Climatology Notes - Booklet 2 PDF

Page 1 of 27 2. Physical Geography Climatology Pressure cells and global winds This page provides an overview of pressure cells and global winds. It is important to note where South Africa is located with regards to the pressure and wind systems. Study the diagram on th...

Page 1 of 27 2. Physical Geography Climatology Pressure cells and global winds This page provides an overview of pressure cells and global winds. It is important to note where South Africa is located with regards to the pressure and wind systems. Study the diagram on the right. Seasonal movement of pressure cells and wind systems The positions of the pressure cells and wind systems migrate with the heat equator. The heat equator (ITCZ) is the zone of hottest temperatures.  In December the sun is overhead the Tropic of Capricorn. It is summer in the Southern hemisphere. The heat equator, inter-tropical convergence zone (ITCZ) pressure cells, etc. lie south of the real equator.  In June the sun is overhead the Tropic of Cancer. It is summer in the Northern hemisphere. The heat equator, ITCZ, pressure cells, etc. lie north of the real equator. This movement influences where tropical cyclones and frontal depressions will develop. It is important to understand how this seasonal movement influences South Africa. Favourable conditions for the Favourable conditions for frontal development of tropical cyclones in the depressions influencing South Africa Southern hemisphere  Page 2 of 27 Tropical Cyclones Location  Tropical cyclones occur over tropical oceans.  They develop about 5° North and South of the equator, and travel pole-wards to approximately 25°-30° North and South, where they dissipate. Occurrence  As the tropical cyclones travel westwards, they affect the east coasts of continents.  They seldom travel far inland, but curve away until they reach cooler latitudes. Names Tropical cyclones are known by different names in different parts of the world:  Hurricanes – North America, Caribbean Sea (North Atlantic Ocean)  Willy Willies – Eastern Australia (South Pacific Ocean)  Typhoons – China, Japan, Philippines (North Pacific Ocean)  Tropical cyclones – East coast of southern Africa, Madagascar (Indian Ocean) Tropical cyclones are named alphabetically, with the first of the season beginning with A. Once storms develop sustained wind speeds of more than 33 knots (61 km/h; 38 mph), names are generally assigned to them from predetermined lists. Season  Tropical cyclones occur from late summer to early autumn (in the Northern hemisphere from July to October, in the Southern hemisphere from January to April).  Page 3 of 27 Movement of the entire system  Tropical cyclones move east to west as they are carried by the tropical easterlies.  They recurve eastwards, moving further from the equator as they are picked up by the westerly winds. Circulation of air within the cyclone  These cyclones are characterised by fierce winds, which rotate very fast around the central low pressure.  The air rises in a clockwise direction in the Southern hemisphere.  The air rises in an anti-clockwise direction in the Northern hemisphere. Remember that according to Ferrel’s law, the Earth’s rotation causes Coriolis force, which deflects winds to their left in the Southern hemisphere and to their right in the Northern hemisphere. Average figures The list below gives average statistics for tropical cyclones:  Speed and movement - 25km/h  Wind speed - 200-300 km/h  Pressure in eye - 940 hPa  Vertical extent of cloud - 15 km  Diameter of eye - 30 – 50 km  Diameter of storm - 500 km Categorising Tropical Cyclones In order to categorise tropical cyclones around the world, the Saffir-Simpson Hurricane Wind Scale is used, defining events by their wind speed and impacts. The Saffir-Simpson Hurricane Wind Scale consists of a five point scale of hurricane intensity and starts at 74 mph. Tropical cyclones with wind speeds up to 38mph are classified as tropical depressions and those with wind speeds from 39 - 73 mph are classified as tropical storms.  Page 4 of 27 Monitoring and forecasting Tropical Cyclones The most important method of monitoring and tracking Tropical Cyclones is by satellite imagery. An array of geostationary satellites (those that remain over a fixed position on Earth) is operated by a number of countries. Each of these satellites provides continuous displays of Earth’s surface in visible light and in infrared wavelengths. It is the latter that are most important in tracking the stages of tropical cyclone development. Infrared images show the temperatures of cloud tops.  Page 5 of 27 Requirements for formation Tropical cyclones do not occur by chance; specific conditions must be present to enable them to develop. Requirement Reason Surface sea temperatures  Much evaporation occurs when the water is warm. of 27 °C or more  Warm temperatures provide thermal energy.  