Wind and other Meteorology

Winds - BOM Marine 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.

SEA BREEZES

Nature and Causes

Sea (and land) breezes are caused by unequal heating and cooling of adjacent land and sea surfaces. A sea breeze is one that blows from the sea to the land in consequence of this differential heating.

During the day, especially in spring and summer, solar radiation causes the land surface to become warmer than the sea surface. From about August through to December, off the NSW coast, cooler sea water temperatures prevail, while the warming effect of solar radiation is increasing to its maximum. Therefore the contrast between land and sea surface temperatures becomes considerable during, the day, being greatest around mid-afternoon.

Without going into the mechanics of the matter, suffice it to say that the warmed air rises over the land surface and a local circulation commences, with cool air from the sea being drawn in over the land. At the same time the ascending air returns seaward in what is known as the upper return current (See Fig. 1).

Just as a fire in a grate causes an up-current in a chimney, the updraught becoming progressively stronger as the fire burns more vigorously (and vice-versa), so the sea breeze gains in momentum and spreads over a greater area of land. A normal sea breeze in late spring or early summer may extend 80 to 160 km inland during the afternoon (complex orography can complicate this somewhat). Under very favourable circumstances the sea breeze may penetrate as much as 200 to 300 km inland by about midnight, and on rare occasions to 400 km. The sea breeze is detected by its maritime characteristics, such as lower temperature, higher humidity, and as well as by actual tracking of the wind change.

Sea Breeze Development

With weak general wind circulations (around the centre of a high for example), a sea breeze (pure sea breeze) will commence over the coastline soon after the land temperature begins to exceed the sea temperature (late morning to early afternoon). As the difference increases, so the sea breeze will become stronger and will extend farther inland. It will also increase in depth from about 100m to as much as 450 to 800 m in a well developed breeze. Maximum wind speed (14 to 16 kt) will occur during the few hours after maximum temperature has been reached. This would generally be during the mid to late afternoon.

A weak sea breeze will die away soon after sunset, but a better developed sea breeze will persist at the coast till 8 to 10 pm., usually dying away fairly suddenly at the coast and slowly moving seawards as a cut-off sea breeze circulation.

With stronger general wind circulations, coupled with the required temperature gradient, the development of the sea breeze can be complicated to say the least. Obviously a moderate to strong , prevailing off-shore surface wind will delay the onset of the sea breeze and if strong enough, say over 20 kt, will prevent the sea breeze from developing at all.

On-shore surface winds are generally enhanced by the sea breeze component. On the other hand, if we have a light to moderate prevailing off-shore 900m (900m above the earth's surface, commonly known as the Gradient level) wind, with say a direction from the northwest through to the north, then we can generally expect a re-enforced sea breeze at the surface-Sydney's super sea breeze. In this situation, our strongest sea breezes will occur on the south coast of NSW, with speeds well in excess of 30 kt.

The rule is that 900m wind directions from offshore tend to favour sea breeze development, whilst those onshore tend to either complicate the sea breeze or make sure that we don't get a sea breeze at all.

Wind speeds either at the surface or at 900m in excess of 20 kt will generally keep a sea breeze out, providing that speeds don't drop out over the late morning or early afternoon period.

Sea breeze Direction

It is found that a sea breeze will generally start up as a light onshore surface wind, at roughly right angles to the coast, before moving in a counter-clockwise (southern hemisphere) direction as the breeze increases in strength.

The start-up and final sea breeze direction (and speed) will depend on the orientation of the coastline as well as the complexity of the coastline (steepness, headlands, bays, estuaries, etc).

Cloudiness

On cloudy days convection currents over the land (due to solar heating) are prevented or restricted. Thus the sea breeze circulation does not develop to any appreciable extent, and on many occasions of cloud cover will be non-existent.

Building Your Weather Skills - Recommended reading by Frank Rendell Endevour Class Association

Wind Terms

The wind is a continuous succession of gusts and lulls associated with equally rapid changes of direction over a range which may exceed 30°. The mean wind speed over a period of time is therefore the mean of many gusts and lulls. Usually only the mean wind is forecast, unless the gusts are expected to be a significant feature. For instance, Fresh, gusty southwest winds indicates that the mean wind speed will be between 17 and 21 knots and the mean wind direction will be from the southwest, but that there will also be gusts to speeds significantly higher than the mean.

Gust: A gust is any sudden increase of wind of short duration, usually a few seconds.

Squall: A squall comprises a rather sudden increase of the mean wind speed which lasts for several minutes at least before the mean wind returns to near its previous value. A squall may include many gusts

Wind descriptions (derived from the Beaufort Wind Scale)  Wind speeds are given as the  equivalent speed at a standard height of 10 metres above open flat ground

Sea and Swell

Sea Waves: Waves generated by the wind blowing at the time, and in the recent past, in the area of observation.

Swell Waves: Waves which have travelled into the area of observation after having been generated by previous winds in other areas. These waves may travel thousands of kilometres from their origin before dying away. There may be swell present even if the wind is calm and there are no 'sea' waves.

Wave Period: The average time interval between passages of successive crests (or troughs) of waves.

Wave Height: Generally taken as the height difference between the wave crest and the preceding trough.

Wave Length: The mean horizontal distance between successive crests (or troughs) of a wave pattern.

Port Phillip Bay

Detail overview of Port Phillip Bay Winds: Follow this link

Conclusion

It would certainly pay the keen yachts person to spend a great deal of time studying the sea breeze as it can be quite complex on any one day . This article hopefully whets the appetite and a further article will home in on the complexities of the sea breeze.

References:

Batt, K, 1995: "Sea breezes on the NSW coast", Offshore Yachting, Oct/Nov 1995, Jamieson Publishing.
Bethwaite, F, 1996: "High Performance Sailing", Waterline Books, UK
Houghton, D, 1992: "Wind strategy", 2nd edition, Fernhurst Books.