As sea lovers winds can be our friends or enemies, filling our sails and helping us to our destinations, whipping up the waves into an angry sea, or being frustratingly light, or “on the nose”, making it difficult to go anywhere. Knowing what to expect from prevailing winds can help us to be prepared, but there is always that unexpected regional wind that can take us by surprise.
Wind is the flow of gases caused by differences in atmospheric pressure. The air moves from the higher to the lower pressure area, resulting in winds of different speeds. As ours is a rotating planet, the air is also deflected by the Coriolis effect. Winds blowing over mountain areas, through mountain passes, over desert areas, over the sea and even through wind farms, will all act differently. These are some of the winds you may encounter as you travel the seas.
The Tropics
The Trade Winds are found in the tropics, towards the equator and are the prevailing pattern of easterly surface winds. They blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, acting as the steering flow for the tropical cyclones that form over the world’s oceans.
A Monsoon is the name given to the seasonal prevailing wind in the tropics that lasts for several months. Usually it refers to the rainy phase of a seasonal changing of atmospheric circulation and precipitation. The major monsoon systems are those of the West African and the Asia-Australian monsoons. The name was first used in English to describe the large seasonal winds blowing from the Bay of Bengal and Arabian Sea which brought heavy rainfall to the areas of India, Bangladesh and Pakistan.
The Middle Latitudes
Westerlies are the prevailing winds in the middle latitudes (35 to 65 degrees). As their name suggests they blow from west to east and steer the extratropical cyclones which can produce anything from cloudiness and mild showers, to heavy gales and thunderstorms. They are predominantly from the southwest in the Northern Hemisphere and the northwest in the Southern Hemisphere and are strongest during winter when pressure is lower over the poles and weakest during summertime when the pressure is higher. They cause the development of strong ocean currents on the western sides of oceans in both hemispheres. The Westerlies can be particularly strong where there is less land to slow them down. The strongest are within a band between 40 and 50 degrees latitude, south of the equator, known as the Roaring Forties.
It was the Trade Winds and the Westerlies that provided a possible round-trip trade route for sailing ships crossing the Atlantic and Pacific oceans.
The Poles
Polar Easterlies are dry, cold, prevailing winds blowing from high-pressure areas at the north and south poles towards low-pressure areas. They usually blow from east to west and are often weak and irregular.
Natural Phenomena Which Affect Winds
Local Sea and Land Breezes can importantly affect a location’s prevailing winds. As the sea is warmed up by the sun, more slowly than the land, the warm air above the land is less dense, so it rises, causing the cooler air above the sea to flow inland. At night the land cools off more quickly than the sea, causing the pressure over the water to be lower than that of land, establishing an offshore wind.
Hills and Valleys can substantially distort airflow by increasing friction between the atmosphere and landmass and acting as a physical block, deflecting the wind upwards. Where there is high ground, the localised differences in the heating of the land can change the wind circulation of the region. If there is a pass, in a mountain range, the winds tend to rush through it with considerable speed and the airflow can remain erratic and turbulent for large distances downwind, often affecting an offshore wind over the sea. Strong, downslope winds flowing from high elevations of mountains, plateaus and hills are called Katabatic winds. These are caused when air in contact with high ground is cooled by radiation, increases in density and flows downhill and across the lower ground or sea. Some are referred to as Foehn winds.
Some Local Winds With Specific Characteristics
The Abroholus – Brazil. Violent squalls between May and August caused when the SE Trade Winds acquire heat while traversing the warm current of E Brazil.
The Barat – northern coasts of Indonesia. A squally, strong, west to north-westerly wind blowing during December to February. Known for its rapid changes in wind-speed and direction as it is deflected and channelled by numerous islands and gaps of the archipelago. Sudden violent gusts can cause considerable damage.
The Barber – North America. A severe wind known at sea for freezing the decks and rigging of boats. In the Gulf of St Lawrence it takes the form of a blizzard with wind-borne ice particles which can almost cut the skin.
The Bora – eastern side of the Adriatic Sea. A cold, dry and often gusty katabatic wind from the NE. Sometimes very localised, extending only a few miles offshore, while at other times dominating the whole Adriatic Sea
The Calima – Canary Islands. A hot, dust and sand laden wind blowing from high-pressure over N Africa and the Sahara. Sometimes the eastern Atlantic can be blanketed with a dense cloud of Saharan sand, which can reach as far as the Caribbean, or further and severely affect visibility.
The Khamsin – N Africa, E Mediterranean and Red Sea. Like the Sirocco it usually blows ahead of a depression. Its name comes from the Arabic word meaning fifty (the approximate number of days during which it blows).
The Levante – Spanish east coast. It flows from the E or NE and is funnelled through the Straits of Gibraltar. It is usually mild and humid along the SE coast of Spain and the Balearics, but can reach gale-force at the eastern Gulf of Cadiz.
The Meltemi – Aegean Sea. The Greek and Turkish name for the well known wind that blows from north or northwest across the Aegean. Numerous islands and mountain gaps channel the wind causing gusty jet-effects, lee eddies and local katabatic winds. The meltemi season is usually May to October and is associated with dry weather, clear skies, a drop in humidity, good visibility and a rise in atmospheric pressure.
The Mistral – Mediterranean coast. A strong, cold, dry and squally wind blowing offshore, often persisting for several days. It is stronger in winter and is increased by the funnelling effect of the Rhone valley.
The Poniente – Straits of Gibraltar and NW Mediterranean coasts. A warm, dry, westerly or northwesterly wind usually bringing hot, clear and dry weather.
The Quas – Persian Gulf. A southeasterly moderate to gale-force wind, most frequent between December and April. It is usually accompanied by gloomy, damp weather and squally thunderstorms.
The Sirocco – Mediterranean Sea and coasts. The hot and humid winds originating as dry air over N Africa, which flow north into the S Mediterranean basin. The winds have different characteristics depending on location and many different names. They have no favoured month or season, but the strong, gale-force siroccos are most common during spring, when they can last for 10 to 12 hours and occasionally up to 36 hours. Sometimes they bring hot, humid weather, with high overnight temperatures, causing insomnia and headaches.
The Thehuantepecer – S Mexico. A violent, squally north or northeasterly wind on the Pacific coast of S Mexico caused by areas of high pressure over the plains of N America extending downwards over the Gulf of Mexico and the Caribbean during winter. Strong, cold winds blow through three gaps in the Central American mountain ranges with wind speeds up to 50 knots and lasting up to a week. They tend to cool the sea surface quickly and produce gusty eddies that can be felt up to 160 km offshore.
The Tramonta (or Garigliano) – west coast of Italy. A cold wind from the north or northeast tinged with icy air from the Alps, associated with the advance of an anticyclone known as the “Genoa Low”. Blowing frequently in winter, but also sometimes from mid-September through to April, it may reach force 8 or even stronger during night and mid-morning. In winter it can pose a substantial wind-chill hazard to ship’s crews.
The Vendaval – Straits of Gibraltar and S Spain. A strong, blustery, wet, SW wind usually found during November to April and often accompanied by violent squalls and thunder.
The Williwaw – Strait of Magellan and the Aleutian Islands. The sudden violent, cold, katabatic wind descending from the mountainous coast. The name is of Native American origin referring to erratic gusts of wind. A williwaw gust poses constant danger for any vessel trying to sail around The Horn. Williwaw katabatic winds are sometimes preceded by a rapid rise in temperature, a warning that a sudden gust may whip up the waves into frenzy.
Author – Dee White