Convection currents occur when the warm air rises.  Low pressure develops as the air rises. High humidity  After condensation, latent heat is released which provides thermal energy. Unstable air  The air must rise so that the pressure drops.  The rising air cools, condensation occurs, and latent heat is released. Very low pressure and  Rapid convergence assists in causing the air to steep pressure gradient rise.  The steep gradient strengthens Coriolis force and causes the air to rotate. Coriolis force  Coriolis force causes rotating winds, therefore the pressure continues to drop. Divergence in upper air  This removes the air at higher altitudes and maintains the low pressure on the surface. No friction  Winds can reach great speeds, therefore Coriolis force remains strong, so that rotation continues. These requirements can be fulfilled over tropical oceans, polewards of 5° N or S. If these conditions cease to exist after formation, the tropical cyclone will dissipate. Formation The meteorologist has likened a tropical cyclone to an engine. Energy source  Provided by latent heat from condensation and from adiabatic heating in the eye. Starting mechanism  Provided by upper air divergence and falling pressure. Mechanism to maintain the  Convergence and vorticity. circulation  Divergence in the upper air.  Subsidence in the eye.  Pressure dropping. Transmission to generate  Instability of the air. motion Cooling system  Reaches cooler latitudes.  Crosses land – no latent heat. Vortex / vorticity: circulation of air around a core of low pressure.  Page 6 of 27 Stages of development 1. Formative (tropical depression) Southern hemisphere  An easterly wave in equatorial latitudes deepens to form a separate low pressure cell.  The pressure drops, but it is still more than 1 000 hPa.  Temperatures and humidity are high.  The air rises.  There is convergence and the vortex develops.  There are cirrus and cumulus clouds, and light rain occurs. 2. Immature (tropical storm) Southern hemisphere  The pressure has dropped to less than 1 000 hPa.  The storm is still small in size.  Converging air continues to rise.  Divergence takes place in the upper air.  There are spiral bands of clouds and light rain occurs.  Hurricane-speed winds occur within 50 km of the eye.  Gales occur within 500 km of the eye. 3. Mature (tropical cyclone) Southern Hemisphere  The pressure stops dropping – pressure in the eye is ± 940 mb.  The size of the storm has increased.  Cumulonimbus clouds and heavy rain with thunder and lightning occur.  Divergence in the upper air increases.  The air subsides in the eye, warming adiabatically as it subsides.  Gales occur on the outer edges of the storm.  Hurricanes are found within 160 km of the eye.  The worst weather occurs within the ‘dangerous semi-circle’, as here the direction of winds within the cyclone is the same as the direction of forward movement (the forward left-hand quadrant in the Southern hemisphere).  Page 7 of 27 4. Dissipation  The pressure rises when the cyclone reaches land, as friction weakens Coriolis force and air converges on the low pressure.  When the tropical cyclone moves over land, there is less moisture and therefore less latent heat.  The tropical cyclone reaches cooler latitudes and cold air enters the system. EYE  Page 8 of 27 Weather associated with a tropical cyclone The diagram below shows a cross-section of a mature tropical cyclone. As the cyclone As the eye passes As the cyclone moves approaches (first vortex): overhead: away (second vortex)  The temperature and  The temperature will  The pressure will start humidity are high. increase slightly and to rise and winds will  The pressure will start the wind will suddenly reach hurricane force. to fall and conditions drop.  Conditions will be will be relatively calm.  The pressure will similar to those  Cirrus clouds will reach its lowest point. experienced in the appear on the horizon,  There will be first vortex, although and these will thicken scattered cloud, rear winds are often into cumulus and sunny conditions and more destructive. cumulonimbus as the no rain.  There is a reversal of storm approaches.  These conditions will wind direction as the  The wind strengthens, prevail for up to two second vortex passes. and will increase to hours. hurricane force.  Torrential rain, hail, thunder and lightning will occur.  The temperature will drop as the pressure continues to fall.  