Deserts How Are Deserts Created Learning Objective To

Deserts How Are Deserts Created?

Learning Objective • To explain how deserts are created. Success Criteria • To create annotated diagrams showing relief rainfall and a Hadley cell. • To explain how relief rainfall and air pressure create deserts. • To explain the formation of either rain shadow deserts or trade wind deserts in an exam-style question.

Key Words desert an area the receives less than 25 cm of rainfall per year tropics areas of the Earth 30 degrees North and South of the Equator the circle around the centre of the Earth between the North and South poles evaporation the process of liquids turning into vapour Hadley cell a diagram of a convection current relief the shape of the land

Starter Use the key words and the diagram below to remind yourself how rain is formed. Keywords evaporation condensation precipitation surface run-off

Starter 1. The Sun heats the water and turns it into water vapour. Warm, moist air rises. This is called evaporation. 2. As air rises, it cools and the water vapour condenses into clouds. This type of rain is called convectional rainfall. 3. When clouds are saturated with water vapour, it rains. This is called precipitation.

Desert Formation Deserts areas with very little rainfall (less than 25 mm of rain per year). We are going to look at two reasons why an area may have very little rainfall. Rain shadow desert Trade wind deserts

Rain Shadow Deserts Relief rainfall is when warm, moist air is forced to rise by the relief (shape) of the land. as it rises, it condenses and rains cold, dry air sinks back down the other side of the hill warm, moist air forced to rise by hill this creates deserts

Your Task Draw and annotate (label) a diagram of relief rainfall. In your own words, write an explanation of how rain shadow deserts are created by relief rainfall.

Your work should look like this: as it rises, it condenses and rains cold, dry air sinks back down the other side of the hill warm, moist air forced to rise by hill this creates deserts A rain shadow desert is formed when the relief rainfalls on one side of a hill/mountain range, leaving dry air sinking down the other side of the hill or mountain. dry, sinking air = no rain = desert

Trade Wind Deserts are found on the tropics because of the patterns of air flow shown in the Hadley cell diagram.

Trade Wind Deserts Step 1: The Sun heats the Earth at the Equator, causing air to rise. This creates an area of low air pressure at the Equator.

Trade Wind Deserts Step 2: As the air moves away from the heat source (the Earth), it cools. The troposphere forces the air to spread out high up in the atmosphere.

Trade Wind Deserts Step 3: Cool air sinks back down to Earth over the tropics. This creates high pressure. Rain cannot form under high pressure as evaporation cannot happen.

Trade Wind Deserts Step 3: Cool air sinks back down to Earth over the tropics. This creates high pressure. Rain cannot form under high pressure as evaporation cannot happen.

Trade Wind Deserts Step 4: The cool air spreads out as it reaches Earth and is drawn back towards the Equator to fill the void left by the rising air. This creates strong winds moving across the surface of the Earth called trade winds.

Trade Wind Deserts At the Equator, where we have warm, rising air, we get a lot of rain. This creates rainforests.

Trade Wind Deserts At the tropics, where we have cold, sinking air, we get no rain. This creates deserts.

Your Task Draw and annotate (label) a diagram of a Hadley cell. In your own words, write an explanation of how deserts a trade wind desert is formed.

Your work should look like this: rain forest desert A trade wind desert is formed by the cold air sinking over the tropics. The sinking air creates high pressure and prevents warm air rising to form rain. As no rain can form in areas of high pressure, a desert is created.

Exam Question Name and explain one process that leads to desert formation. You may draw a diagram to illustrate your answer. (5 marks) Mark Scheme: 1 mark for correctly naming a process (rain shadow/trade wind). NB – this must be a process, not simply a reason, such as ‘high pressure’. 1 mark for drawing an appropriate diagram (must be correct for the named process). 3 -4 marks for description and explanation of the process – this can either be in written sentences or annotation of the diagram. You should explain: • how and why the warm air rises; • how and why the rain (precipitation) falls where it does; • the impact of this process on the desert areas.

Plenary Quiz Write three quiz questions based upon what you have learned today. Don’t forget to write your answers down as well! I will choose 10 of you at random to share one of your questions.