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9.24 Winds - General

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Gradient and Surface Wind

Wind is often described by two characteristics: Wind direction and speed. Wind speed is the velocity attained by a mass of air travelling horizontally through the atmosphere. Wind speed is often measured with an anemometer in nautical miles per hour. Wind direction is measured as the direction from where a wind comes from. For example, a southerly wind comes from the south and blows to the north. Direction is measured by an instrument called a wind vane. Both of these instruments are positioned in the atmospheric environment at a standard distance of 10 metres above the ground surface.

Wind is described by the sixteen compass direction bearings. Thus, a wind from the north blowing toward the south is called a northerly wind. Most meteorological observations report wind direction using one of these sixteen bearings. However within aviation wind direction is expressed as a bearing using three figures, 050 or 250 or 005 degrees relevant from the true North Pole.

Wind is expressed in degrees coming from a direction, relative to the True North Pole and speed given as nautical miles per hour e.g. 090T 20kt or simply 09020.

Pressure Gradient Force

The pressure gradient force is the force that is usually responsible for accelerating a parcel of air from a high atmospheric pressure region to a low pressure region, resulting in wind.

The pressure gradient force acts at right angles to isobars in the direction from high to low pressure. The greater the pressure difference over a given horizontal distance, the greater the force and hence the stronger the wind.

The pressure gradient force, however, is not the only force that acts on a moving parcel of air — if it were, then low and high pressure regions would eventually disappear. Other forces acting on a moving parcel of air include friction and/or the Coriolis force.

Coriolis Effect

The earth has a circumference at the Equator of 40,075 kilometres. Since it rotates once every 24 hours, objects at the Equator are travelling at 40,075 divided by 24 = 1670 kilometres per hour. However at the North Pole, objects do not have any significant speed of rotation. In between the Equator and North Pole, objects have intermediate speeds of rotation. The Earth's Coriolis Effect arises because of the differences in the Earth's speed of rotation at different places.

In a non-rotating world an object could simply move from one place to another by accelerating in the direction required. This is our normal experience on the earth because the Earth's Coriolis Effect is small on a human scale and because we are usually firmly attached to the Earth. However over larger distances for objects not attached to the Earth, the effect is significant.

Consider a missile fired from the North Pole towards a place on the Equator. Since the Earth is rotating, the missile must also pick up the rotational speed of its intended destination, otherwise its destination will rotate away from under it as it travels south. Without a correction, observers on the Earth will see the object apparently being deflected from its destination. However the force producing this apparent deflection is an illusion; the earth merely rotates as the missile is travelling. Nevertheless an allowance would have to be made for this illusion when aiming the missile.

The scale of the effect can be seen by taking the distance from the North Pole to the Equator, which is a quarter of the circumference, roughly 10,000 kilometres. The missile, or any other object, must gain 1670 km/h divided by 10,000 = 0.167 km/h on average for each kilometre travelled south. This small difference in speed for each kilometre travelled is too small to be noticed by people attached to the surface of the earth. However weather systems occupy large areas and are not attached to the surface of the earth. Consequently the Coriolis Effect is an important factor in meteorology.

Like the missile in the Northern Hemisphere, a mass of air travelling south has less speed than the air at its destination and so appears to be deflected west. Conversely air travelling north has excess speed and appears to be deflected east. As the wind blows from all sides to fill an area of low pressure, the Coriolis Effect therefore creates rotation around the low pressure system. The winds around areas of low pressure circulate counter-clockwise in the Northern Hemisphere, while in the Southern Hemisphere this circulation is clockwise. The rotation produces the characteristic swirls that can be seen on satellite photographs of weather systems, and of hurricanes in particular.

A common fallacy is that the Coriolis Effect affects the rotation of water flowing through plug-holes. The water in a bath is less than one hundredth of a kilometre across. Consequently difference in the speed of the Earth's rotation from one side of the bath to the other is insignificant and cannot appreciably affect the rotation of the bath-water down the plug-hole. The rotation of the water is chiefly governed by the plumbing and random eddies in the water.

Although the rotation of the Earth provides the most obvious examples of the Coriolis Effect, it arises in other rotating systems. For example, someone standing at the edge of a rotating carousel could throw a ball to someone standing at the centre. If thrown without making allowance for the Coriolis Effect, the ball would not reach its target.

Air Flow around a Low Pressure Area

The force balance is largely between the pressure gradient force acting towards the low-pressure area and the Coriolis acceleration acting away from the centre of the low pressure. Instead of flowing down the gradient, the air tends to flow perpendicular to the air-pressure gradient and forms a cyclonic flow. This is an example of a more general case of geostrophic flow in which air flows along isobars. On a non-rotating planet the air would flow along the straightest possible line, quickly levelling the air pressure. Note that the force balance is thus very different from the case of "inertial circles" which explains why mid-latitude cycles are larger by an order of magnitude than inertial circle flow would be.

This pattern of deflection and the direction of movement represent Buys-Ballot's law. The pattern of flow is called a cyclone. In the Northern Hemisphere the direction of movement around a low-pressure area is counter-clockwise. In the Southern Hemisphere, the direction of movement is clockwise because the rotational dynamics is a mirror image there. Cyclones cannot form on the equator, and they rarely travel towards the equator, because in the equatorial region the coriolis parameter is small, and exactly zero on the equator.

Air flow around a low-pressure area in the Southern hemisphere. The pressure gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows. If a low pressure area forms in the atmosphere, air will tend to flow in towards it, but will be deflected perpendicular to its velocity by the Coriolis acceleration. A system of equilibrium can then establish itself creating circular movement, or a cyclonic flow.

Surface and Gradient Wind

Surface winds on a weather map do not blow exactly parallel to the isobars as in geostrophic and gradient winds. Instead, surface winds tend to cross the isobars. Close to the Earth's surface, friction reduces the wind speed, which in turn reduces the Coriolis force, resulting in the spiralling of air into the centre of a low pressure system and away from the centre of the high pressure system. This causes convergence in the centre of the low pressure system at the surface. It is this surface convergence which leads to rising air which can create clouds and even cause rain and storms to form.

Friction Layer

From the surface of the earth up to a height of 3000 ft is the friction layer. The wind speed and direction varies depending on the terrain of the earth and the height within the friction layer. In the friction layer over land wind veers away from the isobars up to 30 degrees and slows as much as 2/3 rd. Over water the gradient winds veers away from the isobars by 10 degrees and slow to 1/3 rd of its original speed.