Wind. The Basics and Dynamics

Consider the forces that affect the volume of air and determine its movement in the horizontal direction, along the surface of the earth.


 

Pressure Gradient Force

The pressure gradient is a force (PG) that acts in a direction from higher toward lower pressure. It will usually serve as the initial impetus for air movement.

 

Coriolis force

Coriolis force (СF) acts on the moving air to the right in the northern hemisphere (to the left in the southern hemisphere) from the direction of its movement.

 

Frictional forces

Two forces are mainly considered: the frictional force against the underlying surface(drag), which slows down the movement of the near-surface air, and the frictional force against the overlying layer of air, which, due to the faster movement, on the contrary, accelerates the volume of air. Its value, usually remaining less than the value of friction against the surface, depends on stratification. If the stratification is stable, the force of friction against the upper layer is small because there is little mixing between the layers. If stratification is unstable, then the friction force against the overlying air layer increases due to intense mixing.

The magnitude of the resultant force is determined by the ratio of the mentioned frictional forces. In the absence of friction forces, the air will move in such a way that the equality of two forces is maintained: the pressure gradient force and the Coriolis force. Such air movement, or wind, is called geostrophic wind.

 

The wind at altitude

Geostrophic winds are the real air currents present above our heads. After all, from about 500-1000 meters up, the force of friction against the underlying surface can be neglected. Geostrophic wind is often called differently: synoptic, gradient, background, overhead, meteorologist, and on closer examination all these concepts are slightly but different from each other. But more often these terms denote the air flow conditioned by the spatial distribution of atmospheric pressure, which reflects the synoptic situation that exists over the given territory (water area).

No matter how distant from us the geostrophic wind may seem (indeed, because the wind blowing at an altitude of 500 meters is far from always the wind which determines the wind field on the surface), but its importance is enormous. The movement of lower tier clouds located at heights of 0 to 2 km, by which we can judge about the nearest weather, is almost entirely determined by geostrophic air currents. And weather changes for the next few days are largely determined by high-altitude winds (5-10 km), in the dynamics of which the geostrophic balance also plays an important role. Moreover, geostrophic wind characteristics are the main components of forecast models, based on which we further obtain information about the wind on the ground.

The spatial distribution of atmospheric pressure on synoptic weather maps is illustrated by isobars. These are lines, at each point of which the atmospheric pressure, scaled to sea level and expressed in hectopascals (hPa), has equal values.

Since the pressure gradient force is normal to the isobars (from high pressure to low pressure), the geostrophic airflow is parallel to them. Its velocity is greater the greater the pressure gradient, i.e., the closer the isobars are to each other. With such a dependence of the geostrophic wind on the atmospheric pressure field in regions with relatively high atmospheric pressure, or, as they are called in meteorology in anticyclones (high), there is a circulation of air directed around their center clockwise in the northern hemisphere. In the southern hemisphere, air moves counterclockwise in anticyclones.

In cyclones, areas with relatively low pressure (cyclone, depression, low), air motion around their central parts occurs in the opposite direction, i.e. counterclockwise in the northern hemisphere and clockwise in the southern one.

As cyclones are responsible for the weather deterioration (wind intensification and precipitation), when at sea, it is advisable to know the location of their center. Even without a synoptic map it is not difficult to do by closely observing the wind direction in the open space or the movement of lower level clouds (0-2 km). Remembering that clouds move in the direction of the geostrophic wind, turn so that their movement is from left to right. Then you will be facing an area of low pressure and, if it appears in the western part of the firmament, expect worsening weather.

Of course, you should not forget that this rule will only be true for the northern hemisphere. for the southern hemisphere you must turn so that the clouds move in the opposite direction, that is, from right to left.

Actually, strictly speaking, in anticyclones and cyclones, the wind is not purely geostrophic. Since the air flow is involved, in both cases, in a rotational motion around their centers, another force - centrifugal - appears in the balance of forces. The wind arising in this case is called a gradient wind. Since in the vast majority of cases isobars are not straight-line and the centrifugal force is almost always present, it would be more correct to call the high-altitude wind, which is not influenced by the underlying surface, a gradient wind.

 

Surface Wind

Of course, when we consider the wind near the Earth's surface, the frictional force must be added to the balance of forces. It is clear that the larger it is, the lower the velocity of the air flow. But the effect of friction force is not limited to this, its appearance leads to the fact that the balance of forces is already determined by three forces: the pressure gradient force, the Coriolis force and the friction force. Therefore, as a result, the air flow begins to deviate to the left, counterclockwise (back) from the geostrophic wind direction (in the northern hemisphere). In the southern hemisphere, the deviation will be to the right, i.e., clockwise (veer). The greater the amount or angle of deflection, the greater the friction force.

