1.5 Wind and waves

The wind, a mass of air moving from a high pressure to a low pressure area, in addition to determining the intensity of the surface wave motion. Wave action is one of the environmental conditions affecting life in the sea. Waves are caused by wind acting on the sea’s surface. Wind waves range in size from small ripples (Fig.1_sez1.5) to waves over 30 m high (Fig.2_sez1.5).

Although they are visible on the water’s surface, they affect life below the surface, too. For example, a wave that reaches about 2 me high above the surface can be felt up to 6 m below the surface. Waves cleanse organisms and help to increase oxygen availability (Video1_sez1.5).

The waves created by the wind drag the more superficial plankton and it is followed by predators as basking shark that it moves with open mouth near the surface to eat the plankton. The wind can determine the life or death of the jellyfish that are dragged towards the shore and risk to beach and dehydrate.

Velella velella (Linnaeus, 1758) is a free-floating hydrozoan (Fig.3_sez1.5), which lives in the surface of the sea. It is also known as the by-the-wind-sailor and little sailor due to their small stiff sail that catches the wind and propels them over the surface of the sea. Under certain wind conditions, thousands of them can be washed up on beaches. They are blue-coloured and most are less than about 7 cm long. The stinging tentacles are used to sting and capture their prey and are harmless to humans.

In the Mediterranean basin, depending on the area, it is possible to identify, according to the direction of origin, the particular winds, which then compose the famous “Rosa” (Fig4_sez1.5), also carriers of specific meteorological conditions. The winds of the sea are classified in constants (they blow all the year with the same direction and the same direction), periodic (reverse the sense periodically), local and cyclonic (they blow irregularly).

The eight Mediterranean winds have been set with the Crete Island as a reference point and since ancient times they are important for sailing.

TRAMONTANA
It is the North wind, it comes from the polar regions is a cold and dry wind. It blows in a gust, usually brings clear skies, excellent visibility and dry weather. It usually lasts two to four days.

GRECALE
It blows from the North East, it is a cold and dry typically winter wind. In the Adriatic regions it generally has perturbed time, while in the Tyrrhenian ones it leads to a clear sky.

LEVANTE/ LEVANT
It is a fresh and humid wind of weak intensity blowing from the east. In the Mediterranean area it is accompanied by rain and storm. It is called this because it comes from the East, then from the Levant.

LIBECCIO
It is the wind that blows from the South West, comes from Libya. It is a cold and unstable wind that causes the temperature to drop several degrees. Winter or autumn when it manifests can generate storms.

SCIROCCO/ SIROCCO
The sirocco is a very hot and dry wind that blows on the Mediterranean from the South East, it comes from the North African hinterland, from the Sahara desert. It brings rainfall and mists

MEZZOGIORNO
It is a wind of southern origin with a very weak effect, it brings rains and storms. Also called Ostro, takes its name from the hemisphere from which it comes, the Austral.

PONENTE/ WESTERLY
It is the afternoon wind that blows from the west, summer, it originates from the temperature difference between land and sea.

MAESTRALE/ MISTRAL
It is the main of all winds, typical of the central Mediterranean blowing from the North West with a speed that can exceed 120 km per hour. It is a dry wind with a duration of less than 24 hours up to 4-5 days. Bring storm with violent storms, and announce the arrival of winter.