Aviation Weather: Wind

Wind plays an important role in aviation; the movement of an aircraft is greatly affected by the wind and its severity. The weather is also dependent largely on the wind and its circulation in the atmosphere. Wind movement is caused by pressure differentials between different geographical regions over the Earth, and also due to the Earth’s own rotation. In simplest terms, the horizontal movement of the air over the surface of the Earth is called the wind. It is expressed in terms of the direction from where it is blowing, along with its speed or strength. The direction coupled with the speed is known as the Wind Velocity.

Measurement of the wind

Near the surface of the Earth, the wind is affected by friction with the surface and other obstructions or ground features. The Anemometer is the instrument that is used to measure the surface wind velocity; for getting accurate readings, it is placed in open areas along with wind wanes near the airports and runways. The most common types of wind measuring Anemometers are Cup Anemometers; however, Pressure Tube Anemometers are also used to calculate winds and they work on the same principle as the Pitot tube in an aircraft. Both these types of instruments have certain limitations and lack complete accuracy in some conditions. The wind direction is always mentioned with reference to True North; whereas the wind speed or velocity is expressed in knots. The upper air winds are measured through wind balloons and other methods.

Layers of Winds

Aviators generally differentiate winds into three basic categories as:

  • Winds below 2000 feet above ground level
  • Winds at 2000 feet above ground level
  • Winds above 2000 feet above ground level

Winds at 2000 feet is taken as the differentiating criteria since it is the level where the winds are free from the effect of surface friction of the Earth. 2000 feet is also the approximate level for the universal pressure distribution unaffected by the geography of the landscape below. At and above 2000 feet the Geostrophic Force and the Pressure Gradient Force affect the wind. The difference in pressure between areas cause the Pressure Gradient Force, and the Earth’s rotation causes the Geostrophic Force. The Geostrophic Force deflects the objects moving over the surface of the Earth, and is due to Coriolis Force.

Coriolis Force is a resultant force caused due to the rotation of the Earth on its axis. The Coriolis Force has no effect near the equator, but tends to deflect objects at higher latitudes, to the right in the northern hemisphere and to the left in the Southern Hemisphere; when viewed from the equator.

Wind Considerations during Flying

A pilot should be aware of the affects of head wind, tail wind, and the cross wind on an aircraft during all phases of the flight; however, takeoff and landing are the most critical phases for considering the wind. All Takeoffs and landings are planned into the headwind, the tolerance for a tailwind takeoff or landing in most aircraft is minimal. Similarly, each aircraft is limited by a certain severity of the cross wind factor that it cannot exceed during takeoff and landings, otherwise the safety of the aircraft may be impaired.