Beaufort wind scale: uses observations of
the effects of wind to estimate its speed.
Beaufort Wind Scale
Wind speed is the average speed of the wind over a 10-minute period at a height of 10 metres above the surface. As a guide, double the wind speed in knots to convert to kilometres per hour; for example 20 knots is approximately 40 km/h.
Gusts are increases in wind speed lasting for just a few seconds. The speeds are typically 30 to 40 per cent higher than the average wind speed, but stronger gusts are likely in the vicinity of showers, thunderstorms and frontal systems.
A squall is an abrupt and large increase in wind speed that usually only lasts for minutes then diminishes rather suddenly.
Strong wind warning: 26 to 33 knots.
Gale warning: 34 to 47 knots.
Storm force wind warning: 48 to 63 knots.
Hurricane force wind warning: 64 knots or more.
Wind direction is given in 8 compass points for forecasts and 16 for observations and is the direction the wind is coming from.
Wind direction descriptions
Sea (or wind) waves are generated by the local prevailing wind and vary in size according to the length of time a particular wind has been blowing, the fetch (distance the wind has blown over the sea) and the water depth.
Swell waves are the regular longer period waves generated by distant weather systems. There may be several sets of swell waves travelling in different directions, causing a confused sea state.
Combined sea and swell is also known as total wave height, or significant wave height. Combined sea and swell describes the combined height of the sea and the swell that mariners experience on open waters. The height of the Combined sea and swell refers to the average wave height of the highest one third of the waves.
A lookup table has been developed to outline how to calculate the Combined sea and swell.
Sea state describes the combination of sea (wind) waves and swell.
Wave height (trough to crest) for both sea and swell refers to 'significant wave height' which represents the average height of the highest one-third of the waves. Some waves will be higher and some lower than the significant wave height. The probable maximum wave height can be up to twice the significant wave height.
King or rogue waves are waves typically greater than twice the significant wave height. These very large waves are known to occur in areas where ocean currents run opposite to the prevailing sea and swell and where waves overrun each other, generating steep and dangerous seas. Mariners should be prepared for a rogue wave encounter.
The following phrases for times of the day are used in Coastal Waters and Local Waters forecasts. Timings refer to Local Time in each State/Territory.
Early in the morning: Expected to occur before 6am.
In the morning: Expected to occur between 7am and 10am.
Middle of the day: Expected to occur between 11am and 1pm.
During early afternoon: Expected to occur between 2pm and 3pm.
In the afternoon: Expected to occur between 4pm and 6pm.
During the evening: Expected to occur between 7pm and 8pm.
Later in the evening: Expected to occur after 9pm.