Sea Breezes

In my humble opinion, a keen grasp of how and why sea breezes work is essential for sailors. The coastal zone in fair weather is the environment where most sailors spend their time at sea.

Why does a sea breeze develop?

Sea breezes are the result of differential heating between land and sea. On a cloud free morning in the summer months the land heats up faster than the seas. The diurnal temperature range can be up to 100C or more, whilst over the sea the diurnal temperature range in much smaller, typically 1-20C. The difference in these temperature ranges is due to the different heat capacities of the land and sea.

Land warms up more quickly, but retains less heat than the sea which warms up more slowly and retains more heat. As the sun shines on the land it becomes hotter, convection currents distribute heat throughout several thousand feet above the land. The sideways expansion of the air produces changes in pressure which are transmitted sideways at the speed of sound. The resulting pressure difference at low levels is responsible for the onset of the sea breeze. Therefore, the sea breeze is caused by a pressure gradient at low levels (below 1500 feet), though the distance the sea breeze blows may extend inland and seaward considerably throughout the day. The region where the colder sea air meets the drier land air is termed the sea breeze front and is similar to a mini cold front.

Sea breeze clouds

At the boundary between the cooler, moist sea air and the warmer, drier land air there is a convergence zone and air is forced to rise. The ascending air may reach condensation level and on a clear day this may cause a line of clouds to form in an otherwise cloudless sky. On a day when the sky is full of fair weather cumulus the sea breeze front can be observed as line of greater cloud development. The sea breeze front can also make itself apparent by the formation of ragged fragments of cloud in the moist sea-air. The moist sea-air has a much lower condensation level than that of the land-air, so the ragged fragments of cloud can be seen below the fair weather cumulus.

Strength and direction

Sea breezes start blowing at right angles to the shoreline. During the day, in the northern hemisphere, they gradually veer and by sunset are blowing nearly parallel to the coastline. This change in direction is primarily due to the earth¡¦s rotation. However, the development of sea breezes along coastlines with headlands and bays is more complex.

Headlands produce areas of convergence which result in stronger sea breezes. On the Cherbourg peninsula in France, for example, a line of cumulus clouds indicate the convergence zone formed by the sea breezes blowing in different directions around the peninsula. Initially, the sea breezes will be blowing at right angles to the coast but during the day the sea breezes will show continuous change in direction as they blow towards the main land mass further inland. Therefore on the left hand side of the peninsula the sea breeze would back, showing anticlockwise rotation demonstrating that sea breezes in the northern hemisphere do not always veer throughout the day. In contrast, a bay produces an area of divergence and a lighter sea breeze.

Islands provide further complications in ascertaining the direction of sea breezes. In the UK, the Isle of Wight can act as a barrier to the low level sea breeze in the Solent. However, as the sea breeze circulation develops during the day the breeze can flow over the top of the island.

Forecasting a sea breeze

Sea breezes usually develop in the summer months when the daytime land-sea temperature difference is at its greatest. This is the most important criteria that must be fulfilled for a sea breeze to occur. The temperature difference is important in relation to the strength of the gradient breeze. For example, on a calm day a temperature difference of 10C is large enough for a sea breeze to form, but to over an offshore wind as strong as 8ms-1 a temperature difference of 110C is needed.

However, the magnitude of the excess temperature seems to have no bearing on the time of onset of the sea breeze or necessarily the strength. The timing of the sea breeze is strongly related to the gradient wind strength. The lighter the wind, the earlier the sea breeze will establish itself. The direction of the wind i.e. offshore or onshore and the relative angle of the wind to the coast will also affect the development of a sea breeze. An ambient offshore breeze will intensify the sea breeze by concentrating the horizontal temperature gradient, whereas an onshore ambient wind will weaken the circulation.

Another factor that is thought to affect sea breezes is the state of the tide. Tidal areas where there is a considerable amount of land exposed by the retreat of the water. An un-flooded tidal flat has been known to cause the sea breeze circulation to be more diffuse.

Land breezes

As the daytime heating from the sun decreases in the afternoon the strength of the breeze decreases and eventually dies away. A similar phenomenon called a land breeze can establish itself during the very early hours of the following morning. The land, as well as, heating up more quickly than the sea, also cools faster than the sea. Therefore, the sea can become warmer than the land, especially in calm conditions when there is little cloud coverage. Thus a difference temperature difference is established opposite to that that generates the sea breeze. Land breezes most commonly occur in the autumn when the sea reaches its maximum temperatures.

Forecasting sea breezes can be notoriously hard, particularly in areas with a disturbed coastline. Other areas can be more straightforward. Perth on the west coast of Australia for example where the diurnal temperature rise is huge, you can set your watch by the time the sea breeze arrives. Locally the wind is known as the ¡¥Fremantle doctor¡¦ and arrives at 1:20!
THE LEARNING ZONEWoodmans Cottage, Boxhill Rd, Tadworth, Surrey, KT20 7LF. Email: info@winningwind.com