SEA BREEZES ON THE NSW COAST

By Kenn Batt

Nature and Causes

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.

Canberra not infrequently experiences a sea breeze in summer evenings (Canberra is approx. 120 km inland and at an elevation of around 600 m). The further inland, the weaker and less identifiable the sea breeze becomes, and the shorter its duration.

Sea Breeze Development

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.

This super sea breeze around the Sydney area will have wind speeds in the range from 20 to 25 sometimes 30 kt. It will stay in longer at the coast, say until around midnight, sometimes 1 am, before becoming cut-off and moving away from the coast.

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

A pure sea breeze will by mid-afternoon, over the Sydney area for example, have a direction from the northeast, whilst a re-enforced sea breeze would have a direction more from the north-north east.

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 the other hand a well established sea breeze may contribute to cloud formation, of two kinds in particular:

• large cumulus and cumulonimbus (thunderstorm clouds) which require an inflow of moist air in the lower levels. In fact, the sea breeze can provide the moisture to help form thunderstorm clouds. Once these form on the ranges to the west of Sydney, these clouds are generally steered toward the coast. Once these clouds get close to the coast, the sea breeze decreases in strength, whilst its direction will veer towards the east. Very soon after this, the sea breeze falters, we enter a short calm period before being hit by wind squalls out of the storm clouds. So the moral of the story is that the sea breeze can lead to its own demise.
• low stratus (scud), in the late afternoon as the moist sea breeze air becomes cooled and is also lifted slightly over the land. Alternatively the inflow of moist air in the lower levels may result in widespread fog over the coastal plain later in the night.

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.