Predicting the Weather

Transcription

1 identify that the distance between isobars on a weather map indicates the relative change of atmospheric pressure in an area explain why satellite photographs of cloud patterns have improved the reliability of interpretations of weather regularities and knowledge of global weather patterns Predicting the Weather When Australia was settled in 1788, the colony nearly starved due to a lack of food caused by drought. Soon the variable nature of our weather and climate would be immortalised in poetry. Dorothea Mackellar wrote: I love a sunburnt country, A land of sweeping plains, Of ragged mountain ranges, Of droughts and flooding rains. We have long known why the weather changes on a day-to-day basis. In recent years we have begun to understand why we have these alternating periods of drought and flood. Day-to-day Weather One of the most basic measurements made by the Weather Bureau is air pressure. On weather maps, lines joining places that have the same air pressure are called isobars. The pressure is marked on the isobars. You will notice that some areas have relatively high pressure. The air pressure decreases as you move away from such regions. Such regions are called highs or high pressure areas. Other areas on the map have relatively low pressure. The air pressure increases as you move away from such regions. Such regions are called lows or low pressure areas. Cold fronts are the other important feature of Australian weather maps. These are masses of cold air that normally undercut hot air, forcing it higher into the atmosphere. The air associated with a high-pressure system sinks down from above. As it sinks, it becomes warmer and is very stable. When compared with low-pressure systems, highs tend to cover a greater area, move more slowly and have a longer life. When the high-pressure system is located over land the weather will often be dry and free of cloud. A low-pressure system develops where relatively warm air rises from the Earth's surface. If the rising air contains moisture, clouds will begin to form with associated rain showers as the air cools. Winds arise as air moves between the sinking air of highs and the rising air of lows. Notice the air spirals clockwise out from a high-pressure area and spirals anticlockwise into a low-pressure area. When the isobars are far apart, it indicates a gradual change in pressure. When the isobars are close together, it shows the air pressure is rapidly changing. The closer the isobars, the stronger the wind. A cold front is the boundary between cold air from polar regions moving towards the equator. As the cold front moves north, it undercuts warmer air moving towards the poles. If the temperature difference across a cold front is large, strong winds can result. If the warm air also contains a lot of moisture, it becomes unstable with the development of large clouds as it is forced to rise. As a result, severe weather Weather maps Highs are due to sinking air; lows are due to rising air. Cold fronts are masses of cold air moving over an area (Bureau of Meteorology)

2 such as thunderstorms and severe turbulence may accompany these cold fronts. Long-term Weather Australia is a continent surrounded by oceans. Long term weather changes in Australia are due to changes in the temperatures of these oceans, particularly the Pacific Ocean. In most years a strong current brings relatively cold water northward along the west coast of South America. Under the influence of trade winds, the cold water then flows westward along the equator and is heated by the tropical sun. These normal conditions make the western Pacific near Indonesia and Australia about 3 C to 8 C warmer than the eastern Pacific. Along with the warm water, the trade winds bring warm moist air towards the Indonesian region. Here, moving over normally very warm seas, moist air rises to high levels of the atmosphere. The air then travels eastward before sinking over the eastern Pacific Ocean. The rising air is associated with a region of low air pressure, and the towering clouds bring tropical rain. If this process reaches an extreme, it can bring floods to parts of Australia and is called a La Nina. The El Nino is the opposite extreme. The trade winds reverse and carry the warm water away from Indonesia and Australia towards the Americas. Instead of lows with rising moist air, we have highs with sinking dry air over cooler oceans. This produces little rain, and drought and subsequent forest fires occur throughout Australia and Indonesia. The warm water accumulates in the middle of the Pacific Ocean. Highs and lows Air can move from highs to lows in a spiral. In the southern hemisphere, the air moved anticlockwise around highs and clockwise around lows (Bureau of Meteorology). Cold fronts Cold air forces its way under warm air. El Nino (a) During a La Nina, warm moist air rises over Indonesia bringing rain. (b) During an El Nino warm moist air rises over the Americas bringing rain