Page 9 of 27 The diagram below shows wind reversal: Draw a fully annotated cross-section through a mature Tropical Cyclone  Page 10 of 27 Synoptic Chart: Summer conditions Environmental damage Damage is caused by:  Hurricane force winds  Torrential rain  Storm surges - is an abnormal rise of water generated by a storm, over and above the predicted astronomical tides. The effect of tropical cyclones on developed areas (MEDC) will be different form that on developing areas (LEDC). While developed counties will suffer more financial losses, developing countries will suffer more basic problems.  Flooding occurs due to torrential rain and inundation by sea water.  Structural damage can be caused by gale-force winds.  Agricultural land and crops can be ruined.  Communication links and the infrastructure can be destroyed.  There is drowning, death, disease and starvation – especially in the developing regions. How Nature Responds to Storms Most natural coasts are protected against storms more effectively than the best fortifications people could build. Beaches, rocky headlands and mangrove swamps all play a part in reducing the energy of the waves and thus reducing the destructive ability of them. As storm waves hurl sand about or crash against rocky cliffs, they use up energy. The more energy that is absorbed by the beach or the cliffs, the less there is to damage the land.  Page 11 of 27 Mangrove swamps extending hundreds of meters inland, with their forests thick sturdy trunks, stilt-like roots and dark rubbery leaves that do not tear easily, act as buffers against the wind and the water. Behind the mangroves are marshes which help to soak up excess water. Further inland, dense forests act as resistance (friction) to the wind and thus slow it down. Their canopies also reduce the impact of hard raindrops on the soil, thus reducing soil erosion and aiding infiltration. Nature uses few brick walls and almost no smooth surfaces. Nothing is placed in even rows, so that the wind cannot gain speed in a straight path. Instead, nature uses rough, tough and flexible materials that can readily be replaced. The Benefits of Storms 1. World fishing: plankton get their food as chemicals (nutrients) from the muds of the sea bed. There the remains of dead sea creatures decay and release the vital chemicals on which new life depends. Normal winds do not stir the sea enough to whisk the nutrients towards the surface where most plankton occur, but severe storms create such large waves that the whole depth of the sea is disturbed and the nutrients are stirred up to the surface where they provide food for plankton (which in turn provide food for fish and so support the marine food chain). 2. Forest fires: storms can quell forest fires that fire-fighters cannot control. 3. Balancing temperatures: storms carry heat from the tropics to the poles helping to even out world temperatures. 4. Relieving drought: storms can relieve drought-stricken areas by pushing high pressure systems without moisture away. 5. Helping natural selection: storms push over diseased and old trees making way for young and healthy ones. Problems associated with Tropical Cyclones  They are erratic and may change course at any time and without warning.  Not enough is known about them to build a reliable model which could be used for prediction. Control and precautions  Tropical storms and depressions are monitored by weather bureaus.  Their development is observed via weather reports, reconnaissance aircraft and satellite photographs.  Timely warnings are made to inhabitants to vacate low ground, board up windows etc.  Sandbags can be placed in coastal areas to reduce flooding by the sea.  Disaster management schemes must be operation.  Leave the natural environment intact which forms a natural protection barrier against such storms.  Sea walls and coastal fortifications can be constructed or artificial sand dunes can be created to protect property from the storm surges and swells.  Houses and bridges next to rivers can be built above the flood line.  Page 12 of 27 Test yourself Questions 1. Refer to the newspaper article and the synoptic chart below and answer the questions that follow. The mother of all storms MAPUTO – Cyclone Eline reached a terrified Mozambique late yesterday, ripped roofs of houses, cut water and power supplies in central provinces and drenched an already flood-stricken nation with torrential rain that sent swollen rivers racing through villages. A local radio station reported that sparsely populated fishing villages near Inhambane, a coastal town near where the storm surged ashore, had been completely washed away. There is nothing there. The houses have been destroyed,’ a radio reporter, who had driven to the villages, said by telephone. But the devastating news is that Eline has spawned a daughter whom the Pretoria Weather bureau spokesman is keeping a close eye on. Meanwhile, late last night Cyclone Eline was headed westwards across Mozambique towards Zimbabwe. With some heavy rains forecast for some northern parts of South Africa it is feared that the flood plains on the Limpopo River will be further swamped. (Adapted from the Pretoria News, 23 February 2000). 