During the night the cooling land and the channelling effect of the Vilaine valley funnel the offshore wind and increases it to a good force 4 or so. But, as the land cools in the late afternoon and evening the sea breeze effect weakens and the wind reverts to the northwest around the high to the north. The sea breeze then becomes northwest and strengthens. It is the French term for the solar wind – a stream of particles in the stratosphere.) The name Vent Solaire used by many sailors is, in fact, a misnomer. This causes a weakening of the offshore wind. 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. In settled weather with a high pressure area to the north, the night wind is off the land. A good example occurs between the Vilaine river in Southern Brittany and the isles of Houat and Hoëdic. The “Vent Solaire” is a daily cycle of sea breeze and land breeze. We had a very marked example of this along the east coast of Italy between Vieste and Brindisi, see below. Shortly after setting in, it then veers (due to Coriolis) over a period of a couple of hours or so. With a fairly low lying coast, the sea breeze usually sets in straight onto the coast. Veering Sea Breezes and the “Vent Solaire” The ascent of air at this front creates a line of cumulus cloud. This difference is often referred to as a "sea breeze front". The air coming off the sea is cooler and moister than the air over the land. As we are well aware, the earth is spinning and the Coriolis effect causes air to be deflected to the right (in the northern hemisphere.) Buys-Ballot told us that pressure is lower to the left when standing back to the wind. Pressure differences create air movement as gravity takes effect and air tries to move from high to low pressure. Of course, the atmosphere is far more complicated than the simple concept of highs in cold areas and lows in warm ones. The Winter Siberian high is one example the cooler water in mid-Atlantic compared to further south leading to the Azores high is another. See my page on how pressure differences are formed. Conversely, cooling of the air leads to high pressure. On the large scale, this is the cause of Monsoons, Trade Winds, jet streams and the low that sits over Spain for much of the Summer half of the year. In a nutshell, warming the ground causes warming of the air above it and that leads to a decrease in the pressure at ground or sea level. This is true for winds on all scales from the global, monsoon, scale right down to what happens around Island Barn reservoir at Walton-on-Thames. These arise from the way in which the air is heated and cooled. Sea breezes, like all winds are caused by pressure differences – pressure gradients.
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