wind Archives - Kitesurf Portugal

These are winds that occur and are regularly observed in a certain place due to certain local characteristics (usually the terrain).

What is the difference between a local wind and ‘just wind’?

The main features of a local wind are its regularity and the limited area where it is observed.

Trade winds, for example, are observed regularly, but in a huge area, so they are not ‘local’. ‘Just wind’, the direction of which is reported to us every day in weather forecasts, lacks regularity and can change its direction every day.

The most common example of a local wind are Breezes. During the day they blow from the sea, and at night, on the contrary, towards the sea. This happens regularly, plus they occur in a limited area, on the coast – so these are local winds.

Today we will only discuss coastal local winds, and in the following letters we will talk about other winds!

Coastal local winds

Breezes:

Breezes occur when there’s a land and water boundary (e.g., the sea).

During the day, the land heats up faster than the sea. The air above warms up too, becomes light and rushes up. Cooler air from the sea immediately replaces it, heading towards the land.

In other words, during the day, the breeze blows from the sea to the land. The night breeze blows in the opposite direction, as the sea stays warm while the land cools down quickly at night.

The breeze is a weak (1-5 m/s) and slightly cool wind. Any coast is suitable for it.

EXAMPLE: MISTRAL

Mistral.

But local winds can also be very strong, with suitable conditions only in a particular part of the coast.

For example, the mistral. It can be very destructive and only occurs on the Mediterranean coast of France. Winds such as the mistral occur in other places too, but there they are called differently (bora, nord).

It is a cold and very strong wind. Its speed often exceeds 66 km/h (41 miles per hour). The mistral is responsible for an unimaginable number of broken masts and yachtsmen’s failed plans… Besides, in spring and summer the mistral comes to the region very dry and causes significant damage to agriculture.

Sometimes it lasts only one or two days, often several days in a row, but it can last even more than a week. In Marseille, in southern France, the mistral is observed most often – 175 days a year! It is the local characteristics of the region that make the wind strong and destructive. How does the mistral appear?

On the shores of the Mediterranean Sea alone there are more than a dozen different local winds, both warm and cold. But not only yachtsmen, sailors or coastal residents face them. This topic is also relevant for those who come to the mountains – winds there also have their distinctive features. And of course, we will tell you about these local winds too.

Text: Windy.app team

Cover photo: Unplash

Today we’re going to talk about trade winds.

Trade winds are constant winds that blow all year long in the same direction, i.e. from tropical latitudes to the equator.

The term comes from the early 14th-century word «trade» that meant «path» or «track».

Due to their constancy and strength trade winds greatly simplified the movement of ships coming from Europe to the New World in the Age of Sail.

Why do trade winds maintain their direction throughout the year?

Let’s recall how heat is distributed across our planet. The Earth receives heat from the sun’s rays. The closer the angle of incidence of the sun’s rays is to the right angle, the warmer the Earth gets.

Rays fall at right angles on the equator, but at the poles, they go tangentially. Therefore, during the year, the equator receives more heat, and the area of the equator is the warmest on our planet.

The heated surface heats up the air as well. When it heats up, it expands, becomes less dense and therefore lighter, and goes up.

While warmer (and therefore lighter) air is surrounded by colder (heavier) air, it has the ability to rise. This process is called convection.

Now we have to recall what atmospheric pressure is.

Atmospheric pressure is the force with which air presses on a point or surface. In other words, it is the weight of the ‘atmospheric column’ that is the weight of the entire atmosphere, from its upper boundary to the surface.

Important! Weight is a force, not to be confused with mass. The weight of an object can change even if its mass does not change.

For example, the weight of your hand on the scale without additional pressure will be different from the weight of your hand if you put additional pressure on the scale, although the mass of the hand is the same in both cases.

The same goes for the atmosphere – the weight, i.e. air pressure on the surface, depends not only on its mass.

Yes, when we are climbing a mountain, the total number of air molecules above us decreases (and thus the air mass above us), and therefore the pressure in the mountains is less than at the foot of the mountains.

But the pressure is also affected by air movements in the atmosphere. If the airflow is directed from top to bottom, it additionally pressures the surface and the pressure is high, if there are no vertical movements, the pressure depends only on the air mass, and if the air rises, the pressure on the surface will decrease.

Let's return to the heated surface at the equator:

upward movements of air above the equator (convection) lead to a decrease in pressure there. Therefore, a low-pressure area forms at the equator.

Rising air at the height of about 10-15 km spreads out from the equator towards the poles. As it moves, this air loses its energy and descends in the tropics. As it descends, the air puts additional pressure on the surface. The downward movement of the air forms an area of high pressure in the tropics.

The last thing we have to recall is that the wind always blows from high to low pressure areas. This means that the wind is directed from the tropics towards the equator all year round.

On a non-rotating planet, trade winds would be directed strictly from high to low pressure areas. But on the Earth, their trajectory is influenced by the Coriolis force, so they twist a little clockwise in the Northern Hemisphere and counterclockwise in the Southern.

They meet around the equator. The convergence zone of trade winds is called the Intertropical Convergence Zone (ITCZ, you may have seen the abbreviation).

What happens in this zone? Winds are constantly colliding there, which means that there is always a «surplus» of air that is displaced upwards.

Remember us saying that the rise of air at the equator is caused by heating? It turns out that there is an accompanying mechanism of vertical movements at the equator: when trade winds collide, a «surplus» of air is formed and displaced upwards.

The air rises and as a result, the pressure at the equator drops. Rising air at the equator often results in cloudy weather and heavy rains.