The Fremantle Doctor, the Freo Doctor, or simply The Doctor, is the Western Australian vernacular term for the cooling afternoon sea breeze which occurs during summer months in south west coastal areas of Western Australia. The sea breeze occurs because of the major temperature difference between the land and sea.
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During summer months, it consistently blows from the southwest along the southern half of the west coast, starting between noon and 3 pm. It can penetrate as far inland as 100 kilometres (60 mi), reaching York in the early evening.
In Perth, the capital city of Western Australia, the wind is named the Fremantle Doctor because it appears to come from the nearby coastal city of Fremantle, and it brings welcome relief from the summertime high temperatures.
The Fremantle Doctor often cools the Perth suburbs by several degrees, whilst nearer the coast the wind can be quite strong, often blowing between 15 and 20 knots and making afternoon beach visits unpleasant.
On days when the wind fails, the afternoon temperatures of Perth suburbs are considerably higher, often exceeding 40°C (104°F), although this is also attributable to the influence of a strong easterly wind blowing in hot desert air from the arid interior of the state.
The Fremantle Doctor is strongest in December and January, when the temperature differential between the land and ocean is greatest. In February and March the breeze is not as strong, because the ocean temperature is a bit warmer. Though October and November are not as warm, the ocean temperature is cooler resulting in the sea breeze in these months.
Other months have a weak or non-existent Fremantle Doctor. In the winter months, the land temperature is usually cooler than the ocean temperature, and this sometimes results in the weaker land breeze in the early morning.
When the Fremantle Doctor first arrives it is from the west-south-west (WSW) direction. Later when it is at maximum strength, the direction is usually from the southwest or south-south-west. By the early evening, the direction is from the south. This change in wind direction is due to the Coriolis Effect. Other factors, like the prevailing wind of a particular day, can also have an effect on the direction.
A wind rose showing the different wind directions for Perth can be found at the Bureau of Meteorology's website.[1]