1.1 a) What type of cyclone is Eline? b) Give evidence from the newspaper article to support your answer in question 1.1 a). c) Give a possible reason why this storm has been referred to as ‘the mother of all storms’. d) Excluding Mozambique, which other country neighbouring South Africa has been affected by Cyclone Eline? e) Name TWO of South Africa’s provinces that would most likely have been affected by Cyclone Eline.  Page 13 of 27 1.2 Give map evidence to suggest that Cyclone Eline is still in her mature stage of development. 1.3 Most devastation caused by cyclone Eline was a result of the ‘dangerous semicircle’ (dangerous quadrant), which reached land first. a) Draw a simplified, labelled diagram of cyclone Eline to show the position of the ‘dangerous semicircle’. b) Explain the existence of the ‘dangerous semicircle’. c) List THREE direct effects caused by Cyclone Eline. d) Explain why a coastal town such as Inhambane would experience more devastation than towns situated further inland. 1.4 a) 'Cyclone Eline has spawned a daughter...’. Explain this phrase. b) Give a possible name for Eline’s daughter. c) Explain your answer to Question 1.4 (b). 2. Discuss and compare the effect a tropical cyclone would have on a developing country with the effect that it would have on a developed country. 3. Refer to the figure on the right illustrating a tropical cyclone in the Gulf of Mexico, and answer the questions that follow. 3.1 What is the direction of movement of tropical cyclones in the Southern hemisphere? 3.2 Suggest a possible date (day and month) on which the eye of the cyclone can be expected to pass places C and D respectively. 3.3 What will the direction of the wind be at position E? 3.4 Why do tropical cyclones not originate in the immediate vicinity of the equator? 3.5 Will the effect of the tropical cyclone be restricted to the coastal areas of Florida, or will the effect be experienced in the interior of the USA? Explain your answer. 3.6 Name two areas in the Southern Hemisphere that experience tropical cyclones.  Page 14 of 27 Answers 1.1 a) Tropical cyclone. b) It has a name and is traveling in a westerly direction. c) The villages have been totally destroyed. d) Zimbabwe. e) Mpumalanga, Limpopo Province, KwaZulu-Natal. 1.2 Low pressure of 988 hPa in the eye, steep pressure gradient as indicated by the isobars, which are close together. 1.3 a) b) In the dangerous semi-circle, the direction of the wind within the cyclone coincides with the direction in which the cyclone is moving, and the wind speed is therefore the sum of the speeds of the two movements. c) Houses were destroyed as their roofs were ripped off, water and power supplies were cut, and houses were washed away by the storm surge. d) Coastal areas experience storm surges and high waves, and the storm becomes weaker as it moves inland - the winds are weakened due to friction with the land and rainfall decreases. 1.4 (a) Another cyclone has developed. b) Any name that begins with F. c) Cyclones are named alphabetically, with the name of the first cyclone of the season beginning with A. 2. A developing country would suffer a greater loss of life because:  there is often a dense population, especially on the flood plains;  starvation will occur as there are many subsistence farmers on the flood plains;  these countries have no adequate warning systems, precautions or disaster management schemes;  building structures are poor, and many people are left homeless;  health services are inadequate.  Page 15 of 27 A developed country would suffer economic losses because:  financial losses are caused by the disruption of economic activity;  there is much damage to harbours and ships;  the communication links and infrastructure can be damaged. 3.1 East to west. 3.2 C - 15 August D - 19 August (You may select other dates, but these must be from August to October.) 3.3 South-east. 3.4 There is no Coriolis force, and therefore there are no rotating winds. Air will converge on the low pressure areas, and pressure will rise. 3.5 The effect will be restricted to the coastal areas. The hurricane will dissipate as it moves inland. Friction will cause the wind to slow down, and Coriolis force will weaken. As it moves inland, it will be cut off from its source of moisture, the sea, and less condensation and latent heat will be released. 3.6 Madagascar, Mauritius.  