Grade 9 Geography Unit 2 PDF

Grade 9 Geography 42 INTRODUCTION Geography, as a discipline, helps us to learn about people, their life styles, and the ways their lives are affected by their interactions with their physical environments. Geography also provides us with closely related facts about how such factors affect our own...

Grade 9 Geography 42 INTRODUCTION Geography, as a discipline, helps us to learn about people, their life styles, and the ways their lives are affected by their interactions with their physical environments. Geography also provides us with closely related facts about how such factors affect our own day-to-day lives throughout our lifetimes. In the previous unit, you have learned about the concept of geography and map reading. In this unit you will learn about the physical environment of the world and Ethiopia. The unit is organized in five sub-topics. These are Forces that Change the Surface of the Earth, Weather and Climate, Natural Regions of the Earth, Ecosystem and Villagization of the World through Distance-time Decay. You were introduced to most of these topics in earlier units. This unit presents them in greater detail to give you more information about our physical environment and to help you interactively learn more about them. 2.1 FORCES THAT CHANGE THE SURFACE OF THE EARTH At the end of this section, you will be able to: ÞÞ list the landforms formed by each internal force; ÞÞ describe the process of each internal force; ÞÞ relate some major landforms with their respective internal forces; ÞÞ explain the effects of earthquakes on infrastructure; like buildings, dams, roads; ÞÞ review external forces; ÞÞ state the meaning of weathering; ÞÞ distinguish the different types of weathering; ÞÞ identify land features resulting from chemical weathering; stalactite, stalagmite, pillar etc; ÞÞ explain the types and characteristics of agents of erosion; ÞÞ relate types of erosions; ÞÞ state the effects of erosion on human activities; with various landscapes; ÞÞ explain the process of deposition; ÞÞ recognize erosion-deposited soils and landforms. 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 43 Key Terms ³³ Block mountains ³³ Fissure ³³ Magma (Horst) ³³ Folding ³³ Orogeny ³³ Caldera ³³ Fold mountains ³³ Rift valley ³³ Crater ³³ Focus ³³ vent ³³ Earthquake ³³ Force ³³ Volcanism ³³ Epicentre ³³ Landform ³³ Faulting ³³ Lava Start-up Activity 1 What is a landform? What are the factors that influence the formation of landforms? 2 Look at Figure 2.1 and describe what you can see. 3 Then categorise them into internal forces and external forces. Figure 2.1 Different forces that are shaping the surface of the earth 2.1.1 Internal Forces What are the internal forces that affect the formation of landforms? Those forces that drive energy from the interior part of the earth are called internal forces. Internal forces form the ups and downs on the earth’s crust by breaking and bending (faulting and folding) it. Forces inside the crust cause folding, faulting (cracking), volcanism and earthquakes. 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 44 ÂÂ Folding is one of the internal processes which occurs when two forces act towards each other from opposing sides. Due to this force, rock layers are bent into folds. The process by which folds are formed due to compression is known as folding. There are large-scale and small- scale folds. Large-scale folds are found mainly along destructive plate boundaries. After Figure 2.2 Compression Before forces and their resulting effect Figure 2.3 Tensional Before After forces and resulting effects Folding can be explained using two important parameters FF Axial Plane FF Limbs Types of folding: different types of folds are created, based on the nature of the forces applied on bedrock. If the fold is upward and convex, it is called anticline. If the fold is downward, it is called syncline. Axial plane b L Syncline fold Lim im b Figure 2.4 Syncline fold Figure 2.5 Anticline fold Anticline fold 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 45 Fold Mountains What are fold mountains? How are fold mountains formed? Name some of the fold mountains of the world. Fold mountains are formed by crust which has been uplifted, and folded by compressional forces. They are formed when two plates move towards each other. The compressional force which is created as a result of this movement pushes sedimentary rocks upwards into a series of folds. Fold mountains are usually formed from sedimentary rocks and are usually found along the edges of continents. This is because the thickest deposits of sedimentary rock generally accumulated along the edges of continents. There are two types of Fold Mountains: young fold mountains (10 to 25 million years of age, example, the Atlas, Rockies and the Himalayas) and old fold mountains (over 200 million years of age, example, the Cape Range, the Urals in Russia and the Appalachians of the USA). Many ranges of mountains have been formed by folding. The Andes, the Rocky mountains, the Alps, the Himalayas and the Australian Alps are some examples. The Atlas mountains in north west Africa and the Cape Range in South Africa were formed by folding. This process of mountain building is called orogeny. Faulting What is faulting? Describe types of faults. Movements in the crust of the earth sometimes make cracks. These cracks are called faults. Faulting can be caused by either lateral or vertical forces, which can be either tensional or compressional. Tension causes a normal fault, and compression causes a reverse fault. Activity 2.1 1 Study Figure 2.6 and identify the difference between a reverse fault and a normal fault. 2 Explain the difference between tensional and compressional forces. 3 Is faulting a way by which the surface of the earth is changed? Why or why not? 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 46 strike strike-slip faulting hanging wall block hanging wall block strike dip dip normal faulting dip footwall block reverse faulting footwall block Figure 2.6 Formation of normal and reverse faults Major features formed by faulting include rift valleys and block/Horst mountains Rift Valleys What is rift valley? How are rift valleys and block mountains formed? A rift valley is a linear shaped lowland area between highlands or mountain ranges created by geologic rifts or faults. A rift valley is a valley formed by faulting. When two parallel faults occur on the surface of the earth, and when the land between the two faults sinks down, a rift valley is formed. Rift Valley (African rift valley) New Ocean Basin (Red Sea) Mature Ocean (Atlantic) Mid-Ocean Ridge Sediments Crust Mantle Figure 2.7 Formation of a rift valley 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 47 Activity 2.2 In your group, answer the following question and perform the following task. 1 Which is the largest rift valley in the world? 