3 To see what happens to weather in New South Wales, we can compare normal summer weather maps with a summer weather map in an El Nino year. In a normal summer, the highs move in a relatively low path across the continent bringing moist onshore winds. When the moist winds reach the coastal ranges, rain is produced along the coasts. As well, the highs are not very strong and cold fronts reach the coastal areas bringing cooler air and rain. In an El Nino year, the highs are very much larger in size and move in a path that is further north. As a result, there is little wind when the high covers the state or the winds are hot and dry from the northern inland. As well, the highs are so large and strong that cold fronts that might bring cool weather and rain are forced to the south. Surface temperatures (a) During a La Nina, warm water moves towards Indonesia. (b) During an El Nino, warm water moves towards the Americas

4 Summer weather map (a) Normal years are moist in a New South Wales summer. (b) In an El Nino year the summers are dry. Winter weather map (a) Normal years are dry in a New South Wales winter. (b) In an El Nino year the winters are also dry.. What happens in winter months? In a normal winter the highs are large and slow moving across the middle of the continent.this means that New South Wales is generally dry and sunny. However, with the highs further north it allows cold fronts to regularly move across the state. Since northern Australia is not as hot, winds from that region are not as hot. In an El Nino year this general pattern persists, except the slow moving highs often cover the whole continent so that the dry stable conditions persist for longer periods. Cloudless nights bring many frosts. These very strong and slow moving highs force cold fronts far to the south

5 Questions: 1 What name is used for the lines on a weather map? (A) Isobar. (B) High. (C) Low. (D) Pressure. 2 The air pressure over a city is relatively low and gradually increases in all directions. What type of weather system is over the city? (A) High. (B) Low. (C) Cold front. (D) Isobar. 3 What is indicated when the lines on a weather map joining points of equal air pressure are widely separated? (A) Strong winds will be blowing. (B) There is a gradual change in air pressure. C) There is a rapid change in air pressure. (D) There is a low-pressure area. 4 What is happening to the air to produce a high-pressure area over a city? (A) The air is stationary. (B) The air is rising. (C) The air is sinking. (D) The air is spiraling inwards. 5 What name is given to a large mass of air moving from Antarctica over the southern states? (A) Low pressure area. (B) Warm front. (C) High pressure area. (D) Cold front. 6 Explain why satellite photographs of cloud patterns have improved knowledge of global weather patterns such as the El Nino effect. 7 Describe the movement of air currents between areas of high and low pressure. 8 Explain why isobars that are close together on a weather map indicate strong winds. 9 Describe why a low pressure area will produce rain if the air contains moisture. 10 The air pressure in a coastal New South Wales town changes from low to high during a normal winter. Identify any changes in weather patterns that are likely and explain their cause. 11 During a normal summer, the water temperature around northern Australia is quite high. (a) Explain the effect this will have on air pressure. (b) Explain how this causes extensive rainfall. 12 During an El Nino summer, the water temperature in northern Australia is relatively low. (a) Explain the effect this will have on air pressure. (b) Explain how this causes extensive rainfall

6 describe the relative pressures involved in the formation of tropical cyclones and tornadoes Cyclones and Tornadoes Tropical cyclones are one of the most destructive types of weather to affect Australia. The winds in cyclones are very damaging with gusts approaching 300 kilometres per hour. Rainfall can be as high as one metre within 24 hours. Some of the greatest loss of life has been from storm surges and extremely rough seas that result from cyclones. Cyclones weaken as they move south into colder oceans or move over the inland, but strong winds can still cause disruption while flooding can occur inland. But not everything about cyclones is bad - tropical cyclones are the main source of rainfall in parts of northern Australia. Serious El Nino events sometimes end with a cyclone passing across the continent from the Indian Ocean. Tropical cyclone Satellite photo of a tropical cyclone (Bureau of Meteorology). Cyclone Mahina caused the worst ever disaster in Australia's cyclone history when in March 1899 more than 400 people died on Cape York. Some were victims of a 14-metre storm surge - water swept across lowlying coastal land and carried about 100 people back out to sea. More than 300 died when about 100 pearling vessels sank in the rough ocean. Tonnes of fish and some dolphins were found several kilometres inland. Cyclones A tropical cyclone is a tropical low-pressure system that produces continuous gale force winds of at least 63 kilometres per hour. A severe tropical cyclone produces continuous winds of at least 118 kilometres per hour. Cyclones are classified according to their strongest wind gusts. In other parts of the world, tropical cyclones are called hurricanes or typhoons. In the average cyclone season, ten tropical cyclones develop over Australian waters, of which six cross the coast mostly over north-western Australia and over north-east Queensland. The number of cyclones varies from year to year, from none to as many as 16 in Tropical cyclones in Australian waters are less common in El Nino summers and more common in La Nina summers. This is due to changes in wind patterns and cooler water temperatures in El Nino summers and warmer water temperatures in La Nina summers. Cyclone classification A tropical cyclone will only develop from an intense tropical low when the surface of the ocean is warm enough. The intense tropical low provides the ascending air. There must be a 50-metres depth of water at 27 C or more in order to supply enough evaporating moisture, which provides the initial energy source of the cyclone. The life cycle of a tropical cyclone may be divided up into four stages:

