The uneven distribution of pressure on the globe causes air movement. Its most noticeable form is wind. Wind is the horizontal movement of air. Atmospheric pressure depends mainly on latitude, altitude, and season. Air, being a gas, seeks to equalize pressure, so a movement of air is created, always directed from a pressure with a higher value to a pressure with a lower value.
The vertical movement of air, which is called convection, also plays a major role in the circulation of air on the Earth’s scale called global air circulation. It is caused by varying air temperature, by the different specific gravity of two air masses in contact with each other, or by the forcing of rising air by a characteristically shaped land surface. Convection currents can be divided into ascending currents, and descending currents. Ascending currents are formed under conditions of reduced pressure.
The rising of warm, lighter air takes place then. When the pressure is higher we observe the descent of cooler, heavier air such movement is descending currents. In the atmosphere there is still turbulent motion, which is the result of differences in wind speed in adjacent air layers. This movement consists of dynamic disordered vibrations of individual air masses, causing mixing. The roughness of the ground caused by the terrain, or its uneven heating, is the cause of dynamic turbulence.
Thermal turbulence, on the other hand, is the result of differences in the heating of dissimilar surfaces. Thermal turbulence reaches its highest values during the midday hours on summer sunny days. Wind is defined by two parameters: direction and speed. Direction always means from which direction the wind is blowing not which way. Measurements of the direction, and the speed of the wind are made at weather stations with the help of a so-called wind meter. English nomenclature is used to designate wind directions, where N- means north, S- south, W- west, while E- east.
These are the main directions of the wind rose. The symbols for intermediate directions are two or three letters. Wind speed is usually specified in meters per second (m/s), sometimes km/h is used in aviation, and in sailing the unit of a knot, or nautical miles per hour, is used. At sea, wind speed is also determined using the conventional Beaufort scale.
The main feature of this scale is that the strength of the wind can be assessed based on observations of the sea surface or objects on land, so no measuring instruments are needed. This scale is divided into 13 degrees. 0 means no wind, and a mirror-like sheet of water. Grade 12 defines a hurricane. The wind then blows at speeds of up to more than 118 km/h, at sea the air is filled with fog and droplets of foam, the sea is completely white and visibility is limited to a great extent. On land, the wind uproots trees, destroys and tears off the roofs of houses.
At the Earth’s surface, we usually experience wind speeds of a few m/s, rarely above 15 m/s. During storms and hurricanes, on the other hand, the wind speed exceeds 30 m/s, in temperate latitudes it reaches 60 m/s, and in tropical latitudes even 100 m/s. Air movement at low speeds is almost straight parallel to the land surface. At higher speeds, both the direction and speed of the wind are changed. The so-called wind gusts are then observed.
The main factor that causes wind, and influences its speed, is the horizontal barometric gradient (pressure differences over a given distance). If there were no other interacting forces, air would follow the shortest possible path from places with higher pressure to those where lower pressure values are recorded. However, with the onset of wind, there are additional stimuli that change its current speed and direction. These are the Coriolis force and the frictional force.
All freely moving bodies undergo a deviation from their original direction. These twists are due to the Earth’s rotation around its axis, and the law of inertia. Air molecules, which, along with the mass of the atmosphere, participate in the Earth’s rotation, also undergo deviations. The wind changes its direction due to the Coriolis force. In the northern hemisphere, horizontal air movement deviates to the right, while in the southern hemisphere all wind directions show a deviation only to the left from the initial direction. The movement of air taking place in the lower layers of the atmosphere is affected by the frictional force T, caused by the force of the roughness of the ground, in addition to the force of the baric gradient and the Coriolis force. The frictional force limits the wind speed. Its effect is most pronounced near the Earth’s surface and decreases with altitude.
The general relationship of wind directions to pressure systems is defined by the baric law of wind. According to this law, if in the northern hemisphere we stand with our backs to the wind direction then the lowest pressure will be on the left side slightly in front, while the highest pressure will be on the right side slightly behind. A wind blowing parallel to the isobars will leave the low pressure on the left, while the high pressure will be on the right.
The primary drivers of horizontal atmospheric air movement are thus:
- Uneven heat delivery to different areas of the globe
- Troposphere thickness decreases with increasing latitude
- Different rotation speed of the Earth at different latitudes
- Coriolis force causing bodies to deviate from the direction of motion in the northern hemisphere to the right and in the southern hemisphere to the left
- Frictional force inhibiting wind speed
The general circulation of the atmosphere on the globe
The main causes of the general atmospheric circulation are the different amounts of heat energy provided by the sun at different latitudes, and the rotation of the Earth. A factor that complicates the Earth’s general atmospheric circulation is the uneven distribution of land and oceans. The different roughness of the ground also affects the direction and speed of the moving air. The globe is distinguished by zones of constant low, or high pressure.
