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Airport Windsock

Wind Origins

Pilots preparing for a VFR flight are expected to read and understand aviation weather maps, METARs, TAFs and interpret this information to assess if their flight can be successfully completed. We refresh some of these once learned subjects and discuss, among other things, warm- and cold fronts and the effect on the flight they have in terms of visibility and cloud base.

All of this while avoiding to become a meteorological textbook, there are enough websites doing that, so were are not going too. In the menu on the left you will find pages with weather maps, radar and satellite images suitable for aviation weather flight planning.

Aircraft move in a pocket of air, which itself moves in a certain direction, this is commonly known to us as wind. Here we are going to scratch the surface of the origins and certain effects wind develops during its movement over the surface.

Wind Dynamics

To get the big picture on how winds starts to and develops patterns of air movement we need to step back and look at how wind starts on our planet. The main energy source is the sun, it warms up the earth unevenly. Air at the Equator becomes warmer than at the North- or South Pole due to the higher latitudes and the lower sun angle, hence less warmth thus less expansion.

The result of this is that the atmosphere expands around the Equator and contracts at the Poles. And this will start (ir)regular air patterns around the globe due to its rotation. These are prevailing winds, pressure driven, seasonal and local winds.

Wind Direction & Speed

Pressure Isobars

Wind moves from a high to a low pressure area and the speed is determined by the pressure difference, see image (courtesy: WeatherOnline). When isobars are closer together, wind speeds are greater. It is expressed as a true bearing from the direction the wind blows in weather reports (ATC uses magnetic directions). Its denoted in knots (miles per hour) and measured at ten meters above the terrain as a ten minute average.

A wind anemometer, which are three cups at 120 ° angle mounted on a vertical shaft, and a wind vane are used to measure speed and direction. It must be kept some distance away from obstacles as these slow the wind down and create turbulent eddies resulting in gusts and lulls disturbing the measurement.

Gusts are rapid changes in speed and direction of short duration and squalls might last for several minutes. When the wind veers, its changes direction clockwise, wind backing is the reverse.

Wind Forces

The movement of the wind is subject to a number of forces which result in a deflection of direction and variation in speed. These forces are explained below.

Pressure Gradient

Air normally moves from areas of high to low pressure, the force making this happen is called the pressure gradient force. Due to friction of the surface of the Earth, more so over land (think obstacles) than over open water, the wind speeds slows and direction backs. This layer is some 2000 ft thick. This mostly happens above 15 ° latitude, below that the wind more or less follows the pressure gradient force.

Coriolis Effect

Coriolis Effect

The speed of the Earth at the Equator is higher than at the Poles. Lets say: if you were able to throw a ball from the North Pole to the Equator in a straight line it would land west of the intended point due south of your position. This is caused by the rotation of the planet. See image (courtesy: RWU Uni Press books). The curve is also increasing as the ball moves further south, because the speed of the Earth is increasing nearer to the Equator.

For a parcel of wind this means that when movement starts, from high to low pressure, by PGF and deflected by Coriolis the wind will eventually be Geostrophic or along straight isobars, at right angles to the PGF.

Geostrophic

Geostrophic wind is an imaginary wind that would result from an exact balance between Pressure Gradient Force (PGF) and the Coriolis effect.

Gradient Wind

This type of wind is blowing at a constant speed parallel to curved isobars above the friction layer (>2000 ft) and is a resultant of Geostrophic and Cyclostrophic effects.

Cyclostrophic

This is a force acting outwards like centrifugal force we all are familiar with, and it acts 90 ° to the motion/direction, thus to the left in the NH and to the right in the SH with curved isobars. This means that for the actual wind gradient the resulting force is from friction of the surface (reducing Coriolis), allowing PGF to deflect the air to the low pressure area.

As centrifugal force opposes PGF in a low pressure area (CF points outwards and PGF points inwards) the gradient wind has lower speeds. This effect is more pronounced in the tropics (< 15° Lat.) so that the Cyclostrophic effect could result into a cyclone when CF and PGF are equal and the wind blows along the isobars with increasing speeds.

With a high pressure system PGF and CF act in the same direction, outwards resulting in a higher gradient wind. At the lower latitudes where Cyclostrophic winds predominates the the high pressure area will dissipate more readily. The outward blow of the wind with the descending air from above can create stronger winds around a high pressure system.

written by EAI.



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