Sailing below the coast: how does the wind behave? (Episode 1)

When sailing near the coast, you always have to deal with wind, currents, tides (if sensitive) and sea conditions that are often directly related to the coastline profile and its topographical features.
In this article we will discuss what to expect when passing close to the coast (high or low), shortly in the next one we will address islands and river estuaries.

Low coasts

Low and sandy shorelines, steep cliffs, deep estuaries or islands and islets create different sailing conditions, wind and currents change significantly.
The wind undergoes major deviations whether it is synoptic, thus “gradient,” related to the weather situation of vast geographical areas and pressure differences, but also in the case of a simpler local thermal breeze, from the sea or land.
The actors acting between the solid forms of the earth are two: sea and wind, two fluids that both obey the physical law discovered by the Italian physicist Giovanni Battista Venturi, whereby “The pressure of a fluid current increases with decreasing velocity, and vice versa.”
In summary, for what concerns us most closely for this article: the velocity of a fluid’s flow increases when it is forced through a bottleneck.
Coastlines, when low and sloping, clearly have little effect on wind force and direction, since they are not a significant obstacle.
However, it is often observed that when the wind blows from the coast towards the open sea, it is deflected slightly to the right as soon as it reaches the sea, where the friction is less than on land and the Coriolis effect becomes more sensitive (Coriolis force is a force caused by the Earth’s rotation: a moving body will be moved to the right in the northern hemisphere and to the left in the southern hemisphere. The Coriolis force is zero at the equator and increases in magnitude toward the poles).
In some casesthe deviation is up to 25/30°. Personally, the places where I have noticed this effect the most have been sailing in France, in the parts of La Grande Motte and Sète (near Montpellier), and in Italy in the Upper and Middle Adriatic.

High ribs

The wind undergoes consistent changes near the coast when the coast is high, even if it consists of reliefs of varying size, not only if it is a classic sheer cliff.
If the wind trajectory is oblique to the orientation of the coast, there are two possible scenarios. When the wind blows slightly toward land, it is deflected to follow the coastline while increasing its strength. When the wind blows slightly toward the sea, it is also deflected to follow the coastline, but this time with a decrease in strength (and Mr. Venturi, or rather his law, is always responsible).