Frey Whether the Weather

Aviation Meteorology from A to Z
3. Auflage 2015
ISBN: 978-3-7392-7742-4
Verlag: BoD - Books on Demand
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 180 Seiten

"Whether the Weather" is not only for air sports enthusiasts such as paragliding, hang-gliding and ultralight pilots; it is also an invaluable meteorological guide for anyone interested in weather conditions. The most important safety element is making correct decisions before take-off, because misjudging the weather situation is a common cause of accidents. The correct decision is even more important than flying skills and requires a fundamental understanding of meteorology. Many pilots recognise this and want to learn more about meteorology, without going to a scientific level. "Whether the Weather" fills this gap from A to Z. On 180 pages with innumerable graphics, it explains with outstanding clarity from the most basic to the most complex processes in aviation meteorology.

Autoren/Hrsg.

Frey, Roger P.

Leseproben

3Wind

In meteorology, wind is defined as a physical process of moving air in the atmosphere. Wind is classified according to its force: up to 50 km/h (31 mph) we talk of a “slight to strong breeze”; from 50 km/h to 90 km/h (27 kn to 49 kn or 31mph to 56 mph) the word “gale” is used with a quantifier such as “strong gale”; between 90 km/h and 120 km/h (49 kn to 65 kn or 56mph to 75 mph) is classed as “storm”, and higher as “hurricane”. A short and intense movement is named a gust.

Three main forces are responsible for the development of wind: the gradient or pressure force; the Coriolis force; and centrifugal force. Depending on the proportions of these forces, wind is classified as geostrophic wind, cyclostrophic wind or gradient wind. The weaker the wind is, the more it is influenced by local factors. Since paragliders and hang-gliders only fly in weak conditions, these local effects are crucial for our sport. See also Mountain and Valley Wind (p. 37), Land and Sea Wind (p. 36), Lee (p. 38).

3.1Gradient Force

Gradient force is when an air particle flows from a high to a low pressure zone, i.e. from anticyclone to cyclone, and is proportional to the pressure difference between them. A wind which only blows due to differences in pressure where the other forces do not play a part (e.g. at the equator) is named Eulerian Wind.6

3.2Centrifugal Force

Centrifugal force is a force of inertia, which can be described by the following formula:

Fz = m?2r

Centrifugal force Fz acts on mass m rotating at angular velocity ? which is at distance r from the axis. Because centrifugal force and gravitational force are proportional to the mass of the body on which they act, the centrifugal acceleration can be defined as follows:

az = ?2r

In the following example, we calculate what kind of centrifugal acceleration acts on an air particle at the edge of a cyclone. This assumes the cyclone has a diameter of 1,000km and a very strong rotation velocity of one turn in 45h (i.e. 65 km/h or 35 kn).

3.3Coriolis Force

A body moving relative to the ground experiences a deviation due to the rotation of the earth. The force which produces this deviation is named the Coriolis force. In the northern hemisphere it acts to the right, in the southern hemisphere to the left in relation to the direction of travel of said moving body.

Fig. 3.1: Coriolis Force

The Coriolis force is strongest at the poles and decreases to zero on the equator; it is a a fictitious or apparent force because it performs no work.

In Fig. 3.1 (picture 1) only the gradient force acts. If the earth stood still, the air particle would flow directly in the direction of the arrowhead from high to low pressure. As soon as the particle starts to move (picture 2), the Coriolis force (bottom arrow) acts at a right angle to the direction of movement, diverting the particle to the right. The gradient force (picture 3 bottom arrow) further accelerates the air particle. This acceleration also increases the Coriolis force and the air particle is further diverted. In the end (picture 4), the air particle moves parallel to the isobars. The Coriolis force and the gradient force operate exactly in opposite directions and equilibrium has been restored.

3.4Geostrophic Wind

Geostrophic wind refers to a wind produced when the Coriolis acceleration and the gradient force are exactly in balance and acting in opposite directions.

Geostrophic wind is a mathematical simplification in which friction and centrifugal force are not included, therefore geostrophic wind only occurs at high altitudes in the form of jet streams.

3.5Cyclostrophic Wind

When the isobars are densely plotted or extremely curved, as in a tornado for example, then the centrifugal force is much stronger than the Coriolis force. The latter can therefore be discounted and an equilibrium between gradient and centrifugal force is established. This resulting wind is named cyclostrophic wind.

