Where Does Wind Come From?

From a teeny little breeze that might tickle some leaf on a tree, to a hurricane lifting an entire wall of a building: wind is part of every day life for humans, but especially for aviators. This is why this topic is so high on the future topic request list. For any flight, it could be the difference between life and death. Yes that sounds like I am being a bit of drama queen, but it is actually true! Plenty of accidents have happened because of poor wind awareness, or downwind approaches, both in the fixed wing and rotary wing industry. Not understanding wind, or not knowing how to deal with it, is an overlooked principle. This is why today, we are going to zoom into what wind is exactly, as it dictates most of our days of operating in the air! Take a moment to reflect on just how many ways wind has an impact on the way we fly:

  • Groundspeed
  • Correction angles
  • General Performance
  • Hover Controlability
  • Take Off Distance Required
  • Wind shear
  • Vortex ring
  • Power required
  • Approach directions
  • Landing Distance Required

All the more reasons why we should dig a little further than “what is our wind direction today?” We will be taking things from the absolute root and will work our way up to practical principles. There will be some already known stuff for some, but also unknown or long forgotten stuff for others. What can I possibly talk about regarding wind for an entire article? Well buckle up, there is plenty you should know:

  • Why does wind even exist?
  • What is Geostrophic Wind?
  • What is Coriolis Force and why does it happen?
  • What is Gradient Wind?
  • How does friction affect wind?
  • Some pilot vs wind mental gymnastics

WHY DOES WIND EVEN EXIST?

Now you’re asking! Let’s not take the origins of wind for granted. Instead, we should have a look at what drives not only wind, but also the rest of the weather we sometimes have this love-hate relationship with as pilots. The fact we have weather in the first place is quite remarkable, but how far can we zoom out to fully understand it?

Well let’s start here: You could say wind is simply air flowing from an area of high pressure, to an area of low pressure. Imagine I have a room full of air and attach it to a room with almost no air at all, with a closed door in-between. If I open the door, air will flow from the room with lots of air (high pressure) to the room with almost no air (low pressure) until the pressure is fully equalised.

On earth though, with constant rotation, warming up, cooling down and other effects we will cover today, this “equalisation of pressure” never stops as it is never finished equalising! It’s like running a marathon while the finish line gets pushed further and further. Urgh, how infuriating, be glad you’re not wind!

So now the core question here becomes: how do these pressure differences even happen, why would all of the air on earth not have the same pressure, nice and easy? To answer this, we need to talk about convection. As the sun heats up certain areas, temperature increases. This is where all the drama (mostly) begins:

Different surfaces heat up quicker than others. Soil for instance warms up quicker than water, due to its greater heat capacity (it also maintains that heat for longer). Heat capacity simply means the amount of energy you need to warm up a specific material by a certain unit of temperature. This warming of soil makes the air above it warmer than it surroundings, which causes it to rise. Why? Because the air’s density reduces. Why? Because of this very important law that thermodynamics gave us, modified to our atmosphere (I can go into how to derive it in a future article). Behold: the Ideal Gas Law.

This thing basically says that the pressure of any gas, times its volume is equal to the temperature of a gas times it’s gas constant. A gas constant is a somewhat harder to understand variable but essentially describes the amount of energy that is required to increase the temperature of a substance.

Dividing by volume on both sides gives:

We can rewrite this formula, as volume is very poorly defined in something as big and dynamic as our atmosphere (good luck trying to accurately measure atmospheric volume). Instead, we can use rho, which is mass per volume (density). We know that density is inversely proportional to volume. Imagine a room with a certain amount of air and we make the walls close in, reducing volume. This would increase the pressure and therefore the density. This is why we can assume 1/V = rho. This is a little oversimplified to the scientists in the audience, but we can elaborate on these in the future, for now, remember this formula:

Looking at this formula, we can now understand why temperature influences density. If we want to separate rho, we need to divide both sides by R (the gas constant) and T (the temperature). Considering R for dry air is a constant value, the only real ‘variables’ that density depends on, are pressure and temperature.

If we increase temperature in this formula (as per our example), the total density value will reduce: the air will become lighter and will therefore start to ascend. This rising of air will, in it absence, leave an area of low pressure behind on the surface. This low pressure will drive a force called the Pressure Gradient Force (PGF), sucking air towards itself, just like the example with the 2 rooms and the door. The opposite will happen with cooling air. It will descend, creating an area of high pressure near the surface and driving air away from it. Tadaaa, wind!

That is the situation seen from the side. But let’s have a look from the top now. Low pressure areas have a PGF acting inwards, while the high pressure areas have a PGF acting outwards. Keep in mind that the amount of gradient (the spacing between isobars) determines wind speed. So the bigger the pressure difference per area, the bigger the wind speed. Remember this one, this is our first (and easiest) wind vector: PGF.

THE CORIOLIS FORCE

Up next, the one that so many people find difficult to understand: the Coriolis Force (CF). This one is a little trickier, but not to worry, we will break it down step by step to make you REALLY understand it.

