Cyclone Tracy - Meteorological History

Meteorological History

On 20 December 1974, the United States' ESSA-8 environmental satellite recorded a large cloud mass centred over the Arafura Sea about 370 kilometres (230 mi) northeast of Darwin. This disturbance was tracked by the Darwin Weather Bureau's regional director Ray Wilkie, and by senior meteorologist Geoff Crane. On 21 December 1974, the ESSA-8 satellite showed evidence of a newly formed circular centre near latitude 8° south and longitude 135° east. The meteorological duty officer at the time, Geoff Crane, issued the initial tropical cyclone alert describing the storm as a tropical low that could develop into a tropical cyclone.

Later in the evening, the Darwin meteorological office received an infrared satellite image from the National Oceanic and Atmospheric Administration's satellite, NOAA-4, showing that the low pressure had developed further and that spiralling clouds could be observed. The storm was officially pronounced a tropical cyclone at around 10 p.m. on 21 December, when it was around 200 kilometres (120 mi) to the north-northeast of Cape Don (700 km or 435 mi northeast of Darwin). Cyclone Tracy was first observed on the Darwin radar on the morning of 22 December. Over the next few days, the cyclone moved in a southwesterly direction, passing north of Darwin on 22 December. A broadcast on ABC Radio that day stated that Cyclone Tracy posed no immediate threat to Darwin. However, early in the morning of 24 December, Tracy rounded Cape Fourcroy on the western tip of Bathurst Island, and moved in a southeasterly direction, straight towards Darwin. The bureau's weather station at Cape Fourcroy measured a mean wind speed of 120 kilometres per hour (75 mph) at 9 a.m that morning.

By late afternoon on 24 December, the sky over the city was heavily overcast, with low clouds, and was experiencing strong rain. Wind gusts increased in strength; between 10 p.m (local time) and midnight, the damage became serious, and residents began to realise that the cyclone would not just pass by the city, but rather over it. On 25 December at around 3:30 a.m, Tracy's centre crossed the coast near Fannie Bay. The highest recorded wind gust from the cyclone was 217 kilometres per hour (135 mph), which was recorded around 3:05 a.m at Darwin Airport. The anemometer (wind speed instrument) failed at around 3:10 a.m, with the wind vane (wind direction) destroyed after the cyclone's eye. The Bureau of Meteorology's official estimates suggested that Tracy's gusts had reached 240 km/h (150 mph). The lowest air pressure reading during Tracy was 950 hectopascals (28 inHg), which was taken at around 4 a.m, by a Bureau staff member at Darwin Airport. This was recorded during the eye of the cyclone. From around 6:30 a.m, the winds began to ease, with the rainfall ceasing at around 8:30 a.m.