2 Identify the location of the largest rift valley on a map. The largest rift valley in the world is the East African Rift Valley. It extends from Syria to Mozambique, passing through the Red Sea, Eritrea, Ethiopia, Kenya, Tanzania, DR Congo, Rwanda and Burundi. What other countries are touched by this rift valley? The total length of the East African Rift Valley is about 7,200 km, of which 5,600 km is in Africa. The Ethiopian Rift Valley is a part of the East African Rift Valley. It extends from northeast to south west. Features found in the Rift Valley include active volcanoes, lakes, hot springs and fumaroles. Afar Triangle Figure 2.8 East African Rift Valley 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 48 Block (Horst) Mountains What is block mountain? How do block mountains form? Block mountains are formed when land between two parallel faults is pushed upward due to pressure from inside the earth. If there are two parallel faults, the crustal block between them may either rise to produce a Horst (block) mountain, or fall, to produce a rift valley. Examples: FF The Sierra Nevada mountains in North America. FF The Harz Mountains in Germany. FF The Afar block mountain in Ethiopia. FF The Ruwenzori in Africa. Fault Fault Pressure inside the earth Rocks are pulled apart Formation of block mountain Figure 2.9 Formation of block mountains (Horst) Activity 2.3 1 Prepare models representing FF Types of folds FF Anticline and syncline FF Rift valley and block mountains using mud, paper mach’e, wood blocks, etc. 2 Explain the meanings of FF Syncline and anticline FF Tensional forces and compressional forces. Volcanism What is volcanic activity? What landforms are associated with volcanic activity? Volcanic activity is another internal force which changes the surface of the earth. It is caused by internal movements within the earth. Volcanism is the process by 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 49 which magma, gases, water vapour, ashes and other solid materials are forced out to the surface. Inside the earth the temperature is very hot. This high temperature changes rocks into molten magma. When this magma reaches the surface, volcanic activity takes place. When the magma emerges on to the surface, it cools and hardens. It is then called lava. Figure 2.10 The process of volcanic activity Magma reaches the earth’s surface through two kinds of holes. They are vents and fissures. Magma may force its way violently through a small hole called a vent. If lava emerges via a vent, it builds up into a volcano (cone-shaped mound), and if it emerges via a fissure, it builds up to form a lava plateau or lava flow. Magma may pour quietly through long cracks (fissures) onto the earth’s surface. If the magma flows to the surface through a vent, acrater is formed. Sometimes a volcano erupts very forcefully. When this happens, the top part of the volcano is blown away. This forms a large crater called a caldera. Water collects in the crater or in the caldera and forms a lake. We call this a crater lake or caldera lake. In Ethiopia there are many crater lakes such as Zuquala, Wonchi and Dendi. 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 50 If an eruption begins again in a caldera, a new small cone-shaped volcano is formed inside the caldera. These are known as caldera cones. Part of the magma may not reach the earth’s surface, and when this magma cools, solidifies and forms rocks inside the crust, features such as batholiths, lacoliths, sills and dikes, are formed. Figure 2.11 The most important types of volcanic intrusion and extrusion features FF A batholith is a very large mass of magma which accumulates in the crust. It is the largest structure. FF A laccolith is a mushroom shaped body of intrusive igneous rock. Smaller than a batholith. FF A dike is formed when magma solidifies in a vertical or near-vertical crack. FF A sill is formed when magma solidifies horizontally or nearly horizontally along a bedding plane. Table 2.1: Types of volcanoes Erta’li, Fentale Dubbi and Active volcano Erupts from time to time Damiali Has not erupted for a long time Dormant volcano but may erupt again in the Tatali and Dabbahu future Has not erupted within historic Mt. Zuquala, Ras Dashen Extinct volcano time and Batu 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 51 Importance of volcanic eruptions: FF Give us some ideas about the interior of the earth. FF Provide fertile soil. FF Provide hot springs (with medical value). FF Generate geothermal energy. FF Help in the formation and concentration of minerals. FF Help in the creation of new land. Activity 2.4 Make models of volcanoes and show the different parts of a volcano, using wooden blocks, mud or paper mach’e. Earthquake What is an earthquake? Why do earthquakes occur? Earthquakes are sudden movements in the earth’s crust. They are caused by internal movements deep down inside the earth. Earthquakes are frequently associated with faults. They take place along fault lines where the earth’s crust is weak. When an earthquake occurs, vibrations from the centre spread out in the form of waves in all directions. The point at which an earthquake originates is called the focus. The point on the earth’s surface immediately above the focus is called the epicentre. As the vibrations spin out from the centre, the damage they cause becomes less and less. Figure 2.12 An earthquake’s waves, focus and epicentre 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 52 Figure 2.13 The wave pattern and strength of an earthquake How do we determine the intensity or magnitude of an earthquake? The intensity of an earthquake is measured by an instrument called a seismometer, and is recorded on a seismograph. It records the vibrations produced by an earthquake. The scale which gives the magnitude is called the Richter