Cyclone Tracy began as a tropical low several days before it hit Darwin. Up until a few hours before, it was not expected to affect the town, but unexpectedly turned and headed straight for Darwin on Christmas Eve, making a direct hit in the early hours of Christmas Day. See the Web Link below for more information. The site has a number of other pages, including all the warnings issued in the preceding days.
A cyclone - which is the Australian name for a hurricane - caused Cyclone Tracy. It actually began as a tropical depression in the Arafura Sea, north of Darwin. It developed into a small, but very intense tropical storm, which then developed cyclone status, passing right over Darwin on Christmas Eve 1974. Cyclones are intense low pressure systems which begin rotating and form a central eye. Part of the reason for the high damage level was the unpredictable way that Tracy moved. It had been travelling south-west, but turned 90 degrees to the south-east and headed straight for Darwin, catching many people by surprise.
See the Related Link ' Cyclone Tracy Website' below and follow the links within the site to learn about how all cyclones develop and how Tracy behaved over the hours leading up to and following landfall. Cyclone Tracy intensified from a storm that developed out in the Arafura Sea. Cyclones can only form if certain conditions are present: e.g. the surface temperature of the ocean needs to be 26.5°C or higher, and there must be a tropical low present. The low air pressure system then begins to pull in clouds and rotate.