What does the friction force depend on? First of all, it depends on the braking effect of the underlying surface, or its roughness. The greater the roughness, the greater the friction force. Over land, the friction force is greater than over water. Therefore, on average, the angle of wind deflection over land from the geostrophic direction is considered to be 30-40°, as opposed to 15-20° over sea. Over a hilly surface, the force of surface friction will be greater than over a flat surface. Over a surface covered with bushes or forest, it will be more than over a meadow or a field. And, of course, the amount of roughness as well as the surface friction force will be greatest over populated areas.

One of the factors determining the magnitude of the friction force between the air layers, including the air at the surface and the air above it, will be the stability characteristic. The higher it is (especially at inversion, when the lower air layer is colder than the upper one), the less interaction between the layers, the less friction force between them. And then the total frictional force will be greater. In this case, the wind speed at the surface will be less, and the rotation angle relative to the geostrophic wind will be greater.

Let us dwell a little more on the change of the wind with altitude. The frictional force decreases with height, so the wind velocity at height increases, and the wind itself rotates relative to the surface wind to the right in the northern hemisphere, approaching the geostrophic direction (to the left in the southern hemisphere). This change in wind with altitude is reflected in the so-called Ekman spiral. smoke from a campfire may move in one direction; a flag streaking downwind will show rightward shifted wind direction at flagpole height; smoke from a chimney will spread in a direction even more clockwise shifted from the first two; and finally, clouds will move almost in the direction of the geostrophic wind.

A noticeable turn may be noted even within the mast height of small cruising boats. Experienced athletes, medalists of big international regattas and Olympic Games note, that under some conditions, such turn, even at mast height of "Laser" (less than 6 meters) can be 2-3°! Therefore, without any doubt, this factor should be taken into account when setting the sails.

If for some reason (dynamic or thermal) the air from high altitudes is near the surface, it will inertially transfer its speed and direction of motion downwards. Therefore, often wind gusts, representing masses of high-altitude air, which find themselves at the surface, are characterized not only by higher velocity, but also by the direction of motion deflected clockwise (to the right) relative to the prevailing wind. Remember: for the southern hemisphere this deviation will be to the left!

Now consider the change in wind as it moves over surfaces of different roughness and, consequently, different frictional strengths. On the coast, there are two types of underlying surface: land and sea.

If the air flow is directed from the sea to the land, it experiences braking when it enters it, which has almost no effect over the water area. But if it is directed in the opposite direction, from land to sea, then over the water surface, due to the reduction of friction force, the air flow accelerates and gradually turns to the right (in the northern hemisphere). The space within which the wind experiences these changes depends on many factors: wind speed, roughness of land and sea, stratification of the air over them, and much more.

Sometimes, a right turn of wind can be observed even over the course of the race, exposed to the shore. Thus, information about wind direction at the starting line and upper course mark can be very important to the competitor just before the start of the race.

The most interesting cases are those, where the airflow is directed along the shoreline. To begin with, consider the situation where the wind is directed in such a way, that if you stand facing it, the coast will be to the right. Then, because it is deflected to the right relative to its direction over the land, the divergence zone of the air currents will be located over the land-sea boundary. And if standing against the wind and the shore is to the left, over the coast and the adjoining water area there will be a zone of the convergence of air currents .

In the wind field over coastal water areas like straits or large bays or over inland water bodies (lakes, rivers and reservoirs) the convergence and divergence zones are located on the opposite shores. The same influence is felt by air flows over large islands and also when the wind from the sea blows over peninsulas on their opposite shores located on the right and left relative to wind direction.

But the most important thing is not the convergence-divergence zones themselves, but the effect they have on the wind. It seems obvious, that convergence or the convergence of wind streams, should lead to wind strengthening and divergence, on the contrary, should lead to weakening.

This is exactly what happens when there is stable stratification, for example, when the water is colder than the air. And we can observe this in spring and summer, the most popular seasons for traveling and sailing.

But in the fall and winter, unstable stratification develops over water with its relatively high temperature. Under these conditions, convergence leads to intense air rise and weakening of the wind, while divergence, on the contrary, leads to lowering of air from higher altitudes and, accordingly, possible strengthening of the wind. Thus, the effect can be opposite.

The zones of wind strengthening and weakening may not be limited to the coastal areas of water areas. For example, if this is an island, they will influence the wind dynamics on its leeward side for a significant distance. And the value of this distance will already depend on specific conditions.

But stratification and its stability characteristic may experience not only seasonal changes, but also change over shorter time intervals. For example, a consequence of diurnal changes of stability is the course of the wind during a sunny, low cloudy day, when they say: "The wind follows the sun". Let us stop here in detail, as this peculiarity of wind dynamics is important both for inland water bodies and for coastal water areas, especially when wind is directed from land to sea.