scale. It ranges from 0 to 9. Each number of this scale indicates a tremor that is ten times stronger than the next lower number. An earthquake with a magnitude of 4.0 is ten times stronger than one that measures 3.0. Readings of 7.0 or higher indicate a strong or major earthquake. The strongest ever recorded earthquake was the Valdivia earthquake in Chile that occurred on May 22, 1960 (9.5 on the Richter scale). Table 2.2: Richter scale values and the corresponding magnitude of earthquakes The Richter Scale Effects < 3.5 Detected only by instruments (seismometers) 3.5 – 4.8 Feels like a lorry passing 4.9 – 5.4 Loose things fall 5.5 – 6.1 Walls crack 6.2 – 6.9 Chimneys fall, some buildings collapse 7.0 – 7.3 Many buildings fall, landslides 7.4 – 8.1 Most buildings and bridges are destroyed > 8.1 Total destruction 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 53 Effects of Earthquakes What are the effects of Earth quakes? Indian Ocean Earthquake In December 2004, an earthquake with a magnitude of 9.1 to 9.3 rocked the Indian Ocean and caused a tsunami, known as the Boxing Day Tsunami. The initial death toll reported by the Associated Press was 100,000, but after the final count, it was reported that the death toll reached up to 225,000. In addition to destruction of life and property, an earthquake causes: FF displacement of parts of the earth’s crust vertically or laterally. FF landslides and deep cracks in surface rocks. FF the devastation of cities, fires and diseases. FF the rise or lowering of the sea floor. Legend Meeting place of the principal plates Figure 2.14 The major earthquake and volcanic belts of the world 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 54 About 80% of all earthquakes occur in three regions. They are FF Around the Pacific Ocean zone. The largest earthquake and volcano zone lies along the edges of the Pacific Ocean. This zone is known as the Pacific Ring of Fire. FF Across Southern Europe and Southern Asia. FF The west-coast areas of North and South America. The two most recent earthquakes in Ethiopia measuring more than 5 on the Richter Scale occurred: FF July 14, 1960, near Lake Shalla with a magnitude of 6 on the Richter Scale. FF June 2, 1961, in Karakore. Activity 2.5 In your geography work group, perform the following tasks. 1 Describe the effect of earthquakes on people, buildings and the infrastructure. 2 Name ten countries within the Pacific Ring of Fire. 2.1.2 External Forces Weathering What is weathering? What is the effect of weathering on landforms? External forces can lower the level of the land by washing it away, and this process is called denudation. They also can raise the level of the land by deposition. Denudation consists of weathering and erosion. Weathering includes disintegration (physical weathering), which breaks rocks into smaller pieces and decomposition (chemical weathering), which forms new substances. Physical (Mechanical) Weathering What is physical weathering? What are the main agents of physical weathering? Physical weathering breaks the rocks into smaller pieces. Its main agents (causes) are temperature changes, frost action and the action of plants and animals. 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 56 Figure 2.17 Breaking of rock by a plant Chemical Weathering (Decomposition) What process is important in chemical weathering? What are main agents of chemical weathering? Chemical weathering is a process that forms new substances, and it is affected by the minerals in the rock. Its main agents are rain action and plant and animal actions. As rain water passes through the atmosphere, it takes in carbon dioxide (CO2) and forms a weak carbonic acid. When this acid water comes into contact with rock, it begins to dissolve minerals in the rock. The rate at which rock dissolves depends on the type of rock. Limestone, for example, dissolves very quickly. This process is known as carbonation. H2O + CO2 ⇒ carbonic acid ⇒ dissolves and erodes limestone and forms caves. Example: Sofomer Cave along the River Weiyb in Bale. 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 57 Figure 2.18 Sofomer Cave, in Bale formed by the River Weiyb and seeping rainwater In underground rivers, seeping rain water continues to dissolve the limestone beneath the surface, gradually forming passages and caves. These caves contain features such as stalactites, stalagmites and pillars. Stalactite Pillar Stalagmite Figure 2.19 Underground cave in a limestone area FF A stalactite is a limestone column that hangs down from the ceiling of the cave. FF A stalagmite is a limestone column that builds upwards from the floor of the cave. 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 58 FF A pillar is formed when a stalactite and a stalagmite join together. When rain water dissolves oxygen and reacts with iron in rocks, the rocks become rusty. Pollution in towns and cities increases chemical weathering. How do plants and animals act as agents of chemical weathering? Plants absorb minerals, and decaying vegetation produces organic acid, which causes a further breakdown of minerals. Bacteria in the presence of water breaks down certain minerals in the soil. Leaching is a major soil-forming process. It occurs when substances are dissolved in water that percolates through soil. Such substances include soluble chemicals that move out of biological tissues into soil - for example, rainfall causes potassium and other ions to be lost by foliage. Erosion What is erosion? What are the major agents of erosion? What are the major types of erosion? What are the characteristics of the agents of erosion? Erosion is the transporting of weathered material by various natural forces such as moving water, wind and moving ice. Erosion occurs when particles of rock or soil are: FF washed away by a river FF removed by waves of the sea FF crushed under a glacier FF blown away by the wind Erosion by Running Water How does running water cause erosion? What processes are included in erosion? Rivers are the most important of all natural agents which help in shaping the earth’s surface. The work of running water includes eroding, transporting and depositing eroded material. 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 59 There are three types of running water erosion: 1 Sheet erosion: occurs when surface water moves in a wide flow. 2 Rill erosion: occurs when surface water cuts relatively small channels. 