Page 16 of 27 Frontal Depressions Location Frontal depressions occur in temperate latitude between 40-60° North and South. Occurrence As the temperate depressions travel eastwards, they approach the continents in the temperate latitudes from the west, bringing winter rain to these parts. This is referred to as a Mediterranean-type climate. Alternative names Many different terms are used for these low pressure cells. They are also called temperate depressions, temperate cyclones, extra-tropical cyclones or mid-latitude cyclones. Season They form all year, but are better developed in winter and spring (Northern hemisphere from November to March, Southern hemisphere from April to October). Movement of the entire system Frontal depressions move from west to east as they are carried by the westerlies. Initially they move towards the equator, but curve back towards the poles. This is due to:  the faster-moving, more forceful cold front;  their deflection by the sub-tropical high pressures.  Page 17 of 27 Circulation of air within the frontal depression Rotation around the low pressure centre is clockwise in the Southern hemisphere and anti-clockwise in the Northern hemisphere. Average figures and characteristics The list below gives average data for frontal depressions. Speed of movement 30 – 50 km/h Life span 4 – 14 days Pressure at centre ± 1 000 hPa Pressure Extensive low pressure centre Diameter of whole system Up to 3 000 km Isobars Oval shaped Pressure gradient Not normally steep Dependent on the thermal equator (40 - 60° Position North and South) The cold front of an older depression is joined to Occur in families the warm front of a younger depression.  Page 18 of 27 Stages of development In the same way as tropical cyclones, frontal depressions also have a life cycle. The diagrams below show these conditions in the Southern hemisphere. 1. Initial stage This is divided into two stages:  Polar front stage: Front exists between two unlike air masses moving in opposite directions.  Wave stage: Friction develops at the polar front. Rotary air movement begins. 2. Maturity / warm sector stage  Well-formed warm and cold fronts have developed.  The warm front is the leading edge of the warm air. The warm air at the front rises gradually over the cooler air ahead of it.  The cold front is the leading edge of the cold air. The cold air wedges in under the warmer air ahead, forcing the warm air to rise sharply.  The westerlies have become north- westerly, causing the warm air to move polewards.  The polar easterlies have become south-westerly, causing the cold air to move towards the equator. 3. Occlusion stage  The cold front has caught up with the warm front.  The cold front moves more quickly because: - it displaces the warm air ahead of it easily; - the cold air only moves horizontally, and does not have to use energy to rise. 4. Dissipation stage  All the warm air has been displaced.  Only cold air is on the surface.  All pressure differences are reduced.  Page 19 of 27 Location of the initial stage of a mid-latitude cyclone Plan view (from above) of a mature mid-latitude cyclone in the Southern Hemisphere Satellite image of a mature mid-latitude cyclone in the Northern Hemisphere  Page 20 of 27 Weather associated with a frontal depression The diagram below shows a cross-section of a mature frontal depression. As the warm front As the cold front approaches In the warm sector: approaches and passes and passes over: over: The cold front has a steep  The temperature The warm front has a gradient: increases to its gradual gradient: maximum.  The temperature falls  The pressure reaches its  The temperature ahead rapidly as the cold air lowest. of the front is low, but it approaches.  There is unsettled rises gradually.  The pressure starts to weather,  The pressure drops more rise quickly.  Scattered cloud, quickly at the front.  The warm air rises decreasing  The clouds are cirrus sharply, and rain and increasing thickening to cirrostratus, cumulonimbus clouds sunshine. altostratus and form.  The wind drops and nimbostratus.  Heavy showers occur changes direction.  Gentle, steady rain falls over a small area. It becomes northerly or over a large area.  There is a decrease in north-westerly in the  The wind direction humidity. Southern hemisphere. is approximately  The wind ‘backs’, and north-easterly in the becomes south-westerly Southern hemisphere. in the Southern hemisphere.  Later, cloud begins to clear but it is still cold. Except for the direction of the wind, the weather conditions are the same in both the Northern and Southern hemispheres.  Page 21 of 27 Reasons for weather changes at the cold front (in the Southern hemisphere) A common question in the examinations asks you to describe and explain the weather changes at Cape Town as a cold front passes. Changes (Describe) Reasons (Explain) The cold polar air behind the front has Temperatures drop. arrived. Wind ‘backs’ – becomes south- Air rotates around the low pressure in a westerly. clockwise direction. The warm air rises over the approaching Cloud cover increases. cold air; it cools and condenses. The cold air forces the warm air to rise Clouds become cumulonimbus. sharply, therefore condensation occurs to great heights. Pressure is at its lowest just ahead of the Pressure drops, then rises. front, then it rises as the front passes and the cold air arrives. Heavy rainfall occurs over a small Storms associated with cumulonimbus cloud area. occur. Steep gradient of the cold front. Note: the wind veers in the Northern Hemisphere Draw an annotated cross-section through a mature mid-latitude cyclone.  