement of air in Earth’s atmosphere caused by differences in temperature between the equator and the poles

What is the process of the GAC?

The Equator receives more concentrated solar energy, causing warm air to rise and form areas of low pressure & high rainfall. Air movement between pressure zones forms three circulation cells in each hemisphere: Hadley, Ferrel & Polar cells. Rising air at the Equator leads to low pressure & the development of rainforests; sinking air at 30 N&S creates high pressure and the formation of hot deserts. Winds blow from high to low pressure but are deflected by the Coriolis effect, creating trade winds, westerlies & polar easterlies. Ocean currents, driven partly by surface winds, also help transfer heat from the tropics towards the poles

What causes low pressure?

High temperatures heat the air. Warm air expands and becomes less dense. It rises into the atmosphere which leaves less air pressing on Earth’s surface. Therefore, low pressure forms

What causes high pressure?

Cold air is dense and heavy so it sinks towards Earth’s surface. More air presses down on the ground so high pressure forms

What are atmospheric circulation cells?

Circular movements of air that transfer heat around Earth

Describe a Hadley Cell?

Strong heating at the equator causes air to rise, air cools and moves north/south high in the atmosphere. Around 30° N/S the air sinks, creating high pressure and deserts

Describe a Ferrel Cell?

Air moves from subtropical high pressure to subpolar low pressure. Air rises again around 60° which produces unsettled weather

Describe a Polar Cell?

Cold dense air sinks at the poles and air moves towards 60°. It rises where warmer air meets polar air

How does the GAC create winds?

Air moves from areas of high pressure to low pressure. The Earth’s rotation causes winds to curve (Coriolis effect). These surface winds transfer heat, moisture and energy around the planet which creates global wind belts

How does the GAC affect rainfall?

At the Equator, air rises due to intense heating, rising air cools and condenses. This creates clouds and heavy rainfall which results in tropical rainforests

How does the GAC affect deserts?

Around 30° N/S, air sinks from the Hadley Cell and the sinking air warms which prevents condensation. Very little rainfall occurs, resulting in hot deserts

How does the GAC affect the weather in the UK?

The UK is around 55°N where warm tropical air meets cold polar air which creates low pressure. This leads to cloudy weather and frequent rainfall

Why is the equator hot and wet?

The equator receives direct sunlight all year which causes intense evaporation. Warm moist air rises and cooling air condenses into clouds, causing heavy rainfall and hot temperatures

What are tropical storms?

Intense, low-pressure systems that form over warm tropical oceans

Why are tropical storms areas of low pressure?

Warm moist air rises rapidly and rising air creates low pressure at the surface. As air cools, condensation occurs and huge cumulonimbus clouds form. This releases latent heat energy and strengthens the storm

What conditions are needed for tropical storms to form?

Ocean temperatures above 27°C, water at least 70m deep, low wind shear, located between 5°-30° N/S

How do tropical storms form?

Tropical storms form over oceans where the surface temperature is at least 27°C. Rising warm, moist air creates a low-pressure area that draws in more moisture. As the moist air rises, it cools and condenses to form cumulonimbus clouds and heavy rain. The rotation of the Earth causes the rising air to spin, forming the storm’s circular motion. The storm is powered by condensation and is steered by prevailing winds across the ocean. Once the storm reaches land or cooler water, it loses its heat source and begins to weaken

What is the Coriolis effect?

The spinning movement caused by Earth’s rotation. It causes tropical storms to rotate and winds to curve

What is the eye of a tropical storm?

The central part of the storm, about 30-50km across, calm conditions, little or no rain

What is the eyewall of a tropical storm?

A ring of intense clouds with strong upward air movement and powerful winds/rainfall

Describe the weather during a tropical storm

Before the storm, temperature rises and pressure falls. The eyewall arrives and there’s torrential rains, strong winds, thunder and lightning. The eye arrives and there’s calm conditions with no clouds and the temperature rises. The second eyewall arrives and the violent weather returns. As the storm passes, winds weaken, pressure rises and showers decrease

How could climate change affect the distribution of tropical storms?

Rising sea temperatures means more oceans may exceed 27°C so tropical storms could form in new locations

How could climate change affect tropical storm intensity?

Warmer seas provide more energy, increasing wind speeds and making tropical storms more powerful/destructive

How could climate change affect tropical storm frequency?

Warmer oceans lasting longer into the year may extend the tropical storm season, increasing the time a storm can form

What hazards are caused by thunderstorms?

Torrential rainfall, flash flooding, travel disruption, property damage

What causes droughts?

Prolonged periods of little rainfall and high temperatures - rivers dry up, reservoir levels fall, water shortages occur, aquatic ecosystems are damaged

Why are heatwaves dangerous?

Extreme heat can cause dehydration, increase deaths among elderly people, damage crops and increase wildfire risk

Why does prolonged rainfall cause flooding?

Persistent rainfall saturates the ground which causes increased surface runoff and rivers exceeding bankfull, leading to flooding

What impacts do heavy snow and extreme cold have in the UK?

Travel disruption, school closures, difficulty accessing shops/work, slipping injuries

How has rainfall changed in the UK?

Rainfall has increased by 17% in the last decade, rainfall intensity has increased

Why might extreme weather events be increasing globally?

More heat energy in the atmosphere causes stronger storms, more evaporation, increased flooding and more intense heatwaves

What did the IPCC conclude about extreme weather?

Global warming is likely increasing the frequency of extreme weather events and climate change increases the risk of severe storms, droughts, and floods