3 Gully erosion: occurs when floods cut deep wide gorges. The course of a river, from its source to its mouth, can be divided into 3 stages. The action of the river is different in these three parts. The stages are upper course, middle course and lower course. Upper Course Lo vertical erosion ng weathering pr headward erosion oo. v-shaped valley le. waterfalls Middle Course. rapids. gorges lateral erosion tranportation Lower Course. oodplain transportation. meanders deposition. river cliff. large channel. oxbow lakes. large oodplain. deltas/estuaries Figure 2.20 The three stages of a river Activity 2.6 In pairs, discuss the followinq questions. 1 What are the main characteristics of the river stages shown in Figure 2.20? 2 Which stage has a steeper slope? Upper Course In this stage the river water is usually small in volume. As the river flows very fast down steep-slopes, a V-shape valley, waterfalls and deep gorges are formed. The fast flow of the river causes vertical erosion and destruction. The V-shape valley has steep sides and a narrow floor. The fast flowing river cuts down deeply into the land. 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 60 interlocking spurs river spur Figure 2.21 Features of the upper course of a river Waterfalls are caused by sudden drops in the level of rivers. Waterfalls are formed when water flows over hard rock which cannot be eroded easily, while soft rocks are easily eroded. The hard rock produces an overhang, and the water flows over it as a waterfall. Figure 2.22 Waterfall formation Tis Isat water fall The Middle Course During a river’s middle course, the river valley becomes wider and larger. The river may receive waters of many tributaries, which increase the volume of water. Wide-floored valleys with gentle slopping sides are the main features of the middle course of the river. Instead of taking the most direct course possible, the river begins to meander. Meanders are pronounced curves in the course of a river. 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 62 Activity 2.7 In your group, perform the following tasks and answer the following question. 1 How do rivers transport materials? 2 If there is a river near your school, your teacher will organize a field trip to it. At that site, identify landforms such as meanders, waterfalls, gorges, floodplains, V-shaped valleys, etc. Then write a short note on your findings and present it in your classroom discussion. 3 Prepare charts or models representing major landforms associated with a river. Erosion and Deposition by Sea Waves What are coastal landform features produced by wave erosion? Waves are formed when wind moves over the surface of the sea. This causes the particles of water to move in a circular motion, which forms a wave. This movement of water in the sea clashes against coastal lands and picks up rock particles and throws them into the sea as sediments. The work of the sea along the coast includes erosion, transportation and deposition. Some of these features formed along the shoreline are beaches, spits and lagoons. Figure 2.24 Features of sea wave erosion 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 63 Beach is a strip of land along the sea coast covered with various types of sediment. A Spit is a narrow ridge of sand or shingle. It projects into the sea but is attached to the land at one end. Lagoon is an area of saltwater separated from the sea by loose sandbanks. Wind Erosion and Deposition What is the most active agent of erosion in desert regions? What is the most common type of wind deposit? Wind erosion is common in desert and semi-desert areas. Wind erosion and deposition form different landforms such as sand dunes, barchans and loess deposits. Direction of wind and movement of dunes Barkhane (above) and plan view of barkans (below) Sand dunes in the Namib Desert Figure 2.25 Landforms associated with wind erosion FF Sand dune is a small hill of sand formed by the action of the wind. FF Barchan is a sand hill that has a crescent-moon shape. FF Loess deposit is a deposition of fertile soil in the desert by wind. Activity 2.8 In your group, perform the following tasks and answer the following questions. 1 Describe the difference among spit, lagoon and beach. 2 Compare and contrast barchans, sand dunes and loess deposits. 2.1 Forces That Change The Surface Of The Earth  Grade 9 Geography 64  Exercise 2.1 Word Game Down 1 The method by which the strength of an earthquake is measured. Across 2 Sleeping volcano. 3 The opposite of compression. 4 Young fold mountain in North America. 5 Young fold mountain in Asia. 6 Sudden movement in the earth crust. 7 Magma may force through this narrow hole. 8 Types of erosion occur when surface water cuts relatively small channels. 9 Downward fold of bedrock. 10 Large crater at the top of volcano. 11 Circular funnel-shaped depression produced by volcanic eruption. 12 The side of an upfold. 13 The transporting of weathered materials by water, wind, etc. 1 2 3 4 5 6 7 8 9 10 11 12 13 2.1 Forces That Change The Surface Of The Earth  Unit 2: Physical Environment of the world and Ethiopia 65 2.2 WEATHER AND CLIMATE At the end of this section, you will be able to: ÞÞ explain the meaning of atmosphere; ÞÞ discuss the composition and layers of the earth’s atmosphere; ÞÞ explain weather and climate; ÞÞ express the concept of temperature; ÞÞ appraise the variation of temperature; ÞÞ demonstrate how to measure and record temperature data; ÞÞ compute normal temperature lapse rate; ÞÞ interpret temperature data; ÞÞ explain the formation of rain; ÞÞ discuss the types of rainfall; ÞÞ relate the various roof slopes of houses in various climatic regions to the respective types of rainfall; ÞÞ explain what cloud is; ÞÞ describe types of clouds; ÞÞ practice measuring and recording rainfall data; ÞÞ differentiate types of winds (local, monsoon and planetary winds, including cyclones and anticyclones); ÞÞ relate direction and deflection of winds to the earth’s rotation; ÞÞ interpret wind speed and direction from wind gradient map; ÞÞ explain how conditions of wind affect structures of buildings and crop production; ÞÞ Identify types of atmospheric pressure; ÞÞ relate atmospheric pressure with temperature and altitude; ÞÞ demonstrate the pressure belts of the world: ÞÞ develop the