Wind

The wind, normally, is a movement of air that occurs within masses of large extension also in motion but on a planetary scale, the movements are determined by the reciprocal position of the centers of action represented by cyclones and anticyclones, ie air masses whose conditions of temperature and pressure determine the direction and intensity of winds. The large masses of cold air, heavier and at higher pressure, following the more general conditions of atmospheric circulation, tend to occupy the areas where warmer and lighter air is stationed: this causes a more or less intense and sudden flow of air, the wind, on the movement of which also acts the earth’s rotation diverting the wind to the right in the northern hemisphere and to the left in the southern hemisphere.

Winds can be classified in various ways; according to their regularity we distinguish constant or permanent winds if they blow with almost invariable direction and origin (for example trade winds); periodic winds, if they blow with alternating direction in certain periods of time (for example monsoons, long period; sea and land breezes, short period); irregular winds, if they are characterized by variability of direction and origin (connected with the succession of atmospheric disturbances).

Depending on the geographical distribution we distinguish planetary winds and regional or local winds. The former participate in the general circulation of the atmosphere and are related to the distribution of the vast areas of high and low pressure at different latitudes: in the band between 25º of latitude N and S constant and regular winds (trade winds) blow towards the equatorial depression area; between the subtropical high pressures there are winds that blow towards the circumpolar low pressures from SW in our hemisphere and from NW in the southern hemisphere (westerly winds); from the high pressures on the polar caps towards the circumpolar depressions winds blow from NE in the northern hemisphere and from SE in the southern one (polar winds). Regional winds are instead related to baric situations that are established during the year with some regularity on some regions, often determined by particular topographic configurations; among the most characteristic are the föhn, the mistral, the bora, the tramontana (also called cathabatic winds, i.e. descending winds), the libeccio, the scirocco, the ghibli, etc.

Direction and speed

The wind can be considered as a vector quantity and is therefore defined by its speed and direction. The direction is identified by means of special anemoscopes (wind vane, wind sleeves) and is indicated by the point on the wind rose from which the wind comes, or in degrees counted clockwise from north. The speed, expressed in meters per second, in kilometers per hour or in knots, is measured on the ground with anemometers, or with anemographs if you want to follow the changes over time, and at high altitude by observation with theodolites of pilot balloons or by means of radiosondes. In the absence of instruments, the speed can be identified by observing the effects of wind pressure on common objects (smoke, leaves, trees, etc..), referred to conventional empirical scales, among which the most used is the Beaufort scale; finally it can be determined by measuring the pressure gradient.

The speed is not constant in the vertical sense but varies with altitude with a certain regularity: in contact with the ground there is almost calm; in the immediately higher layers the wind follows the roughness of the ground with consequent variations of speed more or less; then the speed increases rapidly up to 500-600 m and decreases slightly for another 200 m; then it increases again slightly and irregularly up to about 1500 m above the ground; above this altitude it increases regularly. There are therefore two main areas: up to 1500 m where there is the lower wind, which is more or less affected by the morphology of the surface, and above 1500 m, where there is the upper wind, whose speed depends essentially on the barometric conditions.

On the course of the wind, in direction and speed, exert a considerable influence on the mountain ranges with effects that extend to a great height from the peaks (up to 0.3 times the height for isolated mountains, up to 3 times the height for mountain ranges) and begin several kilometers away from the reliefs. The wind, in fact, hitting against the sides of the mountains is forced to follow the trend forming upwind an inclined current having a vertical component in an ascending direction, and downwind a similar current with a vertical component in a descending direction; in both cases we will have the creation of vortical motions.

On mountain tops, moreover, the wind blows with greater speed than on the plain, because between the ridges and the overlapping air layers is determined a bottleneck that forces the air mass coming from the plane to increase its speed. Finally, it should be noted that the wind only rarely consists of a constant current in speed and direction; usually its speed passes in short intervals of time for relative maxima and minima, and so too the direction undergoes continuous oscillations. From this point of view, the wind can be defined according to its structure in the following three types: laminar wind, regular in velocity and direction, formed by particles flowing according to parallel threads; turbulent wind, characterized by strong and continuous variations of velocity; gusty or intermittent wind, characterized by weak currents interspersed with sudden violent gusts of wind.

Morphology

Wind shaping activity is particularly evident in arid regions, characterized by an abundance of fine, incoherent debris derived from the physical disintegration of rocks (desert areas), marine inputs (beaches), or fluvial inputs (floodplains). As with other shaping agents, the mechanisms through which the shaping activity of wind is expressed are destructive and constructive. In the first case, the processes that lead to the lifting of particles from the ground (deflation), the reciprocal rubbing and the repeated impact of the particles themselves against fixed obstacles (corrasion), generally represented by spikes or rock walls, fall into the category of constructive processes.

The category of constructive processes includes the deposition and accumulation of particles left by wind currents along their path, particles that are well selected from the point of view of size and usually rounded (dune sands). In relation to the prevalence of destructive or constructive processes, the characteristic shapes of the wind modelling landscapes are very diversified among them, so as to allow the reconstruction of the routes of the wind currents. In the context of landscapes characterized by the dominance of the phenomenon of deflation, for example, the petrous desert, which is formed precisely as a result of the total removal of medium and fine particles, becomes eloquent.

As secondary forms indicative of the same environment of deflation we can also have the classic triqueter pebbles, that is, those with the three upper faces frosted by particles dragged by the wind. In the lowlands and in the gorges usually crossed by wind currents charged with particles, on the other hand, “rock mushrooms”, honeycomb sculptures, yardangs, etc., become typical. These forms owe their origin to the corrosion, which is notoriously more intense near the ground and rapidly decreases in altitude. As far as the areas of predominant aeolian sedimentation are concerned, it can be observed that the most typical form is that of the dune, with an extremely variable plan and cross-section according to various local factors (transversal dunes, longitudinal dunes, barcane, etc.), which also condition its greater or lesser mobility. Still in connection with the aeolian deposition, we can also mention the extensive and thick loess deposits that accumulated during the Quaternary period at the edges of the main glacial tongues and blankets.

Wind wave

In fluid dynamics, wind waves, or wind-generated waves, are water surface waves that occur on the free surface of bodies of water. They result from the wind blowing over an area of fluid surface.

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