Being Weatherwise for Sailing (Part 4)

Many people are quick to surmise that the weather forecast must be wrong if on your short stretch of water it does not do exactly as they suggested on the national weather! However, if you take the time to study, learn and understand some of the many local effects often felt around our coastline, you'll soon see that the forecaster has an almost impossible task of portraying every weather event which is forecast to happen that day.

Sea breezes are a classic example of just such a case, especially those on a complicated coastline.

Take for example a bay which occurs on the end of a peninsular, such as Hyeres, France where the sailing team was a few weeks ago or Salcombe in Devon. Under slack synoptic conditions, such as the centre of a high or in a col (the area between high and low pressure systems), sea breeze can form on all coasts.

As the land heats up, the air above it rises causing pressure to fall. Air is drawn in off the sea to replace this rising air, and a return flow exists aloft with this cooler air sinking out to sea, the whole process creates the sea breeze cell. During the day this onshore breeze shifts right, looking upwind, due to the turning of the Earth (not as commonly misquoted to follow the sun).

Many sailors will be familiar with this coastal weather feature, however you must always remember to think about the land around your venue, not just the area you can see. Which coast will be the dominant coast?

As shown on the figure, sea breezes on other coast lines may be larger and therefore stronger and more dominant than the small local sea breeze within your bay which may form late morning...

This will then be replaced mid to late afternoon by another sea breeze. We had such a situation in Hyeres, where a local southerly sea breeze tried to form late morning but then a south westerly forced its way onto western courses, whilst a south easterly blew onto eastern courses, with a battle zone in between. This situation continued for much of the day, with the unusual situation of neither of the sea breezes winning and taking over the whole bay.

If there was a north westerly gradient wind, the south westerly would have been stronger, and if there was a NNE gradient wind the south easterly may have forced the south westerly wind back. An offshore gradient wind of less than ~18 knots can drive the offshore flow of the sea breeze cell (B)