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This article explores the origins of wind and the forces that drive it. We then apply this knowledge to weather maps, helping sailors to interpret surface-level wind patterns for better navigation insights.

We also explore vertical wind. Though not directly felt by sailors, it can influence conditions at sea. Finally, we examine global wind circulation and its influence on weather systems.

The wind is air movement caused by a difference in air pressure. The air is pushed from high to low pressure. If you punch a hole in a bicycle tire, the pressurized air inside will blow out until the pressure is equalized between the air inside the tire and the air outside. This is Mother Nature trying to balance itself.

Wind physics is similar. At the Earth's surface, where there is an area of High pressure and an area of Low pressure, air is pushed from the high pressure to the low pressure. This force is called the Pressure Gradient force" It is proportional to the pressure difference over a distance. So, the more the pressure diminishes over a short distance, the greater the force.

One would think that the wind would blow from High to Low in a straight line. However, this is not as simple as it seems. The rotation of the earth creates another force called the Coriolis force.

This force is not specific to the wind, this force applies to any moving object on a rotating system. To understand this, let's forget the wind for a moment and take the example of a carousel turning counterclockwise with 4 people sitting at the edges of the carousel facing each other. If one person throws a ball to the person sitting in front of him, the ball will not end up with the person he is aiming for because that person would have moved out of the way due to the rotation of the carousel. Instead, the person sitting at his right will receive the ball because it was deflected to the right by a force called the Coriolis force.

See the Coriolis force presented in this 25-second video:

The Earth also turns counterclockwise like the carousel, but it turns at a much slower speed, once in 24 hours. The consequence is that the Coriolis effect is not felt when we throw a ball. On Earth, the Coriolis effect can be seen in objects moving at high speed and/or over great distances (like sniper bullets and artillery shells) or in air circulation over great distances.

Let's apply the above example to meteorology and imagine a low-pressure system sitting at the North Pole:

The Earth is not flat like the carousel, and it makes the Coriolis force vary depending on the latitude. The Coriolis force is the greatest at the North Pole and deflects to the right. It is null at the equator. It is also the greatest at the South Pole but in the opposite direction, thus deflecting to the left. To understand this, one needs to imagine at each latitude a carousel cutting the earth at this latitude and needs to project the air speed onto the carousel plane:

Fun fact: if the earth were not rotating, the air would flow along the straightest possible line, quickly eliminating pressure gradients, and meteorology would be much simpler.

The wind is never blowing in a straight line, but instead, the wind curves or turns. The curvature of the wind creates a centrifugal force on the air particles that pushes them outside of the turn. This is the same centrifugal force that you experience when you take a fast turn in your car and your body is pushed to the outside of the turn.
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When adding those three forces — pressure, Coriolis, and Centrifugal — and assuming they balance themselves out, we refer to this theoretical wind as the Gradient wind. Assuming the Gradient wind, like the Geostrophic wind, blows parallel to isobars, the wind speed is either reduced or increased depending on the curvature.

When the wind turns around a Low-Pressure area, the Centrifugal force opposes the Pressure force. Therefore, the Gradient wind for cyclonic curvature is less than the Geostrophic wind (wind is typically overestimated by only considering pressure gradient force around low pressure).

The air particles experience friction at the surface (both over land and water, but more over land). This slows down the air particles. So Friction slows wind near the surface, resulting in a weaker Coriolis and Centrifugal force, but pressure force stays the same. The flow is therefore imbalanced with the pressure force "winning" and finally attracting more of the air particles. Therefore, wind is “deflected” in toward low pressure and away from high pressure. For sailors, it is important to know the following:

Since friction deflects air toward Low pressure, it means that the air will converge toward the center of the low pressure from all directions. The air will have nowhere to go but upward, and therefore, the air will rise (upward vertical motion) at the center of the Low.
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Since friction deflects air outward of High pressure, the effect will be the opposite, and the air will sink (downward vertical motion) at the center of the High.

This is summarized in the picture below:

In this section, we apply the theory of the previous section to weather maps for the Northern Hemisphere to describe the horizontal wind at the surface, which is key for the sailor.

The map below shows an isobar map at the surface. The brown lines are lines of equal pressure. Here are some key concepts you can see in the image below:

The image below is another example of a pressure map from Bracknell. Notice in the top left corner a scale to estimate the geostrophic wind at the surface.

The map below shows a Low pressure over land. The wind at the surface, coloured green in the image below, is not flowing parallel to the isobar but is deflected toward the center of the Low pressure due to the friction effect.

The map below shows an area of Low pressure over land. The wind at the surface, as indicated by the direction of the wind barbs in the image below, is not flowing parallel to the isobar but is deflected toward the center of the Low pressure due to the friction effect.

Clouds are a great indicator of vertical winds, and we recommend reading the article about clouds - Marine Meteorology: Clouds ($$$ insert help center hyperlink $$$).

The vertical wind and horizontal winds are linked. The vertical wind has an influence on the stability of the horizontal wind. This stability means a lot to sailors because if the horizontal wind is unstable, you will be sailing in shifty and/or gusty conditions.

The stability refers to the atmosphere's tendency to decrease or accelerate the vertical motion of the air. One important driver of the vertical wind is the Thermal driver.

The Thermal driver corresponds to the vertical temperature profile of the atmosphere. The main concept is that warm air tends to rise as it is less dense than the surrounding colder air.
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Let's take an air particle at the surface and increase its temperature by a few degrees compared to the surrounding air. This air particle rises. As it rises, the air pressure and the air particle temperature diminish. When the air particle reaches, for example, 10 meters high, if the air particle is still warmer than its surrounding, it will keep rising and even accelerate. On the other hand, if the particle finds surrounding air at the same temperature, it will stop rising. So we can remember,

Here are key takeaways for sailors sailing into a stable or unstable thermal atmosphere:

In this final section, we will apply all the knowledge we have gained about horizontal and vertical winds to explain global wind circulation over the earth. We will focus especially on the global wind circulation at the surface of the oceans, which sailors use to cross oceans with the wind.

The source of global air circulation is due to the fact that the Earth is heated unevenly by the Sun. The equator receives more heat and the poles less. The global air circulation acts as an air conditioning system, redistributing the heat from the equator to the poles. It is important to link air temperature and air pressure at this stage. Warm air is less dense, and it rises, resulting in lower pressure. On the other hand, cold air sinks, making the air pressure higher.

Warm air rises above the equator and moves to the poles at high altitudes. As it moves away from the equator, it cools and sinks. The cool air flows back to the equator at the surface, where it gets warmer again. This simplistic model has one such circulation cell for each hemisphere, which is represented below, comparing a hemisphere to a house.

However, the Earth is rotating, which causes circulation due to the Coriolis force. As the air moves away from the equator at high altitudes, the Coriolis force deflects the wind. When reaching 30° latitude, the wind is parallel to the equator and stops moving further north or south. This causes the air to sink at 30° latitude rather than at the pole. This results in three cells for each hemisphere, as shown below.

Zone A: Areas where the air rises are characterized by low pressure at the surface. Those correspond to areas with clouds and rain (e.g., equatorial forests and the northern UK).

Zone B: areas where the air sinks are characterized by high pressure. This is where the sky is clear with little rain (e.g., the Azores high, the Sahara desert and the Poles).

Those 3 cells generate winds at the surface that are also deflected by the Coriolis force.

Those winds (Zones C and D) are represented in the picture below for the Vendée Globe race.

To learn more, read on! In the next article, Marine Meteorology 3: Mid-latitude Weather, we explore Ferrel Cells (30° to 60° North or South). These areas are mid-latitudes and correspond to where most of the human population lives, making them very important weather-wise.