skills of measuring and recording atmospheric pressure; ÞÞ analyze the position of the sun at various latitudes at noon time of Dec. 22/ June 21; ÞÞ examine the impact of latitude on temperature; ÞÞ justify the effect of altitude on the characteristics of temperature, rainfall and air pressure; ÞÞ compare and contrast the condition of rainfall and temperature between places of coastal and interior areas; ÞÞ express the meaning and types of ocean current; ÞÞ identify the impacts of ocean currents; ÞÞ recognize the effects of ocean currents on temperature and rainfall on land surfaces; 2.2 Weather And Climate  Grade 9 Geography 66 ÞÞ discuss the types and location of pressure belts of the world; ÞÞ state seasonal movements of pressure belts in relation to the apparent movement of the sun; ÞÞ relate movements of planetary winds with pressure belts; ÞÞ predict the impact of cloud cover on temperature. Key Terms ³³ Aerosols ³³ Jetstream ³³ Radiation ³³ Atmosphere ³³ Lapse rate ³³ Stratosphere ³³ Convectional ³³ Mesosphere ³³ Thermosphere ³³ Cyclonic ³³ Ocean current ³³ Troposphere ³³ Evaporation ³³ Orographic ³³ Exosphere ³³ Precipitation 2.2.1 Earth and Atmosphere What is atmosphere? How do you explain the importance of atmosphere for human beings or for all life forms? The air that surrounds the earth is called the atmosphere. It is an envelope of transparent colorless, tasteless and odorless gases found above the earth’s surface. Composition of the Atmosphere The earth’s atmosphere is a mixture of gases, suspended dust particles and condensed moisture droplets which are collectively known as aerosols. The gases are different in their volume. Table 2.3: Gases of Earth’s atmosphere Major gases Minor gases Rare gases Nitrogen 78% Argon 0.93% Hydrogen, ozone, methane, neon, helium, krypton , Oxygen 20.95% Carbon dioxide 0.03% xenon, carbon monoxide 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 67 Activity 2.9 In your geography work group, perform the following task and answer the following questions. 1 Is carbon dioxide useful? If yes, how? 2 Why are carbon dioxide, oxygen, and ozone important to the earth’s organisms? 3 How could plants or agricultural crops and animals in your locality be affected by the atmosphere? Structure of the Atmosphere The earth’s atmosphere is divided into four layers based on temperature variation. They are troposphere, stratosphere, mesosphere and thermosphere (see Figure 2.26). 1000 Exosphere 600 500 Ionosphere 100 Thermosphere 90 Mesopause Altitude in Kms 80 70 Mesosphere 60 Stratopause 50 40 Highest concentration 30 of ozone Stratosphere 20 Tropopause 10 Jet Streams Troposphere Mt. Everest 0 0 90 80 70 60 50 40 30 20 10 0 10 20 -10 - - - - - - - - - Temperature in oC Figure 2.26 Vertical structure of the atmosphere 2.2 Weather And Climate  Grade 9 Geography 68 Table 2.4: Atmosphere layers and their characters Name of the Average height Major characteristics layer FF Contains 75% of the atmospheric mass. FF Uniformly, temperature decreases with Extends from sea an increase of altitude. Troposphere level to 8/16 kms FF The top boundary is known as the tropopause, which is characterized by jet streams (high velocity winds). FF Has constant temperature. Extends up ward Stratosphere FF High concentration of ozone gases. to 50 kilometres FF Its upper limit is called the stratopause. FF It is the coldest part of the atmosphere. FF Its upper surface is known as the Extends from mesopause. Mesosphere 50 to 80/85 FF Temperature decreases to nearly kilometres –100 C at the top of the mesosphere. o FF Meteorites burn and disintegrate because of friction here. FF Have extremely low density FF Very little heat can be absorbed, held or conducted. FF Temperature rises as high as 1200oC. Thermosphere Extends from FF The ionosphere is a layer of electrically FF Ionospheres 80/85 kilometres charged particles. These electrons and FF exosphere upward into ions are useful for communication space because they reflect radio waves. FF The exosphere begins at an altitude of about 500 to 700 kilometres above the earth’s surface and extends to interplanetary space. Activity 2.10 Discuss the following issue and questions in pairs. 1 The main characteristics of the layers of the atmosphere. 2 Which layer of the atmosphere contains ozones? Discuss the use of ozone gas. 3 What is the coldest layer of the atmosphere? 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 69 2.2.2 Meaning of Weather and Climate What is the condition of the atmosphere today? What is weather? What is climate? How is climate different from weather? Weather is the condition of the atmosphere over a short period of time. Weather includes daily changes in precipitation, air pressure, temperature, wind, etc. Weather refers to atmospheric conditions in a given location. What is the weather like in your locality today? Climate is the average of all weather conditions of an area over a long period of time. These conditions include average temperature, air pressure, humidity, and days of sunshine for a period of 30 years. Climate tells us what it is usually like in the place where we live. Major Elements of Weather and Climate The major elements of weather and climate are temperature, rainfall, winds, air pressure, clouds, etc. You will learn more about these elements of weather and climate. Temperature What is temperature? Temperature is the amount of hotness or coldness of an object. The sun is the primary heat source for the earth and its atmosphere. The sun’s energy is called insolation or solar radiation, and this turns into heat energy at the earth’s surface. How is energy transferred in the atmosphere? Not all the energy that originates from the sun reaches the earth’s surface. Incoming Solar Radiation 100% Figure 2.27 Global modification of incoming solar radiation by atmospheric and surface processes 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 71 Measuring and Recording Air Temperature What is the instrument that is used to measure temperatures? Explain how air temperature is measured and recorded? We measure temperature with thermometer. There are two types of thermometers: maximum and minimum thermometers. A maximum thermometer is a mercury-in-glass thermometer that has a constriction near the bulb end. When the temperature of air rises, the mercury in the thermometer expands and forces its