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©Kondinin Group Reproduction in whole or part is not permitted without permission. Climate and sustainability Weather tools 100E FreecalL 1800 677 761 110E 120E 130E 140E 1012 1016 1012 20S Low pressure centre L 1012 1008 30S 1004 1000 H 1012 1008 Ridge of Higer Pressure 1000 1004 1008 40S H Strong southwesterlies 996 980 975 50S 80E 1020 Lindsay 996 Smail 1016 1004 Consultant 1008 1000 992 992 1000 1008 1004 996 984 110E 120E 130E 140E 90E 100E Forecasts: A typical winter weather map gives enough information for a forecast of several days ahead. 30S 1004 1000 40S 996 Cold front 20S High pressure 1020 centre H Jargon busted in weather maps 10S Isobar for 1012 hectoPascals 10S Source: Bureau of Meteorology. 150E 50S L 988 150E 984 988 992 160E Lindsay Smail of Geelong Weather Services unravels the jargon of weather maps to make ‘reading’ these important published charts easier. Everybody recognises the TV weather map but, despite all its fancy colours and graphics, it still remains a mystery to some. The weather map, or synoptic analysis chart, as the meteorologists call it, shows the current weather and what can be expected in the next few hours or even days (see Figure 1). But be careful to check the date and time first. Maps that are more than a few hours old will need that fact taken into consideration. In Australia, south of the Tropic of Capricorn, most weather systems tend to move from west to east. At a glance When interpreting weather maps, take note of the date and time, even if they are only a few hours old, and take the time difference into consideration. What farmers like to see at the right time is a cooling lowpressure system because that means rain is on the way. While a high means little chance of rain, this could change if a low occurs above the high-pressure system. So, while it might not be true to say that Melbourne gets Perth’s weather one week later, it is often the case that the system causing Perth’s weather on one day could be over Victoria in about 5–7 days. Deciphering the map The first feature people notice about the map is the wiggly lines, or isobars. These lines are drawn by a person or computer connecting together all places that have the same barometric pressure. A barometer measures the pressure of the air in hectopascals — how heavily the air is pressing down. The places where the barometric readings are identical are then joined together by the isobars. On old barometers, the measurements were in inches of mercury. The higher up a hill or in a plane, the more the air pressure decreases. But so that the isobar pattern does not end up showing only where the mountains are, all pressures are corrected to what they would be at sea level. 40 Farming Ahead October 2008 No. 201 www.farmingahead.com.au Reading between the lines What the maps end up with is a series of amoeba-like shapes across it with ‘H’ or ‘L’ inside them. ‘H’ (high) means an area of higher pressure than the surrounding air. It is composed of heavier, or sinking air. There are several useful pieces of information that can be obtained from seeing a high on the weather map. Firstly, the air tends to be stable or gently sinking downward. This means there will be little chance of rain because, for rain to occur, the air has to be cooling and this is usually achieved by rising air. So, a high pressure centre (such as the one over northwestern New South Wales in Figure 1) indicates an area of fine, dry weather, with few clouds. This situation could extend for several hundred kilometres around the centre. Some rain could still fall where a high is shown: drizzly showers along a coastline, or a low-pressure system could occur above the high. Are windy conditions forecast? The high also indicates wind direction. In the southern hemisphere, high pressure air circulates anti-clockwise around the centre, following roughly the isobars and tending to spread outward. This is due to the earth’s rotation. On the map pictured, light north-westerlies are blowing at Adelaide around the high. These will be fairly light, perhaps gusting at 20–30 kilometres per hour. Wind speed is shown by the distance between the isobars, indicating the pressure gradient. At Port Hedland, Western Australia, conditions are likely to be calm, but at Melbourne, the winds are likely to be increasing in strength as the isobars become packed closer together. Climate and sustainability Weather tools The cool down A low (L), on the other hand, indicates lighter, or rising air. If there is sufficient water vapour available and it is forced to rise, it will start to cool. If this cooling is sufficient, or if something else assists the cooling, like the existence of a cold pool of air higher up, then precipitation, usually rain, will result. Precipitation occurs because, as air cools, it is less able to hold water vapour and lower temperatures will force the molecules to condense, or form a cloud. Tiny visible droplets form around a speck of salt or dust in the air and grow