Understanding the sea breeze: Part 1
It’s the beginning of October. Winter ended a month ago and summer is two months away, yet already there’s a feeling of transition. Aside from the rising temperature the most obvious change is the wind: the persistent south-westerly regime of winter is relenting and in its place is the coastal seabreeze - warmer, gentler, more localised. Outside of the tropics, seabreezes characterise the Australian coast during the warmer months.
But what causes the seabreeze and how does it affect surfers?
Seabreezes, as their name implies, only affect the coastal margins of the continent. Rarely will they travel more than 100 kms inland, although Canberra, 120 kms from the coast and 600 metres elevation, will feel the occasional sea breeze during summer.
The genesis of the seabreeze is a temperature difference between coastal land and the adjacent ocean. In spring and summer, the sun heats the land during the morning while the ocean remains the same temperature. As the land heats up, the air above it rises, as all hot air does, leaving a void underneath that needs to be filled, and this comes in the way of the relatively cooler air from over the ocean - the sea breeze.
Though they're common features of the spring and summer seasons, sea breezes are generally stronger during late spring and early summer when the difference between land and sea temperature is at its greatest. A big disparity between hot land and cool water amplifies the effect. In Sydney, by Christmas time sea surface temperatures have usually risen enough to temper the strongest sea breezes though they'll still blow till autumn.
When a seabreeze is fully established it moves in a cycle: Hot air over land rises and cooler air from the ocean moves in to replace it, while in turn the hot air cools and moves out to sea where it descends and then once more blows toward land with the seabreeze.
The seabreeze will often only be felt up to approx. 300 m above sea level, while the air blowing back out to sea - called the return flow - is above 600m. The strength of the return flow is always less than the seabreeze below it.
A fully established sea breeze will move in a cycle with a return flow back out to sea
Though the dynamics are the same, seabreezes come from varying directions depending upon the alignment of the coastline. When the seabreeze first begins it blows directly from sea towards land, however due to the Coriolis Force the wind moves counter-clockwise as it strengthens, so what we have in Sydney is a nor-easter, in Perth and south-west WA a sou-wester (the Fremantle Doctor) with each coastline in between having varying angles of seabreeze.
The Coriolis Force comes from the spinning of the Earth. It’s the same force that makes low pressure systems spin clockwise and highs anti-clockwise. In theory, wind should blow directly from an area of high pressure to low pressure as the atmosphere tries to find equilibrium. This would mean wind blows directly across isobars. However, the Coriolis Force intervenes and ‘knocks’ the wind to the left (in the southern hemisphere) so that synoptic wind roughly follows the isobars.
The same force is applied to seabreezes though on a much smaller scale.
With that in mind, it’s worth thinking about the area over which a seabreeze works. As stated earlier, a strong sea breeze can push over 100 kms inland. It’s easy to record this from the numerous weather stations on land. Harder to record is how far out to sea the sea breeze extends, and this is important as it has a direct effect on surfers.
You see, the further out to sea a sea breeze extends, the longer it takes for the swell it creates to reach the coast. And as most seabreezes slow down during the evening and stop before midnight, much of the swell - albeit small windswell - is hitting the coast during the unsurfable hours before sunrise.
Yet if we know a seabreeze has developed further out to sea then it means its influence will remain for longer during the surfable daylight hours.
Anecdotal evidence from yachtsman suggests the nor-easter seabreeze at Sydney can extend up to 60 kms out to sea. In Perth it may extend further, the reason suggested is that the Great Dividing Range limits the scope of the seabreeze on the East Coast while in WA there are no topographical barriers. It’s felt further inland, and it starts further out. Again, that remains anecdotal.
But however far out to sea a sea breeze extends there are ways for surfers to maximise its effects. The most important is to understand that while it may only extend 60 kms out to sea, that distance is measured in a straight line while the wind blows at an angle to the coast. Some parts of the NSW coast can have fetches over 100 kms, meaning that a jumpy little windswell at home acquires new dimensions with a drive to somewhere with favourable alignment.
Mid-January 2015, Whippy surfs a jumped up nor-east windswell (Tim Bonython)
The only downside, however, is timing. Seabreezes are notoriously short lived. As soon as the breeze stops blowing, the waves start dropping, so you don’t want to be driving when you should be surfing. Fortunately there’s an (almost) surefire way to predict when a sea breeze will blow. If during spring and summer you wake to find light dew on the grass then the sea breeze will blow that afternoon. It’s not a guaranteed method but then that’s what the BOM’s for. We live in the Information Age after all.
All of the above applies to periods when the seabreeze isn’t unduly affected by synoptic winds. At times, the seabreeze can either be nullified or amplified by the prevailing synoptic wind. This topic is covered in Part 2.