Strongly heated air in the circumpolar zone expands and rises upward. In this area, a low-pressure belt, also called the equatorial quiet belt, forms, changing its position in the wake of the sun’s summer position. It moves northward in July and southward in January. This zone is also called the Hadley Cell. This area is characterized by weak or no winds. At the same time, storm clouds form as a result of the drop in temperature, which cause at the zenithal position of the sun the so-called zenithal precipitation. Air masses separate and head north and south towards the tropics, taking a westerly direction thanks to the Coriolis force. Around 30-35° north or south latitude, these winds take on a zonal character, assuming a westerly direction in the northern hemisphere, while an easterly direction in the southern hemisphere.
Thus, they take a latitudinal direction. In the upper troposphere, and in the lower stratosphere at latitudes of 30 to 35°, there are zones of particularly high pressure gradients. They are the cause of the huge velocities of geostrophic winds. These are called jet currents. These are narrow streams of air with a horizontal axis, and very high lateral velocity variations. The length of these currents is thousands of kilometers, the width is hundreds of kilometers, while the thickness is several kilometers. They move mostly from east to west. They resemble a river flowing in bends. These air movements were detected during World War II by US pilots. At these latitudes, some of the air descends, causing the formation of tropical high-pressure belts, which overlap with tropical quiet belts. As the height of the troposphere and the circumference of the Earth decreases with increasing latitudes, air is compressed and cooled. From this zone, air flows partly downward toward the equatorial low pressure belt and partly toward higher latitudes. The winds that blow toward the equator are the pasatas.
These are steady, warm winds of moderate speed. In the northern hemisphere, the pasat blows from the northeast and in the southern hemisphere from the southeast. The place where the pasatas from the two hemispheres meet is called the Intertropical Convergence Zone, or low pressure furrow. Those winds that occur in the higher parts of the troposphere blowing toward the tropics are called antipassates. They occur at altitudes of 2000- 3000 meters.
These winds transport hot air from the circumpolar region toward the tropics. Antipassates, as a result of the Earth’s rotational motion, change direction to the east and descend, after which, as pasatas, they return towards the equator. Currently, some scientists have given up calling currents blowing from the equator toward higher latitudes antipassates. Radio observations show that winds in the upper troposphere lose their meridional direction completely and become currents blowing parallel to isobars, taking a latitudinal direction at these altitudes. The belt and antipassate circulation is also known as the intertropical circulation. This area is characterized by the relative constancy of weather types, which is disturbed only by tropical cyclones.
Between subtropical and circumpolar highs, low-pressure areas are formed at latitudes of 60-65°. Warm air masses flow into them from subtropical highs and in winter from circumpolar highs. Under the influence of the Coriolis force, these winds change direction to the west. Thus, westerly winds blow there. Westerly winds are particularly strong in the southern hemisphere because there are no large land areas in temperate latitudes. These regions are called by sailors “roaring forties” and howling fifties” because the strongest winds are formed between 40-60° south latitude. The 40-70° latitude belt especially in the northern hemisphere is the most disturbed because cyclonic activity develops the most in these areas causing huge variability in wind direction. Since the least amount of heat is received by the circumpolar areas, the lowest temperatures occur there, so the air there is heavy and dense.
It falls downward, creating areas of high pressure. These winds flow toward lower, temperate latitudes, where there are areas of low pressure. The winds, thanks to the Coriolis force, take a northeast. and east direction in the northern hemisphere and a southeast and east direction in the southern hemisphere. They are called easterly winds. The zones of occurrence of easterly winds are located above 70º latitude. In these zones, the pressure is high, with heavy cloud cover and the presence of fog. Wind strength can reach more than 100 knots. Due to stormy winds, low temperatures and flooding, these are areas of difficult navigation. Air currents occurring on lines of convergence between circumpolar highs and subtropical high-pressure belts cause ascending movements that lift air into higher layers. The ascending air thus flows away both toward the lower latitudes and toward the poles.
The general atmospheric circulation presented above is only a theoretical pattern of wind directions on a global scale. Under real conditions, that is, taking into account the inhomogeneity of the Earth’s surface, this distribution is strongly disturbed by the different influence of the land and water substrates. Within these zones, areas of increased, and decreased pressure are formed. Such closed areas are called centers of atmospheric action. Some of them take place in both seasons and are therefore called permanent, while others are marked in January, or in July, so they are said to be seasonal, or periodic.
