The Sea Breeze Effect
The sea breeze is an onshore thermal wind. It is generated when the air over land is warmed, expands and rises, then flows out seawards, cools and moves downwards again and replaces the air that starts to move across the shore.
The sea breeze needs several requirements for building. Not every thermal wind is also a sea breeze. Often thermal breezes are mistaken as a sea breeze which may lead to wrong decision on the race course as the characteristics of sea breezes and other thermal winds vary.
It normally starts with a sunny and bright day. Then the following happens:
- The air over the land is warmed more than the air over the sea and the sea breeze mechanism is able to start. A rise in temperature of 1-2° C above sea temperature is enough to start the process.
- The rise of air causes and excess of air at some higher level – between 300 and 1000 m above sea Level.
- From there the air flows out seawards to remove the imbalance.
- A few kilometres out over the sea the air sinks down again and pushes air across the shore. This is what the sailor experiences as the sea breeze.
- Later during the day the wind increases in strength and slowly turns right (left on Southern hemisphere) due to the Coriolis effect.
The Quadrant Theory / The influence of the Gradient Wind
The Quadrant Theory was developed by David Houghton, a British meteorologist, who was known as a weather legend. Over decades he supported the British Olympic team. With his Quadrant Theory Houghton describes the effect of the normal (gradient) wind to the sea breeze. Depending from which quadrant the gradient wind blows there is a higher or lower possibility for a development.
As described above the classical sea breeze starts perpendicular to the coast and then builds and turns right. In Quadrant 1 (Q1) at the start the normal wind and sea breeze component are opposite to each other. The sea breeze has to fight the normal wind at the surface. Most of the time this results in a period of no wind before the mechanism begins to develop. As the day progresses the gradient wind helps the circulation and a sea breeze is able to develop. At the same time the Coriolis force then drags the sea breeze component right. The sea breeze component falls more in line with the normal wind in Q1, and the two will enhance and increase each other.
In Quadrant 2 (Q2) the opposite happens: as the day progresses the sea breeze component and the gradient wind will fight each other more and more. As a result there will be very light wind or no wind at all.
Applying the same line of thinking in Quadrant 3 (Q3) as the day progresses the gradient wind becomes more inline with the sea breeze. There is a high possibility of a well developing sea breeze. The difference to Quadrant 1 (Q1) is that prior to the start there will not be a period of no wind.
Again in Quadrant 4 (Q4) the gradient wind is opposite to the sea breeze component. Therefore the two winds will not enhance each other. As a result the wind will be lighter than forecasted.
Note: Especially a very strong gradient wind (25 knots or more) from Quadrants 2 and 4 will prevent the sea breeze mechanism to start building.
Sea Breeze Facts
- On a sea breeze day there will be no wind in the morning. As the day progresses, the heating effect gets stronger. The earliest start of a sea breeze is mid day as the mechanism needs some time to develop.
- Normally you can sea the air rising by appearance of cumulus clouds over the land.
- The wind always clocks to the right the later it gets in the afternoon (left on the Southern hemisphere) due to the Coriolis effect.
- The breeze builds in strength during the day.
- There will be no sea breeze if there is a fairly strong gradient wind blowing onshore.
- There will be no sea breeze if the gradient wind is stronger than 25 knots.