Page 22 of 27 Cross-sections for occlusions Cold front occlusion Warm front occlusion Air behind the cold front is colder and Air behind the cold front is warmer and therefore more dense than the air ahead of therefore lighter than the air ahead of the the warm front. warm front. The cold front remains in contact with the The warm front remains in contact with ground, and lifts the warm front. the ground, and the cold front rises.  Page 23 of 27 Influence on South African weather It is important to interpret specific conditions on synoptic charts. The diagrams below show various synoptic conditions. Deflection by the South Atlantic and South Indian high pressure systems These high pressures can prevent the depressions from reaching South Africa, and can cause them to move in a SE direction away from the land. Cold snap on the plateau  The South Atlantic high pressure ridges in behind the cold front, thus reinforcing the SW air behind the cold front and pushing it across the land.  Very cold air can be introduced from very far south. South Africa only experiences warm sector and cold front conditions, because the more powerful cold front causes the depression to swing in a SE direction. The warm front is too far south. Snow on the Cape fold mountains  This can occur if the air behind the cold front is very cold, and cools further on rising up the mountains.  Orographic rain occurs when the air of the warm sector is caused to rise up the mountains. Berg winds  These are hot, dry winds occurring when there is a steep pressure gradient between the Kalahari high pressure and the depression (LP), and the air heats adiabatically as it moves down the escarpment to converge on the low pressure. Adiabatically: change in temperature due to a change in pressure.  Page 24 of 27 The South African Berg Wind This local wind is linked to the secondary circulation and the pressure gradient between land and the sea. The berg wind is a small-scale tertiary circulation. The characteristics of the wind are also caused by the topography of South Africa (escarpment). Berg wind Time that it occurs Day or night – chiefly in winter Characteristics Hot and dry  A high pressure over the land (the Kalahari HP) causes subsiding air that heats adiabatically.  The air on the plateau is therefore hot and dry.  This air moves down the escarpment towards a low pressure over the sea (a coastal low or a Conditions under frontal depression). which it forms  As the air descends the escarpment, the air undergoes further adiabatic heating at the Dry Adiabatic Lapse Rate of 1 °C/100 m, and becomes hot and dry.  The berg wind is the air descending the escarpment. Direction of air Downslope – interior to coast movement Influence This dry, hot wind can cause fires – especially as the vegetation is dry in winter. South African coasts Most common on the west coast, but also occurs on the south and east coasts. Diagrams  Page 25 of 27 Label features A, B and C and describe the conditions at Durban and Port Elizabeth.  Page 26 of 27 Satellite image of Southern Africa 06/06/2020 (a) Describe the climatic conditions most likely to be responsible  Page 27 of 27 Comparison between tropical cyclones and frontal depressions Tropical cyclones Frontal depressions Temperature cyclone, mid- Hurricane, typhoon, willy- Names latitude cyclone, extra- willies tropical cyclone Over tropical oceans: At polar front: Origin 5-30° N and S 40-60° N and S Pressure Very low, below 970 mb Low, usually 990 – 1 000 mb Pressure gradient Very steep Gentle East to west – carried by West to east – carried by Movement tropical easterlies. westerlies Very fast – hurricane force Wind speed Less than gale force 118 km/hr 1 000 – 3 000 km in Size 300-500 km in diameter diameter Duration Up to 14 days 4-14 days Occurrence Mid to late summer Winter (in South Africa) First vortex, eye, second Warm front, warm sector, System vortex cold front Warm-to-hot, violent winds, Cool, cloudy, rainy Weather intense thunderstorm activity conditions From latent heat being From the cool air pushing up Energy of System released by condensation of the warm air along the fronts unstable warm, moist air Isobar shape Circular Oval Eastern side of continents in Western side of countries in Areas affected tropical latitudes temperate latitudes