At night, due to cooling, which occurs due to the heat given off by the land surface via long-wave infrared radiation, a stable stratification is formed in the lower atmosphere, often even a surface inversion is observed. The wind is weak, and its angle of deviation from the geostrophic direction is significant. At strong cooling, calm is quite often observed.

When the sun rises, it begins to warm the land surface, from which the adjacent air layer is already heated. Unstable stratification is formed with a large temperature contrast between the warm lower air layers and the colder upper ones. Convective mixing develops. The friction force between air layers increases, the faster upper layers start dragging the lower ones with them.

The total frictional force (surface friction minus friction between air layers) decreases, the wind at the surface increases and turns clockwise to the right. And the "clockwise" direction corresponds to the movement of the Sun across the firmament in the northern hemisphere. So it turns out that the wind, "turning to the right", "follows" the Sun!

Of course, by the end of the day, when the Sun sinks to the horizon and the warming ceases, stratification takes on a steady character again, and the wind, turning back to the left, weakens again, sometimes to a complete calm.

The cause of changes in stability can be not only changes in the temperature of the underlying surface, but also changes in the temperature of the air itself. This is usually observed when atmospheric fronts pass over the land or water area. Behind a cold front, stratification often becomes unstable due to the fact that cold air is over the surface that retains heat. convective processes develop, cumulonimbus clouds appear, and the wind becomes stronger and noticeably gustier.

The opposite situation is observed behind the warm front: as warm air moves over the colder surface, the stability in the lower layers increases. Low clouds of layered forms predominate, fog or dense haze is often noted, and wind speed decreases.

But similar changes in stability can occur not only in time, but also in space. This is associated primarily with changes in the temperature of the underlying surface, including the water surface. When the airflow moves from the warm water surface to the cold one, the stratification in the near-surface layer becomes stable. This leads to weakening of the wind and turning it to the left in the northern hemisphere and to the right in the southern hemisphere. These changes are completely analogous to the changes experienced by airflow as it moves from water to land.

As the air moves from cold water to warm water, there is an inverse change with the air flow: the wind speed increases and the direction changes clockwise (counter-clockwise in the southern hemisphere).

The boundaries defining different water masses are called water fronts in oceanology.  Water masses usually differ not only in temperature, but also in salinity and other parameters, such as optical characteristics: color and transparency. The water of the open sea usually has a dark blue shade and great transparency, while coastal waters are characterized by less transparency and a yellowish-brown shade.

So what are the reasons for the change in water temperature? For example, it is quite obvious that in winter the waters of the open sea have a higher temperature than the water near the coast. In summer, the situation is reversed: the coastal water becomes warmer due to more warming. Therefore, the wind characteristics will also change as the airflow moves from the open sea to the coastal zone or vice versa.

In addition, in water areas near the coast there are often areas of fresh water flowing out of estuaries which in different seasons of the year, has a different temperature in relation to the temperature of sea water. In autumn and winter it is usually colder than sea water, and in summer, if the river is not mountainous and has an extensive mouth, where there is intense heating, river water is, on the contrary, much warmer.

On water areas with high tides, the distribution of fresh river water on the coastal water area is subject to tidal cycles. At low tide, river water will be observed near the shore, and if it is cold, with a noticeable weakening of the wind, while at high tide the water area will be occupied by warmer sea water, accompanied by an increase of wind. A similar change in water temperature, resulting in strengthening or weakening of the wind, occurs during water exchange due to currents through straits.

Of course, the natural factor of sea surface temperature change (increase) is water warming in vast shallow waters. For example, the difference between the water temperature on the coral reef shoals and the water temperature in the adjacent deep ocean water area can exceed 3-5°C. Accordingly, over the reef flats there is a significant increase in wind speed compared to the wind over the open ocean.

Another group of causes of changes in surface water temperature includes hydrodynamic processes. For example, the often observed sharp drop in water temperature in summer on many sea coasts is commonly associated with the so-called phenomenon of surging. This occurs when the wind blowing from the shore for several days drives the surface warm water into the open sea, and the cold bottom water mass rises to replace it from the depths.

The rise of deep, cold water to the surface is called upwelling in oceanology. This phenomenon usually has a dynamic origin. For example, a noticeable drop in water temperature is observed as a result of vertical mixing of the water column behind various obstacles encountered in the path of sea currents: shoals, capes, or peninsulas. As a result, relatively warm surface water mixes with colder water located deeper, the temperature at the surface drops, and zones of weakened wind are formed over such areas.

All of the above examples indicate that the underlying surface has a significant effect on the airflow moving over it.  This can be done both directly, in a purely mechanical way, through the force of surface friction, and indirectly, in a more complex way, through the influence on the structure of the surface layer of the atmosphere and the characteristic of its stability. The contrast of wind conditions due to these processes can be significant.

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