More on Sea Breezes

By Kenn Batt

Now that we are full bottle on the pure sea breeze, let's have a look at sea breeze behaviour under the influence of gradient wind flow - or the flow created by the large high and low pressure systems we can see on a weather map by looking at the isobars.

Following on from the work performed by Alan Watts, John Simpson and a few others back in the 50's, 60's and 70's, mostly in the UK, David Houghton, again from the UK, came up with the idea of explaining sea breeze behaviour by referring to the gradient wind direction and speed, which he called the Sea Breeze Quadrants. Frank Bethwaite of Australia, took Houghton's work and applied it to the Australian region as well as to a number of locations around the world especially during the 70's and 80's (Frank's excellent book "High Performance Sailing", published by Waterline, is a must get book). Houghton proposed that gradient winds blowing offshore at the level of the upper return flow assisted the sea breeze circulation (unless they are so strong that they completely stop the sea breeze), and gradient winds blowing onshore tended to work against the upper return flow and are associated with either no sea breeze or a pseudo-sea breeze flow.

We will now look at the Sea Breeze Quadrants as they apply to gradient wind directions and hence sea breeze behaviour over coastal areas of NSW. Following Bethwaite, gradient winds from between the south and west over the NSW coast are classified as Quadrant 1 (Q1) winds.

A Q1 gradient wind, between south and west, will generally see the sea breeze start up a little later, early to mid-afternoon, than the Q2 case. It won't be as strong, typically peaking at 15 to 18 kt, and will leave the coast about 1.5 hr after sunset. This situation can be messy at times with a calm zone that can oscillate to and fro over the coastal zone, which includes Sydney Harbour. This can lead to a day where there could be a sea breeze operating few kilometres off the coast with an offshore wind inside of the sea breeze, but separated by a calm zone roughly parallel to the coast which could be some 300 to 500m wide.

Q2 conditions are responsible for the best sea breezes. This breeze will start off in a very similar way to that described in the first article ("Sea breezes along the NSW coast") but will attain, in summer, maximum speeds of between 20 and 25 kt by mid-afternoon, somtimes stronger. By late afternoon the direction of the breeze will be more parallel to the coast (say from the NNE or around 020 deg. True) than for the pure sea breeze. A Q2 breeze will last a lot longer at the coast, often up til 2300 hr, before leaving the coast and moving out to sea.

Surface winds produced under the influence of both Q3 (between east and north) and Q4 (between south and east) gradient winds aren't true sea breezes but nevertheless they still blow onshore and affect our sailing!

The Q3 surface wind will essentially be the result of the gradient or isobar-generated wind. This is a broader scale wind flow that will have a smallish sea breeze vector added to it during the afternoon. This means that the wind that is blowing first thing in the morning will typically increase by approximately 5 knots during the middle of the afternoon due to the sea breeze effect.

The Q4 surface wind will generally commence as an isobar-generated wind which will have added to it the sea breeze vector during the late morning or early afternoon. This results in the wind direction being shifted to the left or anti-clockwise progressively during the afternoon. This wind very rarely attains a true sea breeze final direction, but can get close to it. Its speed generally hovers around 8 to 12 knots.

When using this technique, care must be taken to observe changes in the weather pattern that can change the quadrant of the gradient wind.

The above situations hold for gradient wind speeds less than 25 knots and un-complicated coastlines. Speeds greater than 25 kt will generally make sure that a sea breeze does not develop to any great extent. Complicated coastlines, including harbours, lead to a rich diversity of sea breeze behaviour.

Just because the sea breeze has developed it does not mean that it will remain intact! Near the coast, any cloud building up over the land, especially thunderstorm clouds, will generally see the breeze falter. A weak cold front moving into your area during the course of the day will weaken the sea breeze or will make sure that the breeze does not build to the expected speed. A sure sign that the breeze is faltering is a speed decrease coupled with the direction moving more right or clockwise.

By now we hope that you can see that the sea breeze circulation is complex and this means that you should be spending a lot of time in logging its behaviour under a variety of conditions in the areas where you sail most often.

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