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Beaufort Scale
Storm force is measured by the Beaufort Wind Scale. Force 5, defined as a 'fresh breeze', is when conditions begin to become bracing for small craft, with waves measuring 2 metres (6.6 feet) and the chance of spray. Force 8 denotes a gale, with moderately high waves of more than 5m (16ft) and wind velocity of 34-40 knots (30-35mph). Top of the panic scale is hurricane force 12, with 14m (46ft) waves and wind velocity of 64 knots (56mph) or more. Weather blues From latitudes 30° to 50°S, through which the course of the Sydney-to-Hobart race lies, warm north-easterly winds occasionally meet cold south-westerlies from the Antarctic, creating what is known as a polar front. This clash deflects the winds so that they become more easterly and westerly respectively, and each opposing airflow makes incursions into the other. At the same time, the cooler, drier polar air rushes in underneath the warmer, wetter air as the latter rises, creating strong winds and, gradually, the formation of a whirlpool effect. This is known as a depression or low-pressure system. As the warm air rises rapidly, it cools and the water droplets it contains produce storm clouds. The bigger the air masses that meet, the nastier the storm. The one that hit the 1998 race had 'cyclonic' winds at its core. Hurricane force 'Hurricane', 'cyclone' and 'typhoon' are different names for the same type of severe tropical storm. Generally speaking, such storms are referred to as hurricanes in the Atlantic, cyclones in the Indian Ocean and typhoons in the northern Pacific. They usually occur at the end of the hot season, and, as well as generating fierce winds and waves, they can last several days and cover vast distances. The storms tend to form out at sea in areas of low pressure when there is a combination of heat and moist air. The condensation of the water vapour that is produced provides the energy necessary to kick up a storm, which then rotates violently around a central point. The wind, which can reach 115 knots (100mph) or more, travels clockwise in the southern hemisphere and anti-clockwise in the north, and is accompanied by heavy rain. At the centre of the storm is an area known as the 'eye', where the clouds disappear and the winds are relatively light. On either side of the eye, the wind blows from opposite directions, which whips up massive waves. Round the Horn The Southern Ocean – and the area around Cape Horn in particular – has been dubbed by sailors 'the sea of certain death'. In 1992, yachtsman Robin Knox-Johnston, who had made the first solo non-stop round-the-world voyage in 1969, explained in The Times why making it round the Horn is the 'ultimate credential for any seafarer': 'Nowhere else on our planet does an ocean have free passage all around the world, and an almost continuous succession of easterly moving depressions generate enormous waves, up to 120ft [36.6m] high … These waves have an unrestricted passage nearly 2,000 miles wide as they roll ponderously eastwards, propelled by a prevailing westerly wind … at one point [at Cape Horn], they are confined where South America reaches southwards nearly 1,000 miles towards a similar finger of land jutting northwards from Antarctica. Here, the ocean is forced through a space only 600 miles wide, and the winds are simultaneously squeezed southwards by the Andes mountains.' This gap is known as Drake Passage and is also often home to icebergs that have drifted north-eastwards on the Antarctic current. Dire strait The 150-mile wide Bass Strait between Tasmania and south-eastern Australia is often described by sailing folk as 'hell on high water'. As well as being in an area prone to low-pressure weather systems, sea conditions are also influenced by other factors. Prevailing westerly winds push the rolling waters of the Southern Ocean through the narrow channel where they meet other deep-water swells from the Pacific and the fast-moving East Australia Current. This causes steep waves to form. Because the channel is both windy and shallow – 50-70m (165-230ft) – the waves break as they would if encountering land. In summer, ocean-going racers are likely to experience the 'southerly buster', a seasonal wind that affects the New South Wales coast and can gust up to 70mph. When it meets the opposing current, the sea becomes rough. Choppy, confusing seas are also produced in winter by a feature known as the Bass Strait Cascade. The eastward-flowing current is cool and salty. Cool, salty water is relatively dense. When the Bass Strait waters flow out towards the deep Tasman Sea, they plunge down the continental shelf and underneath the warm East Australia Current like a waterfall. |
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