way into the stem past this constriction. But when the bulb cools, none of the mercury above the constriction moves back into the bulb. Therefore, the length of the mercury in the stem remains the same. The end of the mercury thread, which is the farthest from the bulb, registers the highest temperature reached in a day. capillary bulb glass tube mercury metal index vacuum alcohol metal index Figure 2.29 The maximum and minimum thermometers FF The freezing point of mercury is –38.83oC, and the boiling point is 356.73 oC FF Alcohol freezes at a temperature of negative one hundred thirty degree Celsius (–130oC) A minimum thermometer has alcohol as its liquid, and it sets a metal index. When the temperature falls, the alcohol column drags the index towards the bulb end. When the temperature rises, the alcohol column expands and runs past the index without disturbing it. Thus, the end of the index, moves the farthest from the bulb and gives the lowest temperature attained in a day. Alcohol thermometers may be used to measure temperatures from -130oC (freezing point of alcohol) to 785oC (boiling point of alcohol). The standard thermometer for environmental measurements needs only to cover the range between -30oC to 50oC. 2.2 Weather And Climate  Grade 9 Geography 72 Maximum and minimum thermometers are kept in a box-like shelter which is known as a Stevenson screen. 121 cm Figure 2.30 Stevenson screen The temperature of the air changes from time to time. Typically we measure the daily and annual variations. The changes between the highest and the lowest temperatures during 24 hours of a day is known as the daily march of temperature or the diurnal range. The changes of temperature from month to month within a year is known as the annual march of temperature. To describe this temperature variation, we have to use records for a long period of time. We use words like average and range to indicate the variations. Daily average (mean) temperature: is obtained by adding the maximum and minimum temperatures of a day and dividing the sum by two. Example 1: If the maximum daily temperature is 25oC, and the minimum daily 25o C + 5o C temperature is 5oC, daily average temperature = = 15o C 2 FF Monthly average (mean) temperature is calculated by adding all daily averages and dividing the sum by the number of days of the month. FF Annual average is obtained by adding the average monthly temperatures and dividing the sum by 12. FF Daily (diurnal) range is the difference between the maximum and minimum temperature in a day. 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 73 Example 2: If the maximum temperature is 25oC, and the minimum temperature is 5oC, FF Daily range = maximum – minimum = 25oC – 5oC = 20oC. FF Annual range is the difference between the temperatures of the hottest and coldest months in a year. Example 3: If the hottest month is 40oC, and the coldest moth is – 10oC, Annual range = 40oC −(−10oC) = 50oC Table 2.5: Average annual temperature for Addis Ababa Months J F M A M J J A S O N D Annual Max.Temp (oC) 23.3 24.3 24.8 24.2 24.4 22.8 20.6 20.6 21.3 22.3 22.6 22.8 Annual Min.Temp (oC) 9 10.8 11.6 12 12.3 11.2 11.3 11.2 11 9.9 8.7 8.1 Activity 2.12 1 By referring to Table 2.5, a Calculate annual range of temperature. b Calculate annual average (mean) temperature. c Convert the data into graphs 2 When do the maximum and minimum temperatures of the month occur in Ethiopia? Why? Rainfall What is rainfall? Rainfall is liquid precipitation. Any moisture that falls from the clouds towards the earth’s surface is called precipitation. Precipitation may occur in the form of rain, snow, hail, sleet and drizzle. Precipitation is part of the water cycle or hydrological cycle. The water cycle begins as water is changed from liquid to vapour by evaporation and transpiration of water vapour. Once water vapour is formed, it expands and cools. Then, condensation occurs, forming clouds, and the water falls as snow, sleet or rainfall. The whole process is powered by solar 2.2 Weather And Climate  Grade 9 Geography 74 energy and is repeated continuously. This whole process is called the hydrological cycle. Moist air to continent Condensation To ocean and precipitation Condensation Evaporation from falling Evaporation from vegetation Evaporation from Evaporation from ocean soil Evaporation from lakes, rivers, etc. Groundwater to lakes Lake Land Ocean Groundwater to rivers and oceans Figure 2.31 The hydrological cycle FF Evaporation is the process by which liquid water is converted into gases. FF Transpiration is the transfer and change of water from plants to water vapour in the air. FF Evapotranspiration is the combined loss of water through the process of evaporation and transpiration. FF Condensation is the process by which vapour becomes liquid. FF Sublimation is the process in which ice changes into water vapour without first becoming a liquid, and vice versa. Water vapor (gas) Absorbs heat Absorbs heat Releases heat Releases heat Releases heat Ice (solid) Water (liquid) Absorbs heat Figure 2.32 Water exists in three forms 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 75 Water is a unique substance, because it can exist in three states as liquid, solid and gas) in the atmosphere. Water either absorbs or releases heat when changes from one state to another (see Figure 2.32). Types of rainfall What are the types of rainfall? Explain their formation. Rain is given three different names according to the different ways in which moisture is forced to rise. They are: ÂÂ Convectional rainfall ÂÂ Cyclonic rainfall ÂÂ Orographic or relief rainfall 1 Convectional rainfall: When the ground surface is heated by the sun, the air above it is warmed up. At high altitudes, the water vapour cools, condenses to form clouds and falls as rain. This type of rainfall is common in humid areas where temperature is high throughout the year. Figure 2.33 Convectional rainfall 2 Orographic (relief) rainfall: occurs when moist air is forced to rise over mountains. As it rises, it cools, then condenses and falls as rain. Almost all orographic rainfall falls on the windward side of mountains. Cool air depleted Prevailing of moisture sinks wind and warms; relative Moisture condenses humidity decreases as air cools; relative humidity increases Rainy Dry windward leeward RA Moist, warm IN slope slope SH air rises AD OW