7 In the formative stage, clouds start developing over the ocean close to the equator. Cyclones never move across the equator. If the conditions are favourable, in the immature stage clouds collect and move with the winds that start rotating clockwise around a low-pressure area. Winds start to increase in strength. In the mature stage the cyclonic winds reach their greatest speed and destructive power. The power of the cyclone Where tropical cyclones form Map showing average annual decreases during the decay stage frequency of tropical cyclones in the Australian region (Bureau when it moves over land or south of Meteorology). over colder waters. In this decaying stage the winds often decrease rapidly and the cyclone eye and cloud patterns disappear. The eye of a cyclone is in the centre where conditions are almost calm and clear. At the ocean's surface in the southern hemisphere, winds spiral inwards in a clockwise direction towards the eye. Huge bands of cloud spiral in towards the eye and then begin to spiral upwards. As the air ascends, massive thunderstorms form around the eye and in the spiral bands. High in the atmosphere the air spirals outwards from the cyclone. Tornadoes A tornado is a violently rotating column of air in contact with the ground and appearing to hang from a thunderstorm cloud. Tornadoes are much smaller than cyclones, often no more than 150 metres wide. Nor do they last for long, often only a few minutes. Tornadoes can have wind speeds up to nearly 500 kilometres per hour, which is higher than a cyclone. With wind speeds like this, tornadoes are very destructive. In the United States they can totally destroy homes along a narrow path. Tornadoes are much rarer in Australia, with perhaps 12 a year. They have much lower wind speeds, and as they occur in isolated areas, they cause little damage. However, at least one has caused deaths by lifting a car off the ground and dropping it 100 m away. Structure of a tropical cyclone A cyclone in cross-section. No one is totally sure how a tornado forms. However, the worst tornadoes are produced in a very large thunderstorm called a supercell. Supercells are giant long-lasting thunderstorms that are responsible for producing large hail and dangerous winds. Normal thunderstorms have a fairly flat top, while the top of a supercell is dome shaped. The air in a supercell rises at very high speeds and is Tornado A violently rotating column of air

8 turning as it rises, which can give parts of the clouds a corkscrew appearance. As the updraught strengthens, the spinning increases, until the updraught becomes a narrow, rotating column. Tornadoes often drop from the supercell near the edge of the rain-free area under the storm. Less intense tornadoes can occur near the fronts of approaching storm fronts, beneath rapidly swelling thunderstorm clouds, and within hurricanes. These non-supercell tornadoes, or land spouts, form when updraughts in a rapidly developing thunderstorm cloud draw in low-level air that is already slowly rotating. This pre-existing slow rotation of the lowlevel air can be caused by airflow around mountains or ridges or the convergence of sea breezes or gust fronts. The rotation of many non-supercell tornadoes begins near the ground and grows upwards. Comparing Cyclones and Tornadoes Although cyclones and tornadoes are both rotating masses of air, they differ dramatically in size, pressure and wind speed. The graph for a cyclone starts at a normal air pressure of around 1010 hpa (hectopascals) and starts dropping as we move towards the eye of the cyclone. It drops down to around 950 hpa then increases as we move out of the eye. The wind speed also drops from about 150 kilometres per hour to near zero in the eye and then increases again as we move out from the eye. Cyclones are typically 500 kilometres across. Thunderstorms and tornadoes Tornadoes often form around supercell thunderstorms. For a tornado the pressure drops to a much lower value than for a cyclone, as low as 880 hpa. In theory there is an eye of a cyclone, but in practice there is none as it moves in such an erratic manner. The wind speed can be as high as 500 kilometres per hour, but is often about 300 kilometres per hour, which is again higher than for a cyclone. Tornadoes are much smaller than cyclones, typically 100 to 600 metres across. Pressure and wind speed (a) Notice how both air pressure and wind speed drop in the eye of the cyclone. (b) In a tornado the central pressure is lower. As the tornado is part of a thunderstorm the velocity outside is still high. Values are only approximate as accurate readings are hard to take

