Transcript File
Disasters Part 2: Monitoring and Predicting Weather define the term ‘atmospheric pressure’ and describe the movement of air currents between areas of high and low pressure Atmospheric Pressure • The force per unit area that the air is exerting on the body. Atmospheric Pressure Units • Air pressure is measures in hectopascals (hPa). • The atmosphere pushes down on the Earth’s surface with a pressure of about 100 000 pascals. With such large numbers it is common in meteorology to use kilopascals (kPa) or hectopascals (hPa). • 1 kilopascal = 1000 pascals 1 hectopascal = 100 pascals. Atmospheric Pressure • The higher you are from the surface of the Earth, the less dense the air is so the lower the air pressure is. • Sea level average = 1013 hPa • During severe storms = 970 hPa Atmospheric Pressure • High Pressure • Cold air • air gets denser and heavier • Low Pressure • Warm air • air becomes less dense and lighter Movement of Air Write down what you think is happening here. Movement of Air During the day, especially in summer, land areas heat up more than adjoining waters. This warms the air over it and, as the air expands, it rises. Air pressure becomes less over the land. This makes room for cool air to blow from the ocean to the land. The warmed air now moves out over the ocean to take the place of the cool breeze. In doing so it is cooled down and contracts. This creates a higher pressure over the water and so the air descends. Textbook: Spotlight Page 304 Summarise section: “Too Much Pressure” Video: https://www.youtube.com/watch?v=aiYyCurh_SU Measuring Pressure Changes • Barometers are used to measure air pressure in our atmosphere. • Invented in 1643 and used mercury. The mercury would be pushed down by the air pressure, moving it slightly. • Other liquids were tried but moved by the air more easily Measuring Pressure Changes • Modern (aneroid) barometers consist of a closed metal capsule with flexible walls. Only a small amount of air inside the capsule. • When air pressure increases outside the capsule, the capsule squeezes a little • When air pressure decreases outside the capsule, the capsule bulges out a little. identify that the distance between isobars on a weather map indicates the relative change of atmospheric pressure in an area Air Pressure on Maps • Lines are drawn on weather maps joining places which have the same air pressure. These lines, called isobars, show high and low pressure systems. Pressure is shown in hectopascals (hPa). Air Pressure on Maps • For the Southern Hemisphere, in a low pressure system is clockwise and moving inwards, while that around a high pressure system is anticlockwise and moving outwards. Air Pressure on Maps • If isobars are close together the pressure difference between two points will be great and therefore the winds will be strong. • If they are a fair distance apart, wind speeds will not be as great. Air Pressure on Maps • Worksheet: Typical Weather Map Answers 1. The interval between successive isobars is 2 hPa. 2. X is 1008 hPa as it lies between the 1006 hPa and 1010 hPa isobars. 3. a) 1011 hPa b)1008.5 hPa 4. Hobart has the lowest air pressure, being about 1008 hPa. 5. a) Air moving approximately in a northwesterly direction (a South-Easterly wind). b) Air moving approximately in a southwesterly direction. (a North-Easterly wind). 6. Winds are stronger in Brisbane as isobars closer together. Describe the relative pressures involved in the formation of tropical cyclones and tornadoes What is a Tropical Cyclone? A cyclone begins to form when moist air is heated by the Sun and rises from the surface of warm, tropical seas. This air is funneled in a natural updraft. On rising, the moist air cools and condenses into rain. What is a Tropical Cyclone? The condensation of gaseous water vapour to liquid water droplets releases large amounts of heat energy, and this adds to the storm’s updraft. Air continues to spiral upwards, with hot moist air rushing in from all sides to replace it. These winds spiral around an ‘eye’, an area of calm and light rains. What is a Tropical Cyclone? The cyclone begins to revolve because of the Earth’s rotation. In the Southern Hemisphere the rotation of air in a low pressure system is in a clockwise direction towards the centre, where it rises then flows outwards at higher altitude. The energy released from this process goes towards increasing the speed of the spinning column of air. How Do Tropical Cyclones Occur? How Do Tropical Cyclones Occur? How Do Tropical Cyclones Occur? How Do Tropical Cyclones Occur? Cyclone Category Measurements of wind speed and air pressure made before a cyclone reaches a location can be used to predict the size of any storm surge and the likely damage at that location. Where Do Cyclones Occur? • About 80-100 tropical cyclones develop each year from low air pressure areas in tropical oceans. This is usually between the latitudes of 5° and 30° on either side of the Equator. • Cyclones never develop directly on the Equator as there is no Coriolis Effect there to start the required circular motion of warm moist air. Textbook • Spotlight Senior Science • Pg 305-307 • Summarise “Cyclones and Tornadoes”