larger until gravity eventually takes over, producing rain. Winds around the low move clockwise in the Southern Hemisphere. On the map, the tightly packed isobars around a low south of Tasmania are causing strong, cold south-westerly winds and probably rain across Victoria and Tasmania. Running hot and cold Cold and warm fronts are useful features of the weather map. A cold front is the front edge of an air mass that is cooler than the air it is moving toward. In Figure 1, the cold fronts are shown as black barbed lines which generally move east and are associated with lows. Air behind the cold front (to its west) will be relatively colder than that ahead of it (to its east). At the surface, the difference could be five degrees Celsius or more during August, and much higher in summer. Warm fronts are uncommon but do occur sometimes in southern Australia. Waiting for rain Both cold and warm fronts are likely to cause rain, if there is sufficient moisture. Atmospheric instability, condensation and precipitation are caused when a cold front, acting like a gigantic bulldozer, forces cold air underneath the warmer air ahead of it. This lifting process can lead to rain, if enough water vapour is present. Under these circumstances, the low pressure centred near Perth, as shown in Figure 1, is likely to be bringing cold, wet and windy conditions, with the wind swinging around, after the low has passed, to blow from the south. As the low moves toward Victoria, conditions are likely to become more wintry at Adelaide and more rain or showers could result from the rising air, particularly as it cools when forced up and over the Mt Lofty Ranges. Troughs and ridges To complete the picture, a word is necessary about troughs and ridges. A trough is an area of lower pressure, sometimes shown as a broken line if clouds are evident. A trough will not usually cause as severe or noticeable a change as a front but, because the pressure dips lower, it is still often likely to bring gusty winds or showers. A ridge, on the other hand, is an elongated isobaric pattern of a high (such as that Glossary Isobars Hectopascals High Low Pressure gradient Condensation Cold front Atmospheric instability Ridge Trough Lines on a map joining places of equal barometric pressure at sea level. Units of pressure used on a barometer and weather map. Heavier, sinking air that tends to be drier, bringing generally fine weather. Lighter, rising air that tends to contain more water vapour per unit of volume than a high, making the atmosphere unstable. The difference in pressure between two places, resulting in strong winds if isobars are close together and gentle if further apart. Invisible water vapour cools to form a visible cloud of tiny droplets. The leading edge of a mass of colder air. The likelihood of air to continue to rise and cool once the process has started. Precipitation is the likely result. An elongated area of high pressure. A line of lower pressure, sometimes associated with a cold front. shown over the eastern part of the Great Australian Bight in Figure 1). More settled weather is usually the result. All these weather patterns need to be interpreted according to a knowledge of local conditions — altitude, location as well as the distance of water bodies, hills and valleys. Contact Lindsay Smail is director of Geelong Weather Services, an operational meteorology and consultancy business. (03) 5241 5332 [email protected] www.geelongweatherservices.com DISC SEEDERS With so many KRAUSE 5200NT Disc Seeders in paddocks this year, GERONIMO FARM MACHINERY are very pleased to announce that all their clients are extremely happy with their purchases FR E E $1000 worth of quality tools with each order. Dawson of “Winyar” Merino Alan (Smoke) Da Canowindra, NSW bought a 15’ Stud at Cano Krause 5200NT disc seeder six months ago from Geronimo Farm Equipment. Smoke said “This seeder has been wonderful. It has everything I could want. It sowed easily straight into an old existing hard packed lucerne flat, grazed by cattle. I also had no trouble sowing straight into a 14 year old pasture paddock. “The seed placement and germination were really great and that is in this very tough season,” he said. Greg McSpadden “Tamaralina” Canowindra is thrilled with the job his new Krause 5200NT disc seeder has done on his country. Greg told David Fagan (Sales Manager of “Geronimo”) “The most amazing thing about the seeder was its versatility. I have done 1500 acres. It included some soft worked country, some hard lucerne stands and some of last years’ stubble, but the most exciting thing was how well it handled the rough stoney ground”. Full range available from 10’ to 36’ pull type and 3 point linkage For more information phone: GERONIMO FARM EQUIPMENT Ring Ashley or David on Pridham St, COWRA (02) 6341 3369 Website: www.geronimo.com.au 1800 242 432 Newell Highway, MOREE (02) 6752 8388 or 0427 423 641 Headerword Ave, COROWA (02) 6033 1933 Email: [email protected] TL1656628 Farming Ahead October 2008 No. 201 www.farmingahead.com.au 41