Since the time of Guericke, it has been known that the air has weight. Under ordinary conditions, a cubic foot of air weighs about an ounce and a quarter (1.25 ounces). As the air has weight, it must follow that, the atmosphere must press upon the surface of the earth, and the pressure at any point will depend upon the amount of air above it. It will be less on the top of a mountain than that at the foot. But even at the sea level the pressure varies from day to day.
Small but distinct pressure differences remain from place to place. If 1,013 mb (29.92 in. or 76 cm) is taken as standard sea level pressure, readings higher than this will frequently be observed in middle latitudes, occasionally upto 1040 mb (30.7 in.) or higher. These pressures are designated as “High Pressures”; ranging down to 982 mb (29.0 in.) or below are “Low Pressures”.
UNITS OF PRESSURE
: 1.013 kg/cm2 = 1.013 bar = 1,013 mb = 760 mm of Hg = 76 cm of Hg = 29.92 in.
1. EQUATORIAL LOW PRESSURE TROUGH
Within a few degrees of equator is a belt of pressure, some what lower than the normal pressure 1013 mb, between 1011 mb and 1008 mb, which is known as the Equatorial Trough or Low Pressure Belt. This is an area of high temperature and high humidity, commonly known as the Doldrums, where the air near sea level is stagnant or sluggish. The low pressure is due to heating; as the pressure of a volume of air decreases when its temperature increases.
It lies entirely north of the equator during July, because this is the summer hemisphere (Northern Hemisphere). The trough is by no means uniform in width, depth or position. It is deepest and most pronounced when it lies farthest from the equator, such as over an area extending from the Persian Gulf eastward to north-western India, and also over northern Mexico and the south-western United States. It is least distinct over the western and central pacific.
Such continental, thermal lows are relatively shallow and have influence only on surface wind patterns.
2. SUB-TROPICAL HIGH PRESSURE BELTS
At about latitude 30 degrees North and South occur the sub-tropical high pressure belts, sometimes known as the “Horse Latitudes”, zones of calm and descending air currents. In the Southern Hemisphere, this belt is clearly defined but contains centers of high pressure termed as “pressure cells”.
In the Northern Hemisphere in summer, the high pressure belt is dominated by two oceanic cells, one over the eastern Pacific and the other over the eastern north Atlantic. Average pressure exceeds 1026 mb in the centers of the cells.
The sub-tropical highs are largely developed by dynamic, rather than thermal causes. Subsidizing air from high levels is largely responsible for their great pressures. Variations in their strength and form are among the most important features of the global energy balance, since the air that diverges from them comprises a large part of the entire air circulation system on the surface.
The summer hemisphere (northern hemisphere) shows only the large oceanic, high pressure centers. The winter hemisphere has a greater number of anti-cyclonic highs and the most active day by day pressure changes along the general sub-tropical pressure zones.
3. SUB-ANTARCTIC LOW PRESSURE BELTS
Pole-wards of the sub-tropical high pressure belts are the broad belts of low pressure, extending roughly from the middle latitude zone (35o to 55o north and south) to the Arctic zone (60o to 75o north and south) but centered and intensified in the sub-Arctic zone (55o to 60o north and south) at about 60th parallel to latitude (horizontal). In the southern hemisphere, over the continuous expense of southern ocean, the sub-Antarctic low pressure belt is specially defined with average pressure as low as 984 mb.
It is one of the deepest and most persistent low pressure troughs in the world bordering the Antarctic continent. It is present, however, at all seasons shifting slightly southwards in January. This low pressure trough marks the zone of energy transfer between warm and cold air. It is a strong frontal zone, and the dynamics of air interchange, implemented by the upward displacement of warmer air, the high condensation, great angular momentum, rotational deflection, and other factors produce an almost continuous succession of deep cyclones that move around the world through this zone. Anticyclones, or high pressures accompanied by descending diverging air, sometimes, occupy positions within the trough and at rare intervals may persist for several days.