9 Questions: Cyclones and Tornadoes 1 Which of the following weather phenomena is the most devastating to humans? (A) Cyclones. (B) Typhoons. (C) Hurricanes. (D) All the above. 2 Why don't cyclones cross the equator? (A) They would no longer be called cyclones. (B) Cyclones form too far from the equator to travel such a long distance. (C) The temperature is too low for a cyclone to exist. (D) They would have to reverse the direction of rotation. 3 In which direction does a cyclone rotate in the southern hemisphere? (A) The same direction as a high-pressure area. (B) Anticlockwise like a low-pressure area. (C) Clockwise like a low-pressure area. (D) Anticlockwise like a high-pressure area. 4 How does a tornado compare to a cyclone? (A) Tornadoes are bigger and have slower winds than cyclones. (B) Tornadoes are smaller and have faster winds than cyclones. (C) Tornadoes are bigger and have faster winds than cyclones. (D) Tornadoes are smaller and have slower winds than cyclones. 5 What is it like in the eye of a cyclone? (A) Little wind and no rain. (C) Winds are very strong and there is torrential rain. (B) Winds are very weak and there is torrential rain. (D) Winds are very strong and there is no rain. 6 Describe the relative pressures involved in the formation of tropical cyclones and tornadoes. 7 Describe the formation of a tornado in a supercell thunderstorm. 8 Explain why satellite photographs of cloud patterns have improved readings of weather maps. 9 Explain why satellite photographs of cloud patterns have improved knowledge of global weather patterns such as the origin of cyclones. 10 Use the data in thetable to answer the questions (a) Plot the movement of tropical cyclone Steve onto the Figure below. (b) Explain why the intensity of the cyclone changed as it moved over the Northern Territory. (c) Explain why the intensity of the cyclone changed as it moved into the Indian Ocean. (d) Identify two areas that may have experienced flooding. (e) What was unusual about the cyclone when in the Great Australian Bight? Justify your answer. Tropical cyclone Steve (Bureau of below. Meteorology)

10 11 Use the Figure to answer these questions. (a) Compare the lowest pressures found in cyclones and tornadoes. (b) Compare the diameters of cyclones and tornadoes. (c) Compare the maximum speeds of cyclones and tornadoes. (d) Which type of storm is harder to measure? Justify your answer. Path of a cyclone A grid to plot the path of a cyclone

11 HSC Style Questions: 2005 HSC Many natural disasters have occurred in Australia since During your course you created a database to present information on these disasters. List TWO examples of Australian disasters you recorded, and for each describe ONE piece of information you collected for the database HSC The following information was published in the Sydney Morning Herald on Tuesday, 7 December (i) What does the distance between the isobars on the weather maps indicate? (ii) Explain the reasoning behind the forecasts for Sydney s four-day outlook. (iii) Explain how information about cloud cover is gathered, and how it has improved our ability to monitor and predict weather patterns

12 2002 HSC (i) Name the lines around the letters L and H on the above weather map. (ii) Describe the movement of air currents from one air pressure system to another HSC (i) Identify ONE Australian disaster where nature and human activity have combined to produce the disaster. (ii) Describe how the human activity contributed to this disaster. (i) What is the lowest air pressure indicated by an isobar on this map? (ii) The Bureau of Meteorology predicted the weather in Melbourne for Tuesday morning as Cooler conditions with strong south-westerly winds and rain. Using the information from the map, suggest why this prediction was made