First imagine a circle, and we give it a whirl so it starts spinning anticlockwise (doodle below). We will look at 3 different points along this circle: A, B and C. As the circle spins 1 rotation, all 3 points have made a full rotation in the same time. But as can see, point C has to travel a LOT further than point B in 1 rotation. The only way point C can accomplish this, is by having a velocity that is a lot higher than point B. Got this so far?

Now this disc represents earth, spinning counterclockwise as viewed from the north pole, which is point A. Point C is the equator and point B is somewhere around let’s say 60° latitude. What we’ve just seen, is that the closer we are to the equator, the greater our velocity is.

So imagine a cloud being generated at the equator (point C). It’s just being a happy cloud enjoying it’s day, but then, due to a PGF we saw earlier, it gets sucked northbound towards point B. The cloud has obviously got the same velocity as point C, as it’s made there. So as it travels northbound, it KEEPS this velocity due to inertia.

What will happen now? Well, it is now flying over land with a LOWER velocity than itself, so the cloud will go FASTER than the ground below: it will look as if it deflects towards the EAST (overtakes the ground). Keep in mind that it is not actually ‘turning’, it just looks that way from someone who is comparing it go the ground, looking from above. It is actually just following a straight line, we just don’t see it that way as we turn with the earth.

The opposite applies to a cloud created at point B. If the PGF sucks it towards the equator (point C), it will then start flying over land that is going eastbound faster than itself, as the cloud maintains the velocity that was present at point B. This makes it look as if the cloud is turning towards the west. This is what it means when literature states Coriolis is ‘not a true force’: It’s only real depending on what you are referencing it to (in this case the earth’s surface). It is also called an ‘apparent’ force.

In summary: Clouds moving AWAY from the equator will always push ahead eastbound (moving faster than the ground below them). Cloud moving TOWARDS the equator will always fall behind (moving slower than the ground below them). This applies to both the southern and northern hemisphere (as the picture below shows).

So coming back to the PGF top perspective, we can now apply Coriolis Force (CF), and you can now see why exactly that little swirl happens. Keep in mind that the following doodles all represent pressure systems in the Northern Hemisphere (the white circles mean areas of equal pressure, also called isobars).

If we merge both the PGF and CF vectors together, we get:

LOW PRESSURE

NORTHERN HEMISPHERE: Anti clockwise

SOUTHERN HEMISPHERE: Clockwise

HIGH PRESSURE

NORTHERN HEMISPHERE: clockwise

SOUTHERN HEMISPHERE: Anti Clockwise

Most of us all heard of Buys Ballot’s law, which you might remember from your ATPL’s: on the Northern Hemisphere, with the wind in your back, the low pressure area will be on your left.

It is also worth remembering that CF is proportional to both wind speed and latitude. If either increases, CF will increase. Why? Here is why, have a look at the equation to calculate the coriolis force:

Wait what? Why is rotation of the Earth even a variable, surely that’s static? Nope, due to friction, the Earth is actually slowing down very slowly. On average, days last 2.3 milliseconds longer every century. While that won’t impact any of our lives, it definitely is a variable. One day there won’t be any Coriolis Force at all, but let’s just say that is ‘not as’ relevant for any of our lifetimes.

So that’s it, we have now covered the effects of CF on PGF. Ready for what the result is called when we merge them together further away from the centres of pressure areas?

THE GEOSTROPHIC WIND

So if we take the PGF and add the CF, around an area with straight isobars (so away from the centre of a pressure area), we get what is called the Geostrophic Wind. This is the wind that blows along the isobars. As air is being accelerated by the PGF from high pressure to low pressure, the CF also increases (remember?). As the CF always acts 90° to the wind direction, and the wind progresses from high to low pressure, the wind will align itself more and more with the isobars. Have a look below at what that eventually looks like (notice that the blue arrows are the same, while the green arrows increase in size as the air accelerates). This continues until PGF = CF and makes the wind direction parallel to the isobars eventually.

The tighter the isobars are, the stronger the Geostrophic wind is. There are a few requirements though for the wind to turn completely Geostrophic:

  • Latitude greater than 15° (otherwise CF is too small)
  • The pressure situation is not changing rapidly
  • Isobars are relatively straight and parallel
  • Wind is above the friction layer (more on this later)

Why the 15 degrees you ask? Well, if you remember the CF equation from earlier, it has the Sine of the latitude as a multiplication in it. If we take the north or south pole for instance, which both have a latitude of 90°, we will get sin(90°), which is 1 (therefore the maximum amount of CF). Filling in a latitude lower than 15° gives less that 0.25, which essentially divides the entire CF by 4. Any further reduction in latitude and you end up with almost no CF. It’s all about that angular distance away from the equator, got it? Onto the next one!

THE GRADIENT WIND

So the Geostrophic wind was what happened further away from centres of high and low pressure, as it requires relatively straight isobars. But what does the wind situation look like when it gets blown around a low or high pressure area with not-so-straight isobars? This wind is called Gradient Wind, and is mainly different to Geostrophic Wind due to the fact that Centrifugal Force is going to be introduced here. We now have:

  • PGF
  • CF
  • Centrifugal Force
  • The resulting wind (Gradient wind)

In the picture below you can see wind around a low pressure area (left) and a high pressure area. We will keep things at the Northern Hemisphere for now, which means low pressure has an anti clockwise rotation, and high pressure has a clockwise rotation. We now have 1 extra variable due to these circular shaped isobars that are closer to the low or high pressure centres: centrifugal force.