4. SUB-POLAR LOW PRESSURE BELTS
Nearer the poles occur the sub-polar low pressure belts. One reason for this pressure distribution is that the rotation of the earth causes a polar whirl and therefore a tendency toward low pressure at the poles. But the intense cold around the poles causes the thermal effect to overcome the dynamic one, with the result that the low pressure belts tend to be around just outside the polar circles.
Since the frontal activity between cold and warm air in these latitudes is weak during the summer season, strong sub-polar lows do not develop.
The weak low-pressure zone bordering southern Greenland and extending across the north Atlantic to Norway is related to shallow summer cyclonic depressions that pass eastward through the region.
5. POLAR HIGHS
The polar zones have permanent centers of high pressure known as polar highs. Both high and low pressure centers are present, which change in intensity and shape seasonally, with the low (located near the continental margin) dominant during most of the year, but less strongly developed during the winter months. A high pressure ridge tends to occupy the highest portion of the continent but is extremely shallow and not well developed.
RELATION BETWEEN WIND AND PRESSURE
Wind may be defined as the movement of air from high to low atmospheric pressure in a virtually horizontal plane. Its direction and strength is the result of following four factors:
1. The Barometric or Pressure Gradient
Regions of high pressure form centers from which winds tend to blow outwards i.e., they are areas of “divergence”. Conversely, regions of low pressure are the face of winds i.e., areas of “convergence”. The change in barometric pressure across the horizontal surface constitutes a “pressure gradient”, its direction is indicated by a broad arrow in the diagram. The gradient is in the direction from higher pressure (left) to lower pressure (right).
Where a pressure gradient exists, air molecules tend to drift in the same direction as that of the gradient. This tendency for mass movement of the air is referred to as Pressure Gradient Force. The magnitude of the force is directly proportional to the steepness of the gradient. When the difference in pressure between two neighboring points on the earth’s surface is great, there is said to be a steep pressure gradient. In such case, the resulting wind is strong. When, however, the barometric gradient is gentle, winds are light. The result is that the air tends to move from areas of high pressure to those of low. Wind is thus the horizontal motion of air in response to the pressure gradient force.
2. The Coriolis Force
If the earth did not rotate upon its axis, winds would have followed the direction of pressure gradient. Instead, earth’s rotation produces another force, the Coriolis Force, which tends to turn the flow of air.
The direction of action of the coriolis force is stated in Ferrel’s Law:-
“Any object or fluid moving horizontally in the Northern Hemisphere, tends to be deflected to the right of its path of motion, regardless of the compass direction of the path. In the Southern Hemisphere, a similar deflection is towards the left of the path of the motion as a result of the earth’s rotation”.
The coriolis force is absent at the equator but increases progressively polewards. The force is proportional to the speed of the moving object and it varies with latitude, being zero at the equator and maximum at the poles.
Applying these principles to the relation of winds to pressure, the gradient force (acting in the direction of pressure gradient) and the coriolis force (acting to the right of the path of flow) quickly reach a balance or equilibrium when the wind has been turned to the point that it flows in a direction at right angles to the pressure gradient, i.e., parallel with isobars.
When a balanced condition develops between the forces exerted by the pressure gradient in one direction and the coriolis force in opposite direction, a steady wind will blow. This is known as a state of geostrophic balance, and the air movement as geostrophic flow or geostrophic wind, in a direction parallel to the straight isobars.
3. Centrifugal Force
When an air-stream moves on a curved course, as in a pressure system with closed or curved isobars, it is subjected to centrifugal force acting outwards from the center of curvature. A gradient wind develops as a result of the balance between the pressure gradient on the one hand, and the coriolis and centrifugal forces on the other. The air thus tends to travel along the isobars, clockwise around the highs and anticlockwise around the lows in the Northern Hemisphere; while in Southern Hemisphere, it would tend to move anticlockwise around the highs and clockwise around the lows.
This relation of the wind to atmospheric pressure was put forward by Dutch scientist Buys Ballot, in the middle of the 19th century:-
“If you stand with your back to the wind in the Northern Hemisphere, pressure is lower on your left than on your right, and the reverse applies in the Southern Hemisphere”.