13 Answers Natural 1 (D). 2 (A). 3 (B). 4 (C). 5 (B). 6 A disaster is an event associated with large-scale environmental or structural damage and/or loss of life. 7 Three of the following: Meteorological, for example, Cyclone Tracy destroys Darwin, Western Sydney Flood. Geological, for example, Newcastle earthquake, Tsunamis of Extraterrestrial, for example, Wolf Creek Crater in Western Australia Biological, for example, Influenza epidemic of 1918, AIDS epidemic. 8 (a) People and businesses insure against those financial losses that they cannot reasonably afford to suffer, such as the replacement of homes or cars. (b) A defined events policy is usually cheaper and sets out a list of what is and is not covered by the insurance policy. For example, if water enters a house through a hole in the roof due to storm damage, the damage is covered. But if the water enters the house under the door as a result of floodwaters from local creeks, the damage is not covered. To get coverage for a flood, extra premium must be paid. An accidental damage policy is more expensive. This is a much broader coverage, and only lists those things that are not covered. Thus, if you spill a can of paint on the carpet it is covered by an accidental damage policy unless it is specifically listed as not covered. 9 Database form (Figure) - many designs are possible. 10 Cyclone Tracy. (a) 1974 (b) Darwin, Northern Territory. (c) 65 people died and about 650 were injured. Most buildings were destroyed or severely damaged; much of the population was relocated to southern states. (d) Building codes were changed to reduce damage in future cyclones. Strong attachments must now be used to attach roofs to walls and walls to the ground. (e) A cyclone watch has been established using satellite imagery to track the movement of cyclones and issue alerts to populated areas in Northern Australia. and Human Activity 1 (C). 2 (B). 3 (B). 4 (D). 5 (A). 6 Three of the following. Terrorist bombings (for example, Bali). Transport accidents (for example, Granville train crash). Bridge collapse (for example, Westgate Bridge, Melbourne). Essential services disrupted (for example, Longford gas plant explosion). 7 Humans allowing overgrazing on farms combined with drought produced the 2002 dust storms. 8 Storms and errors of navigation caused the Cataraqui to be wrecked on King Island in Bass Strait, with the loss of more than 400 lives. 9 Heavy rain combined with poor design of rail embankments led to the Coledale landslide that killed a mother and her child. 10 In this course we define a disaster as events associated with large-scale environmental or structural damage and/or loss of life. The large numbers of lives lost allows road deaths to be classified as a disaster. However, since they occur a few at a time and not in one significant event, they are not normally called a disaster. Studying the Weather 1 (C). 2 (B). 3 (C). 4 (D). 5 (A). 6 Atmospheric pressure is the force per unit area due to the air around us. In studying the weather, the units used to measure atmospheric pressure are hectopascals where 1 pascal is 1 Newton per square metre. 7 Two of the following. Weather radar has allowed the movement of storms to be tracked. This has not only improved our ability to make forecasts of severe weather, but has also allowed us to better understand its origin. The wind movements within storms are studied to better enable us to understand the dynamics of thunderstorms, cyclones and tornadoes. Satellite imagery has allowed us to better predict the path of severe weather such as cyclones. It has allowed us to track the movement of weather patterns across the continent, and to determine the presence of strong winds. This has helped us to better understand the large-scale movements of the atmosphere. Weather radar and satellite imagery both depend on the presence of rain and/or clouds to track movements. They are not able to measure air movements in clear air. The development of ground or satellite-based lidar will help solve this problem. An infra red laser is used to track the movements of tiny dust and aerosol particles in the air. This will help us to better understand both the small-scale and the large-scale movements of the atmosphere. 8 (a) A microwave pulse is sent from the radar dish. If it strikes rain, hail or snow it will be scattered. Some of the pulse will be reflected back to the radar dish. (b) From the time taken for the pulse to return the location of the rain, hail or snow can be determined. Since we know that microwaves travel at the speed of light, we can combine the speed and time to calculate the distance. A computer is used to perform the calculations. (c) As the radar beam scans across the sky, a computer is used to perform the calculations and combine together the many reflections for display onto a map. If the reflections are weak, it indicates light rain. If the reflections are intense, then it may indicate heavy rain or even hail. In each case, the map is colour coded accordingly

