Global circulation, tropical storms and drought

• Global circulation has three cells in each hemisphere: Hadley, Ferrel and Polar. Air rises at the Equator (low pressure, heavy rain), sinks at about 30 degrees (high pressure, deserts), rises at about 60 degrees (low pressure, storms) and sinks at the poles.
• Surface winds blow from high pressure to low pressure. The poles are cold because they get little energy from the sun, not because air sinks there.
• Tropical storms form over warm oceans, about 26 to 27 degrees C or more, roughly 5 to 30 degrees from the Equator. They need that distance for the Coriolis effect to make them spin.
• Warm, moist air rises, condenses and releases heat, so the pressure drops further. The eye is calm. The eyewall has the strongest winds and heaviest rain. Over land a storm weakens, because it loses its supply of warm moist air.
• A drought is a long period of well below average rainfall that leads to a water shortage.
• Droughts are caused by persistent high pressure with sinking dry air, and by El Nino and La Nina changing rainfall patterns. They are most common near the subtropical high-pressure belts.
• In an El Nino, trade winds weaken and warm water moves east across the Pacific. Sinking dry air then brings drought to places such as eastern Australia and Indonesia.
• Effects include water restrictions, crop and livestock losses and wildfires. Adaptation includes desalination plants, recycling water and drought-resistant crops.

Hadley cell

The circulation of air between the Equator and about 30 degrees north or south.

Coriolis effect

The effect of the Earth's spin that makes moving air and water curve and storms spin.

Eye

The calm centre of a tropical storm where air sinks.

Drought

A long period of well below average rainfall that leads to a water shortage.

El Nino

A change in the Pacific when trade winds weaken and warm water moves east, altering rainfall across the world.

Desalination

Taking the salt out of sea water to make it drinkable.

• Saying tropical storms form on the Equator because it is hottest. The Coriolis effect is too weak there to make them spin.
• Explaining extreme wet weather only with El Nino and La Nina. The main cause is the global circulation, such as rising air at the Equator. El Nino and La Nina change where the rain falls.
• Writing about causes or effects when asked for drought adaptation. Say how a method works and how well it worked.
• Mixing up causes and effects of drought. Low rainfall is a cause. Crop loss is an effect.

7 questions, 8 marks. Answer, then press Check. The marking guide explains how the marks are worked out. Nothing is saved.