3.6Gradient Wind

Gradient wind is a mathematical expansion of geostrophic and cyclostrophic wind, taking into account the Coriolis, gradient and centrifugal forces, and defines therefore relatively accurately the real wind. The missing parameters are orology (form of the terrain) and friction.

The wind blows parallel to the isobars. Because this forms a circular flow, the centrifugal force is also enforced. The gradient wind is an equilibrium of these three forces: gradient, Coriolis and centrifugal force.

If the wind blows in an anticyclone, centrifugal force and gradient force add up and this will therefore result in stronger winds. If the wind blows in a cyclone, the centrifugal force works against the gradient force and wind velocity decreases.

Air mass at the equator moves with the earth’s rotation at a speed of approx. 40,000km eastwards. The air at the poles does not have a rotation speed at all. If an air particle moves from the equator to the pole, it takes some of its speed with it. This results in an eastwards shift in the northern hemisphere. The opposite is the case if a wind streams from north to south: it drops behind relative to the earth’s rotation and shifts to the west.

3.7Friction

In addition to all the factors explained above, wind will slow down close to the ground due to friction. Higher than 1,000m above ground level, the influence of friction is almost zero, but below that it causes the wind speed to decrease by as much as 50 %. Due to the resulting lower speed, the influence of the Coriolis force also decreases, as a result of which the wind does not blow parallel to the isobars, but at an angle of approx. 30° (Fig. 3.2). For example a ground wind which blows from 90° at a speed of 15 kn deviates with increasing altitude in the northern hemisphere to the right; thus at 1,000m above ground level it blows at 30 kn from a direction of 120°. In the southern hemisphere the wind deviates to the left meaning it blows at 1,000m altitude from a direction of 60°. Above the sea this influence is smaller, where the deviation is only around 20°.

Fig. 3.2: Wind Flow from Anticyclone to Cyclone

3.8Vertical Wind Circulation

Wind flows from an anticyclone to a cyclone. The cyclone is supplied on the ground from all sides with air which forces the air in the centre of the cyclone upwards. The anticyclone replaces its outflow with air from altitude, creating a circular flow.

Fig. 3.3: Wind Circulation

3.9Wind Measurement

In order to measure wind speed, anemometers or dynamic air speed indicators (Pitot tubes) are commonly used. Wind speed is indicated in different measuring units: metres per second (m/s), kilometres per hour (km/h) or knots (kn) are most widely used. The Beaufort scale7 with its classifications from 1 to 12 is still the most popular system worldwide to define wind speed. It is of little use for para- and hang-gliding pilots, however, since the scale up to 17 km/h is too imprecise. In meteorology it is common to use knots (sea miles per hour),8 where 1kn = 1.852 km/h = 1.1508 mph.

The highest wind gust on earth to date was measured in 1996 on the Barrow Islands in Australia at 408 km/h (254 mph).

Fig. 3.4: Wind Rose

Fig. 3.5: Wind Arrows

Wind direction and force are illustrated by wind arrows. To determine the wind direction, we imagine a wind rose around the arrow (Fig. 3.4) with the arrowhead in the centre. In the example a wind of 15 kn blows from N-NE. The force of the wind is represented by strokes and triangles (Fig. 3.5): a full stroke represents 10 kn, a half stroke 5kn, and a triangle 50 kn. The sum gives us the wind force in knots.

3.10Global Wind Circulation

Due to permanently sinking air, the poles are influenced by high pressure weather. The opposite happens at the equator, where the warm air mass constantly ascends and forms a permanent low pressure belt. Between 30° latitude and the equator, the trade winds blow, from the NE in the northern hemisphere and from the SE in the southern hemisphere. This produces a convergence zone of the two winds at the equator, named the “Inter Tropical Convergence Zone” (ITCZ).

In the ITCZ the air mass is forced to ascend forming powerful clouds and strong tropical rainfall. In the higher latitudes, there is a zone with predominant west winds, the westerlies. Closer to the poles this warm west wind builds a shear zone with the polar cold air. Due to large scale turbulence, cyclones develop here affecting the weather in both westerlies belts.

Fig. 3.6: Global Wind Circulation

Fig. 3.7: Global Wind Circulation (side view)

This shear zone resides between latitudes 30° and 60°. As a result of pressure differences between tropical warm air and polar cold air, a gradient wind is formed. As soon as the stable west wind starts to meander, stronger frontal systems develop. Fig. 3.8 shows a polar front. In the...