Wind is made up of gusts and lulls. The Bureau's forecasts of wind speed and direction are the average of these gusts and lulls, measured over a 10-minute period at a height of 10 metres above sea level. The gusts during any 10-minute period are typically 40% higher than the average wind speed. For example, when the average wind speed is 25 knots, it is normal to experience gusts of 35 knots and lulls of lighter winds. Thunderstorm and squalls may produce even stronger gusts.
Wind speed usually increases with height above the sea-surface, so winds at the surface are not typically as strong as they are at 10 metres, where wind is measured and forecast by the Bureau.
A wind forecast range (for example, 10 to 15 knots) may be given when the wind speed is expected to vary significantly within a coastal area. The wind direction is based on true north orientation and is the direction the wind is blowing from. For example, a northerly wind is blowing from the north towards the south. Wind speed and direction can be influenced significantly by the local environment. Cliffs and other landscape features will affect winds near the shore.
Checking wind conditions is one of the five vital weather safety checks everyone should complete before heading out on the water.
Based on the 40 per cent rule of thumb, the table below shows the potential gust you could expect for different forecast average wind speeds and associated wind warning category.
|Average wind speed
|Gust strength that should be planned for
|Wind Warning thresholds|
|26 - 33||36 - 45||Strong wind warning issued|
|34 - 47||48 - 65||Gale force warning issued|
|48 - 63||67 - 88||Storm force warning issued|
|64 or more||90 or more||Hurricane force warning issued|
Remember, wind gusts can be stronger still in the vicinity of thunderstorms and squalls. Gusts from thunderstorms may also come from a different direction than the average wind direction. Check your local forecasts carefully for mention of thunderstorms or squalls before you go out and be prepared to change your plans.
Read the six things you need to know about wind warnings for more information about wind warnings.
The Beaufort wind scale measures wind speed according to the impact the wind has on the land and sea. Although the system is old (first developed in 1805 by Sir Francis Beaufort), it remains a widely used system to measure wind speed today. The table below describes what can be expected for each level of the scale and the relationship to the forecast average wind speed values.
|Beauf. scale||Desc. term||Units in knots||Description on Land||Description at Sea|
|0||Calm||0||Smoke rises vertically||Sea like a mirror.|
|1-3||Light winds||10 knots or less||Wind felt on face; leaves rustle; ordinary vanes moved by wind.||Small wavelets, ripples formed but do not break: A glassy appearance maintained.|
|4||Moderate winds||11-16 knots||Raises dust and loose paper; small branches are moved.||Small waves - becoming longer; fairly frequent white horses.|
|5||Fresh winds||17-21 knots||Small trees in leaf begin to sway; crested wavelets form on inland waters||Moderate waves, taking a more pronounced long form; many white horses are formed - a chance of some spray|
|6||Strong winds||22-27 knots||Large branches in motion; whistling heard in telephone wires; umbrellas used with difficulty.||Large waves begin to form; the white foam crests are more extensive with probably some spray|
|7||Near gale||28-33 knots||Whole trees in motion; inconvenience felt when walking against wind.||Sea heaps up and white foam from breaking waves begins to be blown in streaks along direction of wind.|
|8||Gale||34-40 knots||Twigs break off trees; progress generally impeded.||Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks along the direction of the wind.|
|9||Strong gale||41-47 knots||Slight structural damage occurs -roofing dislodged; larger branches break off.||High waves; dense streaks of foam; crests of waves begin to topple, tumble and roll over; spray may affect visibility.|
|10||Storm||48-55 knots||Seldom experienced inland; trees uprooted; considerable structural damage.||Very high waves with long overhanging crests; the resulting foam in great patches is blown in dense white streaks; the surface of the sea takes on a white appearance; the tumbling of the sea becomes heavy with visibility affected.|
|11||Violent storm||56-63 knots||Very rarely experienced - widespread damage||Exceptionally high waves; small and medium sized ships occasionally lost from view behind waves; the sea is completely covered with long white patches of foam; the edges of wave crests are blown into froth.|
|12+||Hurricane||64 knots or more||Very rarely experienced - widespread damage||The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected|
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