Ocean Figure 2.34 Orographic (relief) type of rainfall 2.2 Weather And Climate  Grade 9 Geography 76 3 Cyclonic or Frontal rainfall when two air masses (warm and cold) meet, they do not mix freely with each other. They remain separated with a boundary surface between them. The warmer and less dense air is forced to rise over the colder and heavier air. As the warmer air rises, it cools and condenses. Then clouds form and rain falls. The place where warm air and cold air meet is called a front. Frontal rainfall is very common in the middle and high latitudes (60o north and south from the equator). Fronta lb ounda ry Cold air Warm air Figure 2.35 Cyclonic/frontal rainfall Measuring and Recording Rainfall Rainfall is measured using an instrument called rain gauge. A rain gauge consists of a wide-mouthed funnel placed over a cylindrical container. Rain water passes through the funnel into the container below. The water in the container is poured into a measuring cylinder, and then the amount of rainfall is measured in millimeters and is recorded. Funel Jar Metal container sunk in the ground Figure 2.36 A rain gauge 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 77 Activity 2.13 Table 2.6: Rainfall data for Debre Markos Months J F M A M J J A S O N D Rainfall (mm) 18.3 12.1 57.5 55.1 173.1 113 256.5 293.8 210.8 12 91 9.4 Using the preceding rainfall data for the Debre Markos station, perform the following tasks. 1 Calculate the total annual rainfall 2 Identify the season of heaviest rainfall. 3 Draw a line graph to illustrate each monthly total rainfall. Air Pressure What is air pressure? Explain how to record and measure pressure? The air around us has weight. This weight exerts pressure on the surface of the earth. We call this atmospheric pressure. Atmospheric pressure is not the same all over the earth, and it is not the same even in one place all the time. Pressure is measured by a mercury barometer. Normal pressure, at sea level, is about 760 mm/1013 mb. Vacuum Mercury column Glass tube Height 91 cm Air pressure Air pressure Figure 2.37 Mercury Barometer Mercury in dish The distribution of pressure over the earth’s surface depends on (1) the altitude of places above sea level and, (2) most importantly, on temperature. Pressure decreases with an increase in altitude. This explains why air pressure is highest at sea level and decreases with increasing altitude. 2.2 Weather And Climate  Grade 9 Geography 78 High temperature makes air expand, so that it has a lower density and pressure. Low temperature makes the air to contract, resulting in a higher density and creating an area of high pressure. 90oN Polar high 60 o N o N 60 Subpolar low 30 oN o N 30 Subtropical high 0o 0o Equatorial Low (Doldrum) Subtropical high o S 30 oS 30 Subpolar low o S 60 o 60 S Figure 2.38 Air pressure belts 90oS Polar high The distribution of air pressure over the globe is known as the horizontal distribution of pressure. Pressure distribution can be shown on a map. Lines connecting all places that have the same pressure are called isobars. Focus Global Pressure Belts: 1 Equatorial low pressure belt (Doldrums): FF Located from 5o North to 5o South. FF There is tremendous heat, and thus warm air, creating low pressure. Also, the centrifugal force is very high at the equator, because the earth’s velocity of rotation is high. Hence, the air masses tend to be thrown outwards, resulting in low pressure. FF Wind speed is low, that is why this pressure belt is called the doldrums (Belt of Calm). 2 Tropical high pressure belt (Horse Latitude): FF Located from 30o to 35o North and South. FF Except for two months, temperature is usually high. FF Here the pressure is high, because pressure depends on the rotation and movement of air (as winds from the Doldrums rise up and accumulate here. Also winds from the sub-polar low pressure belt accumulate here). 3 Sub-polar low pressure belt: FF Located from 60o to 65o, North and South. 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 79 FF Here the low pressure is created because of intense high pressure at the poles. 4 Polar Highs FF Located near the north and south poles. FF The polar zones have permanent centers of high pressure known as polar highs. The pressure distribution over the earth’s surface is not a continuous belt. Except in the higher latitude of the southern hemisphere, they form belts due to the small land areas which do not affect the free flow of the atmosphere. However the position of the pressure belts and cells does not remain fixed in one position. They move north or south with the apparent movement of the sun. During the northern hemisphere’s summer, the sun is overhead north of the equator. The pressure belts then shift northward by a few degrees from their average position. During the southern hemisphere’s summer, the sun is overhead south of the equator. As a result, the pressure belts move southward by a few degrees from their average position. Activity 2.14 In your geography work group, answer the following questions. 1 What is atmospheric pressure? 2 Why does pressure decrease as there is an increase in altitude? 3 What effect does temperature have on atmospheric pressure? Wind What is wind? Wind is air in horizontal motion. Winds have speed and direction. Wind force (speed) and wind direction are affected by FF Pressure gradient FF Frictional force FF Coriolis force On weather maps, pressure is indicated by drawing isolines of pressure, called isobars. The difference in distance between Isobars is called the pressure gradient. If the isobars are closely spaced, we can expect the pressure gradient force to be 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 81 anticyclones. In the Northern Hemisphere, the Coriolis force causes air in low- pressure areas to spiral counter clockwise and inward, forming a cyclone, whereas air in high-pressure areas spirals clockwise and outward, forming an anticyclone. In the Southern Hemisphere, cyclones turn clockwise and anticyclones, counter clockwise. Anti clock wise Clock wise Anti clock wise Clock wise Figure 2.40 Cyclones and anticyclones Focus Cyclones are atmospheric disturbances which involve a closed circulation about a low-pressure center. They move inward, anticlockwise, in the Northern Hemisphere and clockwise in the Southern Hemisphere. Cyclones are commonly known as lows or low pressure