A Little Theory
As the sun heats the land a horizontal pressure gradient is created, initially at a few hundred feet above the surface. This results in a flow of air from land to sea at those heights and a consequent return flow at sea level. The warming leads to lower pressure over land than over the sea. A light off-shore wind, by encouraging the outflow, helps rather than hinders the process. A light on-shore wind may prevent or delay the sea breeze.
The sea breeze usually sets in more or less directly onshore ie from sea to land. However, the Coriolis effect soon comes into play and, before too long, the sea breeze will veer to be nearly along the coast. An island, such as in the
Balearics, can create, in effect, its own low pressure area and the afternoon winds will tend to behave accordingly ie cyclonic around the centre of the island,
For the sea breeze to be able to develop, a shore needs to have a fairly low coast line. With all their concrete and masonry, large conurbations heat up quicker than grass land and give a stronger sea breeze effect as anyone sailing dinghies in Tor and Weymouth Bays will know.
Waiting near Hurst Castle to go up to the Hamble, I have seen a morning ENE Force 3/4 turn around to a WSW 3/4; at about noon. What looked like being a beat turned into a pleasant spinnaker run.
Alan Watts' book "Wind Pilot" has a description of the sea breeze process and four supplements dealing with a detailed climatology around Europe and the Mediterranean.
Veering Sea Breezes and the Vente Solaire
The Vente Solaire is a daily cycle of sea breeze and land breeze. A good example occurs between the Vilaine river in Southern Brittany and the isles of Houat and Hoëdic. In settled weather with a high pressure area to the north, the night wind is off the land. During the morning, heating of the land starts up the sea breeze more or less in an opposite direction to the night time land breeze.
Someone deterred by the sea breeze from anchoring to the west of Houat might seek shelter on the eastern side only to have a disturbed night in the land breeze. The western side would have been better after the sea breeze died away!
Figure 1.
When formed by a low lying coast, a sea breeze is likely to veer.
Figure 2
A night time wind can be reversed by the sea breeze. In the Vente Solaire. the sea breeze first veers then, in the late afternoon dies away. The offshore wind strengthens due to the land breeze effect.
Topographic effects
A very well known example of the sea breeze being affected by topography is around the Isle of Wight. Here the sea breeze can set in on to the mainland shore driven largely by the heating of Southampton. Because the sea breeze does not easily flow over the Isle, it will be deflected around the ends and flow up the Solent from both ends. This gives the well known curiosity of two yachts running under spinnaker towards each other somewhere off Cowes, only to start beating as they pass each other. See Figure 3.
Along the Devon coast my experience is that the sea breeze will set readily into Plymouth Sound and Tor Bay where the built up areas heat up quickly. The sea breeze will occur but less readily into Bigbury Bay and Start Bay with their fairly low coast lines but little built up areas.
The sea breeze will not blow directly onto the steep cliffs between Bolt Head and Bolt Tail, between Dartmouth and Berry Head nor onto the Babbacombe heights on the north side of Torbay. However, the strong heating of Torquay and Paignton pulls air from around Berry Head and, from the opposite direction, past Babbacombe. Returning to the Dart from Tor Bay can be a stiff beat in such conditions.
Figure 4 show this effect that, again, is clearly shown when two yachts approach each other from opposite directions across Tor Bay. Somewhere towards the north side, nearer to the Ore Stone than to Berry Head, they drop spinnakers, start motoring before beating in opposite directions. The same can happen off Plymouth Sound where a yacht running past the Yealm and the Great Mew Stone will meet one running from the direction of Rame Head.
Figure 3
Schematic of sea breeze being deflected around the Isle of Wight.
Figure 4
Schematic of sea breeze being sucked into Tor Bay around the cliffs of Babbacombe and Berry Head. Between the Dart entrance and Berry Head the sea breeze can easily reach force 4 or 5.
When does it occur and how far out?
Careful study of wind observations do, in fact, show that the sea breeze can be detected, albeit very light, even in December and January at favoured locations.
How far out to sea the sea breeze extends depends on the amount of heating. I have found the effect up to 12 or 15 miles off the coast in the
A very good picture of this effect can be seen on another page of this site. The separation between the two flows is an area of very light winds. Alan Watts calls this Channel Starvation.
Spanish oddities!
I put this down to the relatively near coast of north Africa. I suspect that the African sea breeze sucks all the air away from the Costa del Sol and prevents marked sea breezes on that coast. A similar effect occurs on the south coast of Jersey where the sea breeze forms but is then reversed by the sea breeze into the Bay of St Malo. The mechanism was probably like that shown in the example above - see another page. Off the Costa del Sol, this separation must have been very near the Spanish coast.
Along the east coast of Spain, almost regardless of the morning wind, the sea breeze usually set in around mid afternoon, typically after about 1600 or 1700 local time. However, in terms of "God's" time (i.e. UTC) this is only
Sometimes the Spanish east coast sea breeze lasted for two or three hours and reached a good F 6 or even 7. The effect of significant headlands, such as Cabo San Antonio, can be quite important in strengthening the breeze locally. At other times it died out fairly quickly and only just got to a F 4.
It is not immediately obvious why this should occur, but if the pressure is high then the air will have been subsiding and this can kill or weaken the sea breeze. Alan Watts has suggested that a clue can be had by looking at the western horizon around sunset. Subsidence causes a temperature inversion ie a lid that inhibits convection and the sea breeze. This can be evidenced by a light blue strip caused by dust and pollution trapped at the inversion. This takes very good and careful observation - beyond my abilities!
And elsewhere - some random notes
To the east of Levkas Island, the island sea breeze can be sufficiently strong on the eastern side to counter the prevailing NW winds. Down the east coast of Italy, see above, the sea breeze seemed to be a very regular occurrence.
A very large scale sea breeze effect helps to maintain the strong northerly winds to the east of Iberia - the Portuguese Trades. Down the coast of Croatia, the large scale night time katabatic gives predominately NE winds at night but the warming of the land gives a NW wind during the day. This can be seen in GRIB code output. Try UGrib.