Winds
Winds flow in order to more evenly distribute heat between the equator and polar regions. Wind direction and speed are determined by the patterns of highs, lows and fronts seen on weather maps and by local effects such as sea-breezes and thunderstorm downdrafts. When the isobars (lines of equal pressure) around highs and lows become more closely spaced, then winds increase. That is, the higher (or tighter) the pressure gradient, the stronger the wind speed.
Stronger wind speeds are associated with tropical cyclones, deep lows and cold fronts. Sudden squalls are associated with thunderstorms, heavy showers or the passage of a cold front or low pressure trough and can happen in clear skies (e.g. the Southerly Buster in NSW). The very strongest winds are caused by tropical cyclones, deep mid-latitude low pressure systems and tornadoes/water spouts.
Definitions and Terminology
Wind speed mentioned in forecasts and coastal observations refers to the average speed over a 10-minute period at a height of 10 metres above the surface. It is given in knots. A knot (kn) is equal to a speed of one nautical mile per hour. Note: 10 knots = 18.5 km/h and 10 km/h = 5.4 knots.
Gusts may be up to 40 per cent stronger than the average speed.
A squall is an abrupt and large increase of wind speed with a duration of the order of minutes which diminished rather suddenly.
Strong wind: 25 to 33 kn; Gale force: 34 to 47 kn; Storm force: more than 47 kn; Hurricane force (used for tropical areas only): more than 63 kn (remembering these are all ten-minute averages).
Wind Direction is given in the 16 compass points and is the direction the wind is coming from.
Waves
Wave height is vertical distance between the top of crest and bottom of trough.
Wind (or sea) 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 that were generated by the winds of distant weather systems. There may be several sets of swell waves travelling in different directions, causing a confused seas state.
Sea state is the combination of wind waves and swell.
The wave and swell heights described in Bureau observations and forecasts refer to 'significant wave heights' which represent the average 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/Freak waves can occur when wind waves and/or a combination of swell waves join to produce a very high wave. These can be even higher than the probable maximum wave height, and can result from the added influence of currents, tides, distant weather systems and shape and depth of the seabed.
Time
UTC (Universal Time Coordinate): time references in warnings for high seas are given in UTC. Australian Eastern Standard Time is UTC + 10 hrs. Western Standard Time is UTC + 8 hrs.