Friction tends to decrease air velocities. Air that moves over the earth’s surface has some of its mechanical energy transformed into other forms of energy because of friction, and thus its velocity decreases. Friction with surface, since it decreases wind velocity, also tends to decrease the amount of coriolis deflection, which is proportional to straight line velocity. This decrease in the coriolis deflection, in turn, tends to alter the resultant wind direction with respect to the pressure gradient. The wind direction no longer parallels the isobars, as in geostrophic flow, but crosses them some what in the direction of pressure gradient. The angle of intersection is roughly proportional to the amount of friction encountered at the earth’s surface.
Surface winds usually cross isobars at about 10o angle over oceans and may reach angles of upto 45o over rough land surfaces. Although the loss of energy by friction takes place only at the surface, the breaking effect may be transmitted upwards in the higher levels of the horizontal wind flow. Usually, however, it is contained within 1500 to 2000 ft. of the surface.
Monsoons are the winds, which change their direction in different seasons. This is due to the fact that the surface of the earth, which consists of land and sea masses, acts differently to the insolation applied to it and accordingly the pressure is also different.
Cause of the Monsoons
The unequal heating of land and water areas causes monsoons. During summer, when the land has become quite hot, the contiguous ocean areas are relatively cooler. The air of the land rises up in the form of convection currents and the moist air of the oceans would flow in to take its place. During winter, the conditions would reverse and winds would blow from the lands towards oceans.
In lower latitudes, the temperatures are uniformly high throughout the year while in the higher latitudes, the temperatures over the land are never so high as to disturb the prevailing pressure conditions. Hence these types of circulation only take the form of land and sea breezes. But in the middle latitudes, because of vast range of temperature conditions, these land and sea breezes become semi-permanent and are known as the monsoon winds. These monsoons are the most powerful on the sub-continent of Indo-Pak and China.
Monsoons in Different Seasons
During summer, the interior of Asia and India becomes center of great heat and a powerful low pressure develops there. Comparatively the water surface of Indian Ocean is cooler and the pressure gradient is towards the heated interior. This factor is so important that trade winds are substituted by monsoon winds blowing from the south-west. Since it moves from water to land, the summer monsoon is moist and brings rain.
In the winter season, the interior of Asia and India is very cold and high pressure area exists there. In comparison, the contiguous areas of India Ocean are warmer and the pressure gradient exists towards the outlying ocean. As such the wind begins to blow from the interior of the continent towards the ocean and is known as the north-east monsoon. Coming from the dry interior, it is cold and dry. It only brings rain when it reaches some portion after crossing the sea.
Monsoons Winds of South & Southeast Asia
South Asian lands have mostly a tropical monsoon type of climate. They have a separate and distinct system of winds and air masses. In the extremely northern areas like Kashmir, Nepal and Bhutan, the winters are distinctly cold and wet, so they become a little different from the true monsoon climate while the eastern sides of south Asia i.e., the ASEAN and Bangladesh areas may be called the true monsoon type. True monsoon types of climate lands experience fairly uniform distribution of temperatures and little diurnal range. Low pressures prevail in the summers, which result in the abundance of rainfall by summer monsoons and long drought periods in winter season for more than four months. So summers are quite hot and rainy, while winters in the true monsoon climate are almost dry and warm.
The summer monsoon
comes to South Asia from the Indian Ocean as south-western winds, then it curves towards the north-west upto Ganges plain and then towards the Indus plain.
Usually monsoon winds of South Asia are sub-divided into two parts:
a. The period from June to mid September, a time of advancing monsoon winds.
b. The period from mid September to mid December, a time retreating monsoon winds.
During the hot dry season, due to very high temperatures in the desert and semi-desert areas of South Asia, very low pressure areas are created, which eventually cause the outburst of the monsoon. This low pressure area attracts winds from Indian Ocean, full of humidity, giving heavy rains over the South Asian region.
The duration of this monsoon varies from six months in the extreme south of Madras, to only six weeks in the Pakistan area.
