Knowing the wind is essential for pilots when planning a flight: it allows them to choose the take-off runway, establish the fastest route and avoid possible turbulence.
In this post, we explain everything related to wind in aviation: how it is produced, what types of wind there are, or why planes take off against the wind.
It also includes an explanatory video that we have made especially for you, so… Don’t miss it!
How does the wind arise?
Wind is produced by the difference in pressure between different points in the atmosphere.
This difference in pressure is called the force of the pressure gradient, and causes air to move from areas of high pressure to areas of low pressure. Moreover, the greater this force, the greater the wind speed.
There is also another force, the Coriolis force, which affects winds at height and causes them to move to the right in the northern hemisphere.
When the Coriolis force and the pressure gradient force equalise, the geostrophic wind arises.
Types of surface wind
In aviation, we make a clear distinction between surface wind and wind at altitude. The surface wind is the one that occurs up to 3000 feet, or about 900 metres, while the wind at altitude is the one that occurs above that altitude.
In the lower layers of the atmosphere, the wind changes its behaviour depending on the obstacles (geographical features) in its path. However, at high altitudes, the air is free to move from one place to another.
In addition, at the earth’s surface, due to friction, the wind speed is reduced.
A sea breeze is a meteorological phenomenon that occurs during the day near the coast or large bodies of water.
As the land heats up faster than the water, the air above the surface tends to rise first, thus displacing the cold air above the water.
Sea breezes are more intense than land breezes.
In contrast to the sea breeze, the land breeze is a meteorological phenomenon that occurs close to the coast or large bodies of water but, in this case, it originates at night.
The temperature of the water is higher due to its high calorific value, which means that the air above it tends to rise first this time.
In mountain areas, especially in valleys, there is another characteristic type of wind: the anabatic wind.
This occurs when the sun heats the air in the lower part of a valley, causing it to become less dense and therefore tends to rise uphill.
In contrast, the katabatic wind originates at night, also in mountain areas.
The low temperatures, together with the force of gravity, cause the air to move towards the lower parts of the valleys, giving rise to strong temperature inversions.
The katabatic wind is stronger than the anabatic wind.
How wind is measured in aviation
At airports the wind is measured by using anemometers for speed and wind vanes for direction. Direction is indicated in degrees and speed in knots.
In addition, there are usually windsocks at the runway so that pilots can check the wind visually.
On the other hand, control towers give wind direction with reference to magnetic north, while charts and METARs give wind direction with reference to true north.
Wind charts are reports that tell pilots the different wind speeds and directions according to altitude. In Spain, these charts are produced by the Spanish Meteorological Agency, AEMET, and are valid for three hours.
In this type of chart, wind direction is represented by an arrow, while wind speed is indicated by lines: the smallest indicates 5 knots; the largest, 10; and the triangle, 50.
Wind in METAR reports
METARs allow pilots to know the wind direction and intensity in near real time.
In METARs, the wind usually corresponds to the third group of characters: the first two numbers refer to the direction and the last two to the speed. For example:
LEMG 181100Z 16004KT 9999 SCT025 17/12 Q1021 NOSIG
The above METAR corresponds to Malaga airport and indicates that we have 4 knots blowing from 160º.
In addition, in the case of winds with variable direction, it will be indicated below with values separated by the letter V. For example:
LEAS 181100Z 08008KT 050V120 9999 FEW015 BKN020 10/07 Q1030 NOSIG
This METAR belongs to Asturias airport, where they have 8 knots with a predominant direction of 080º, although the direction is variable between 050º and 120º.
Why do aircraft take off against the wind?
In general, an aircraft, like a boat, prefers a following wind to push it towards destination and reduce travel time. But when it comes take off and landing is a different story.
An aircraft taking off with no wind.
An airplane, like a kite, doesn’t fly due to speed in relation to the ground, but due to the speed of air flowing over the wings.
This is called the ‘Sustaining Principle’ and, yes, it refers to the fact that the air sustains the weight of the plane to keep it in flight. Doesn’t that seem incredible?
An aircraft taking off with the wind.
When taking off with a headwind it slows down the plane in its acceleration respect to the ground, but increases the flow of air over the wings, allowing to take off in a shorter distance and climbing in a greater angle in order to clear any obstacle.
An aircraft taking off with a headwind.
And to make it easier for you to understand, we propose a simple mental exercise. Imagine that you are a passenger in a car and you put your hand out the window.
If you leave your arm loose, the force of the air against it will lift it effortlessly. However, as soon as the car stops and there is no more headwind, your arm will fall off and if you want to keep it up, you will have to do it yourself. Isn’t that logical?
Here’s the video explaining why planes take off in a headwind, which we’ve created especially for you. We hope you like it!