The orography of the land surrounding the sea influences the behaviour of masses of air around it. It deflects the wind and generates phenomena associated with the Coriolis effect through backing and veering.

On this page you will find:

1.     A video with Tomás Molina on the influence of orography in the backing and veering of the wind.
2.     A synopsis of the video.
3.     A case study: One of the effects of orography in the Gibraltar Strait with a video and synopsis.

1. The influence of orography on the wind

2. Video synopsis

Land (the coastline, the ground, the mountains) acts as an obstacle in the wind's path. The interaction between land and the wind generates different changes in wind state. The most obvious is a change in wind speed and direction. Both phenomena are influenced by the Coriolis effect (see the Coriolis effect in Module 1.2).

Two concepts that you must know: veering and backing.

Veering is the deflection of the wind in clockwise motion; that is to say to the right.

Backing is the deflection of the wind in an anticlockwise motion; that is to say, to the left.

Important examples of where orography is an important factor as a weather engine:

A) Backing by deceleration (Northern Hemisphere)

Two case studies:

A.1 A case of winds blowing parallel to the coast

The wind that blows over land loses speed due to the friction with the ground, which is greater than on water.

With less speed than the winds that blow over the sea, there is less Coriolis effect and (in the Northern Hemisphere) it is diverted to the left (backing).

This means, if the coast is to the right of the wind (figure 1.2.A), the flow of current of the diverted wind tends to converge with the winds further out to sea whose direction is not altered, as they experience no changes in speed.

A convergence phenomena occurs at some distance from the coast, which we will look at in more detail further ahead (Module 1.2.3)

A.2 A case of winds blowing perpendicular to the coast

Here we have the same situation: the wind comes into land and slows down (in the Northern Hemisphere) and moves to the left (backing).

As a consequence, the flow of current tends to curve towards the left as it get closer to the coast.

This situation explains the differences between “land side” and “sea side” in coastal navigation. This is a tactical situation that must be taken into consideration for inshore regattas.

3. A case study: One of the effects of orography on the Gibraltar Strait.

Veering by acceleration: the Strait of Gibraltar

In straits the effects of the deflection of of the wind by the Coriolis effect are highly visible.

In straits the wind will speed up in the centre due to the Venturi effect.

A typical case is that given in the Gibraltar Strait when a westerly or easterly wind blows.

As the wind speeds up it moves to the right (veering)

For that reason, a phenomenon of moist advection forms (we will study this further ahead in Module 2.1.3), with cloud accumulation to windward of the coast, which is left to the right of the wind (figure 1.2.)

Recommended links
The Strait of Gibraltar