areas. Anti-cyclones are vast areas of high pressure which have a diverging system of surface winds. The winds in anti-cyclones blow outward in the anti-clockwise direction in the Southern Hemisphere and in the clockwise direction in the Northern Hemisphere. Anticyclones are commonly called highs or high pressure areas. Types of Winds There are three types of surface winds. They are: FF planetary FF monsoon FF local winds. Planetary winds and their relationship with pressure belts: Planetary winds blow over large areas of the earth’s surface. They are closely associated with 2.2 Weather And Climate  Grade 9 Geography 82 the world pressure belts. Winds blow from areas of high pressure to areas of low pressure. The most common planetary winds are trade winds, westerlies and polar easterlies. They have wide influence over the earth’s surface (see Figure 2.41). Polar high Polar easterlies 60o Polar front Westerlies Horse latitudes 30o NE trade winds Doldrums 0o SE trade winds 30o Westerlies Horse latitudes 60o Polar front Polar easterlies Polar high Figure 2.41 Pattern of global wind belts Monsoon winds: They are seasonal winds whose movements are controlled by pressure that differs during different seasons. Monsoon winds are very common in South and South East Asia. Seasonal changes in the direction of these winds are caused by the unequal heating of land and water surfaces. The direction of monsoon winds changes between summer and winter. Figure 2.42 Monsoon winds 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 83 Local winds: They affect only limited areas and blow for a short period of time. They affect climate conditions on a small scale. Local winds are caused by the nature of the physical features of the area. The main local winds are: ÂÂ Land and sea breezes ÂÂ Mountain and valley breezes 1 Land and sea breezes: These winds are common along coastal areas. These winds change their directions daily and affect very small areas. During the daytime, temperature on the land is higher than on the water/sea. So low pressure is formed on the land, while it is relatively high on the sea. Wind blows from the sea towards the land. This is known as a sea breeze. At night the land is colder than the sea. So low pressure develops over the sea. The wind blows from land towards the sea. This is known as a land breeze. Figure 2.43 Daytime and nighttime development of sea breeze and land breezes 2 Mountain and valley breezes: Mountain and valley breezes arise from contrasts in temperature between a valley floor and mountain slopes. During the day the air at the bottom of the valley becomes warmer. As a 2.2 Weather And Climate  Grade 9 Geography 84 result it expands and rises along the mountain slopes. This is known as a valley breeze. Figure 2.44 Daytime and nighttime development of valley breezes and mountain breezes At night the wind over the slope of the mountain becomes cool. Then this cooler and heavier mountain air slides down slopes towards the valley. This is called a mountain breeze. Clouds What are clouds? How are clouds formed? What are the major types of clouds? A cloud is a dense concentration of very fine invisible water droplets, sleet or ice crystals. Clouds are formed by the condensation of water vapour below the dew point in the atmosphere. There are varieties of clouds, based on their height, appearance and shape. 2.2 Weather And Climate  Unit 2: Physical Environment of the world and Ethiopia 85 Table 2.7: Types of clouds Group Cloud type Description Cirrostratus Thin, wispy, appears in sheets. High clouds Thin, wispy, filamentous or curly, mostly Cirrus > 6000 m composed of ice crystals. Small, puffy, patchy and/or with a wave-like Cirrocumulus appearance. Medium-sized, puffy, patchy, scattered clouds Middle clouds Alto cumulus – often in linear bands. 2000 – 6000 m Alto stratus Thin and uniform. Stratocumulus Broad and flat on the bottom, puffy on the top. Uniform, flat thick to thin layered clouds. Low clouds Stratus Mostly composed of liquid droplets. < 2000 m Uniform, dark, flat, low clouds that produce Nimbostratus precipitation. Mostly composed of liquid droplets. Cumulus Puffy and piled up. Vertical clouds < 500 and > 18000 m Cumulonimbus Can cause lightning, hail, strong rains, strong winds and tornados. Source: Focus on Earth science Cirrostratus 6000 m Cumulonimbus Altostratus 4000 m Altocumulus Cumulus Nimbostratus Stratocumulus 2000 m Stratus Figure 2.45 Diagram of cloud types 2.2 Weather And Climate  Grade 9 Geography 86 Activity 2.15 In your group, perform the following tasks and answer the following questions. 1 What are the major controlling factors responsible for the direction and speed of winds? 2 What is the difference between cyclones and anticyclones? 3 What are the effects of sea breezes and land breezes in a given locality? 4 Compare and contrast the characteristics and formation of monsoon winds and local winds. 5 Discuss the basic characteristics of planetary winds. Controls of Weather and Climate What are the major controls of weather and climate? The energy that the earth receives from the sun is not distributed evenly. Many factors affect the distribution. These include latitude, altitude, distance from the sea, cloud cover, ocean current, planetary winds and pressure. Latitude What is latitude? What is the effect of latitude on temperature? On a global scale, latitude is the most important factor determining the strength of heat reaching the earth’s surface. When the sun’s rays are vertical (at a right angle) to the surface, the amount of heat received is the greatest. But when the sun’s rays are slanting (oblique) the heat’s strength decreases. Figure 2.46 Angle of the sun At the equator, the overhead sun is high and of high intensity insolation is received. At the poles, the overhead sun is low, so the amount of insolation is low. The sun is overhead at noon for six months between the equator and the Tropic of Cancer, and it is overhead for another six months between the equator and the Tropic of Capricorn. The sun is directly overhead at the Tropic of Cancer on June 21 (the June solstice). The sun is directly overhead at the Tropic of Capricorn on December 21 (the December solstice). At March and September equinoxes, the sun is directly overhead at the equator. At times between solstices and equinoxes, the sun is 2.2 Weather And Climate