14 9 Satellites take regular images of the Earth's cloud cover using both infra-red and visible light. Successive images provide a picture of cloud movements across Australia and nearby regions and helps in weather forecasting. The movements of clouds also allow wind speeds to be calculated. Regular satellite images are also used to track the movements of cyclones. 10 Several answers are possible, depending on the experiments you have performed. We observed the effects of changing the air pressure inside an empty drink can. We one quarter filled the drink can with water and heated the water until it boiled. The steam forced some of the air from the can. Using tongs, we quickly inverted the hot can over a bucket of cold water. When inverted in cold water, the steam condensed and reduced the air pressure inside the can. The air pressure outside remained constant, and as it was much larger than the pressure inside, it collapsed the can. Predicting the Weather 1 (A). 2 (B). 3 (B). 4 (C). 5 (D). 6 In a normal year, the warm water conditions to the north of Australia cause considerable evaporation and the formation of clouds. In an El Niflo year, the warm water is found further east closer to the Americas. This means the extensive cloud formation is in the central Pacific. This movement of the location of clouds can be tracked using satellite imagery. Satellites can also use modem technology to measure the temperature of the ocean surface. 7 In the southern hemisphere, air spirals anticlockwise out from a high pressure area, and after it crosses the boundary, spirals clockwise into a low pressure area. 8 Isobars that are close together show that air pressure is changing rapidly. Since wind is a result of changes in air pressure, the bigger the difference in pressure over a short distance, the stronger will be the winds. 9 In a low-pressure area, the air is rising into the colder upper atmosphere. As moist air rises, the moisture cools and condenses to form clouds which in turn will produce rain. 10 High pressure areas indicate sinking warm stable air. Thus the weather can be expected to be fine and clear. 11 Warm water will warm the atmosphere and cause it to rise to produce a low-pressure area. Since the water is warm, the rate of evaporation will be high producing moisture-laden air. 12 When moisture-laden air rises into the atmosphere, it cools and clouds form. These clouds are the source of the extensive rain in northern Australia. Cyclones and Tornadoes 1 (D). 2 (D). 3 (C). 4 (B). 5 (A). 6 Both cyclones and tornadoes are formed in areas of low pressure. Cyclones form in tropical lows over warm oceans where the central pressure can drop as low as 950 hpa. Tornadoes form in thunderstorms where there is low pressure due to rapidly rising air. The pressure inside a tornado is lower than that inside a cyclone, and can drop to 880 hpa. 7 The air in a supercell rises at very high speeds and is turning as it rises. As the updraught strengthens, the spinning increases, until the updraught becomes a narrow, rotating column. Tornadoes often drop from the supercell near the edge of the rain-free area under the storm. 8 Satellite photographs allow the tracking of movements of major weather systems and storms. Cloud is often associated with lows and cold fronts. The tracking of large weather systems such as lows and cold fronts allow better everyday predictions of weather. This is particularly true of places such as Western Australia where most weather comes from the oceans to the west. Since there are few weather stations in the ocean, images from weather satellites improve the accuracy of predictions. 9 Satellite photographs allow the tracking of movements of weather systems such as cyclones. Since cyclones have extensive cloud, their origin can be monitored more readily from space as there are few weather stations in the oceans where they form. Cyclones are also unpredictable in their movements. Tracking the movements of cyclones allows people in their path to be warned thus helping reduce damage and loss of life. 10 (a) The movements of cyclone Steve (Figure ). (b) Cyclones obtain their energy from the evaporation of warm water. When a cyclone moves over land, the energy supply ceases and the intensity of the cyclone usually decreases. In this case it became an intense low. (c) As the cyclone moved out over the ocean, its supply of energy from warm ocean waters increased. As a result the intensity of the cyclone increased, reaching category 2. (d) Intense lows formed over Cape York, Northern Territory and Western Australia. All would have produced intense rain and result in flooding. (e) Cyclone Steve remained a category 1 cyclone over Western Australia and into the Great Australian Bight. Normally the lack of warm water over land, and the presence of colder water in the Bight, would have reduced the cyclone to an intense low. 11 (a) The lowest pressure in the cyclone is around 950 hpa, and around 875 hpa in a tornado. (b) The cyclone is more than 600 kilometres in diameter, and the tornado is more than 300 m in diameter. (c) The highest speed of the cyclone is nearly 175 kilometres per hour, while the highest speed of the tornado is close to 320 kilometres per hour. (d) The small size and destructive power of the tornado makes it vary hard to study. A plane can be flown through the much larger cyclones to take measurements