Centrifugal force is the force keeping water in a bucket if you swing it around. It acts away from the center of the circle of rotation. This centrifugal force also acts away from the centre, but there is a difference in how it affects the wind speeds for low and high pressure areas.

Why? Because for low pressure areas, PGF acts inwards, the opposite to centrifugal force. They are basically fighting each other, PGF speeds up the wind (as we have seen before), but centrifugal force then tries to slow it down. We therefore have a gradient wind that is slower than the geostrophic wind here.

For high pressure areas the centrifugal force acts still outwards, but the PGF does too! The result? The gradient wind here will be FASTER than the geostrophic wind:

This principle is the same on both the northern and southern hemisphere, as PGF still acts inwards for low pressure areas, and outwards for high pressure areas, with centrifugal force always acting outwards. Gradient wind is faster around highs compared to lows.

FRICTION

Over to friction then. Most of us know that as air gets assessed near lower altitudes we need to include friction. But how exactly does friction work and affect wind direction? Have a look at the picture below. The low is on the left, high on the right. Under geostrophic or gradient conditions, the air blows almost parallel to the isobars.

But now due to obstacles on the ground, the wind speed will reduce slightly due to the added resistance. This will then reduce CF, as CF matched itself with the wind speed, remember? Now, if CF is reduced, but it was counteracting PGF (as shown below), the result of this is that PGF will win this never ending tug of war and the wind essentially deflects towards the low pressure area.

On average, we can say:

OVER LAND

Wind is deflected towards the low by roughly 30° compared to geostrophic or gradient, and is reduced to about 50%.

OVER SEA

Wind is deflected towards the low by roughly 15° compared to the geostrophic or gradient wind, and is reduced to about 70%.

So to summarise, we start with PGF at the beginning, and end up with the actual wind blowing around at either high altitude, or low altitude, depending on the circumstances, with all the steps in between.

WIND CALCULATIONS

Ok so can we finally get some practical stuff dude? Yes you can. It probably depends a little bit on how much you hand-fly aircraft in IFR as to how comfortable you already are with the following concept. For the pilots who don’t and others in the audience, let’s just go over the basics on how to quickly and easily calculate what effect wind has on us while we fly around, so we can prepare and know what the required adjustments will be before starting a leg or an approach. This is all based on the famous 1 in 60 rule, which states that for a route of 60 nm, a 1 degree track error will push of off track by 1 nm.

Step 1 is to figure out the ‘maximum drift’ for the flight. When would we drift the most? When all the wind comes directly from 90° from the left or right. To make things simpler, let’s find out how many nautical miles we cover per minute. For us rotaty wing pilots, it is rather straight forward, as most helicopters have a cruise speed of roughly 120 kts IAS. Have a look at the table below if you fly something that is significantly faster and want a quick refresh:

  • 120 kts = 2 nm/m
  • 180 kts = 3 nm/m
  • 240 kts = 4 nm/m
  • 300 kts = 5 nm/m

Then, we can fill in the maximum drift equation using:

So as an example, if we have a 40 kt wind today, and our groundspeed today is 120 kts (therefore 2 nm/m), we can simply divide 40 by 2 and get 20° as our maximum drift today. This of course only applies if our wind enroute comes directly from our 3 or 9 oclock.

Step 2 then, is to figure out how that maximum drift applies to us. There are different ways, and if you prefer other ways to remember and apply this while workload is already high, please stick with that. I personally prefer to imagine a clock, with 0 – 60° as pictured below. The orange numbers are the degrees between our track and the wind, the green is how much of the maximum drift we apply to our track. So if our our track is 100° and the wind 160°, the difference is 60°. We therefore take all of the maximum drift (20°) and add it to 100° to get the required heading to stay on track. At 30° difference, we take half of it, etc.

CONLUSION

For this article I want to leave it here! There are loads of other areas we can zoom in to, such as wind shear, wind around mountains and obstacles such as helipads, demarcation lines and more, but those are for future articles. The main focus here was geostrophic wind, coriolis force and gradient wind, as those can be the trickiest for most. For now, if you have any future requests, comments, questions, or feedback, please leave them below or contact me directly.

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3 thoughts on “Where Does Wind Come From?

  1. Hi again

    Your coriolis explanation is exactly as I was taught and it has a big flaw. What about air which is moved exactly east or west by the PGF? It does not go up or down in latitude so doesn’t experience a change between the speed on the surface and the movement of the air. But guess what it still turns! Why?

    You be much better served with keeping your top down view. The first thing to note is that points on the surface move in a curve but the influence of the PGF works in a straight line. If you imagine dropping a breadcrumb trail as you move above the earth in a straight line, your breadcrumb trail would show you followed a curving line over the rotating surface. It doesn’t matter which way you travel.

    In simple engineering terms, it is the result a linear motion in a rotating frame of reference. Would love to see that explanation in your crisp diagrams!

    Liked by 1 person

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