Avgeek Alert: Not Gone with the Wind – What Pilots Need to Know, Part 1


It seems obvious that whoever flies an airplane needs to know perfectly how the flow of air behaves in the atmosphere. But it may be less obvious that it´s also important to understand some of its main properties and effects as well as the forces which act on the wind, thus allowing a better understanding of how it affects aeronautics.

First of All, What Exactly Is Wind and Why Does It Exist?

The natural movement of air in our Earth´s atmosphere is closely associated with atmospheric pressure, meaning it moves due to pressure differences. As we saw in our previous post on isobaric formations (LINK), the surface temperature determines the air´s density of the air. There are also atmospheric circulation patterns which determine its flow, such as dynamic cyclones or anticyclones. In short, air always circulates from high pressure areas to low pressure areas.

But for the purposes of air navigation, it´s more important to be aware of changes in pressure on a given route. This parametre is called pressure gradient, and it´s of interest to pilots because it´s what determines the speed of the wind. In our post on isobars we saw that when lines very close to each other indicate that the wind is moving at high speed (a lot of pressure difference between two points), unlike when they are very far apart (little difference, calm wind).

4 Forces Which Act Upon the Wind

Euler Wind

Pronouced ¨Oiler¨ and named after an 18th-century Swiss scientist and engineer, this type of air flow runs solely due to pressure differences – and in one direction only, toward low-pressure areas. But since it doesn´t take into account the Coriolis Effect, centrifugal force, or frictional force, this is mostly a theoretical model that can´t occur in the real world. For example, if the Earth were not rotating, according to the Euler wind pattern air would always flow from the poles, where the surface temperature is colder, towards the Equator, where it is warmer.

The Coriolis Effect

We already covered this in a previous post, (LINK) a phenomenon generated by the rotation of the Earth and which affects the duration of flights. If we could launch an object from the North Pole towards the Equator, instead of drawing a straight path the constant rotation of our planet will visually make that path appear like a curve.

For practical and air navigation purposes, what happens is that the wind in the Northern Hemisphere will always deviate to the right, so a north-to-south wind will veer west and if it flows from south to north, toward the east. In the Southern Hemisphere it´s exactly the opposite, with wind always deviating to the left.

It is also interesting to know that the Coriolis effect acts more strongly the further we are towards the poles, while at the Equator its strength is zero.

Geostrophic Wind

This model is also a physical approximation of wind in the real world, meaning it uses theoretical models to explain the actual movement of air.

Geostrophic wind combines the effects of the pressure gradient and the Coriolis effect. And here there is a progressive and interesting effect in which the force of the pressure gradient will try to take the wind from a high-pressure to low-pressure area. But the Coriolis effect executes a force in a 90-degree direction (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere).

Thus the wind will progressively deviate towards the direction marked by the Coriolis Effect, which will once again mark an angle of 90º with the new direction of the wind – until the strength of the pressure gradient and that of the Coriolis effect are absolutely opposite; at that point the wind will mark a constant path.

Buys Ballot´s Law

Taking its name from a 19th-century Dutch meteorologist, this is a rule of meteorology that allows us to always know in which direction the wind blows. It holds that if we stand with our back to the direction of the wind in the Northern Hemisphere (that is, what would be the tailwind on airplanes during a flight), the high-pressure area will be on its right and the low pressure on its left. In the Southern Hemisphere, given the Coriolis effect, the opposite would happen: high pressures would be to our left and low pressures to our right.

But there are other effects that act in the direction of the wind and which we´ll discuss in an upcoming post – so stay tuned!