The speed of monsoon winds also varies in different parts of South Asia. There are two main routes of these monsoon winds. One route starts from Arabian Sea and scarcely reaches the north of the gulf of Cambay. The second route starts from the Bay of Bengal as an east wind, upto the Ganges and reaches Punjab from the south-east.
In the eastern side
, i.e., Bengal, the monsoon starts from the middle of June and reaches Punjab in the month of July. South-East Asia receives continuous and the heaviest rainfalls as compared to the others parts of South Asia.
Rainfall reduces the temperatures in most parts of India and Pakistan. The areas of Punjab and Sindh are the driest parts in this season and have the highest temperatures, which remain in June and July.
In northern plain areas, the rainfall is from 30 in. to 60 in. on the leeward side of mountains. In the interior of plateau and valleys of peninsular India, the rainfall received is only 10 in. to 30 in.
When this summer monsoon comes towards the west, rainfall gradually decreases because dry hot air may reach across central India. No rainfall occurs here. Only the Himalayan regions receive heavy rainfall during this time. Kashmir receives 20 in. to 40 in. of rainfall. The Rajisthan and Sindh areas receive no rain during this rainy period.
The winter monsoon
of South Asia is dry with cool air travelling across the Indo-Gangetic Plan from the north-west, and in part, from subsidiary air in the sub-tropical high pressure belt.
The winter monsoon moves from west to east and then turns to become north-easterly over the peninsular India. The mean January temperature remains above 70 oF. The rainfall is less than 5 to 13 cm in Punjab and more than 5 to 13 cm in Himalayan areas with cold air blowing.
Monsoon Winds of North America
North America does not have the remarkable extremes of monsoon winds experienced by South-Eastern Asia, but there is nevertheless an alternation of temperature and pressure conditions between winter and summer.
Wind records show that in summer, there is a prevailing tendency for air originating in the gulf of Mexico to move northward across the central and eastern parts of the United States; whereas in winter, there is a prevailing tendency for air to move southward from sources in Canada. Australia, too, shows a monsoon effect, but being south of the equator, it reverses the conditions of Asia.
EARTH’S SURFACE WIND SYSTEM
When the movement of the air in the atmosphere is in a horizontal direction over the surface of the earth, it is known as the wind. Movement of the wind is directly controlled by pressure.
The Equatorial Belt of Variable Winds
Over the equatorial trough of low pressure, lying roughly between 5o S and 5o N latitude, is the equatorial belt of variable winds and calms or the Doldrums. There are no surface winds here, but a fair distribution of directions around the compass. Calms prevail as much as a third of the time. Centrally located on a belt of low pressure, this zone has no strong pressure gradients to induce a persistent flow of wind.
Trade Wind Belt
North and south of the doldrums are the trade wind belts, covering the roughly the zones lying between 5o and 30o north and south. The trade is a result of a pressure gradient from sub-tropical belts of high pressure to the equatorial trough of low pressure. In the northern hemisphere, air moving equator-wards is deflected by the earth’s rotation to turn westwards. Thus, the prevailing wind is from the north-east and winds are termed as the north-east trades. In the Southern Hemisphere, deflection of the moving air to the left causes the south-east trades. Trade winds are noted for their steadiness and directional persistence. Most winds come from one quarter of the equator.
The trades are best developed over the pacific and Atlantic oceans, but are upset in the India Ocean region by the proximity of great Asiatic land mass. The trade wind belts are not altogether favorable for navigation or flying, because over certain oceanic portions, at certain seasons of the year, terrible tropical storms known as hurricanes or typhoons occur.
Sub-Tropical Belt of Variable Winds and Calms
Between latitudes 30o and 40o north and south are what has long been called the sub-tropical belt of variable winds and calms or horse latitudes, coinciding with the sub-tropical high pressure belt. Instead of being continuos even belts, the high pressure areas are concentrated into distinct anticyclones or cells, located over the oceans.
The apparent outward spiraling movement of air is directed equatorward into easterly trade wind system; polewards into the westerly wind system. The cells of higher pressure are most strongly developed in summer. There is also a latitudinal shifting following the sun’s declination. This amounts to less than 5o in the Southern Hemisphere, but is about 8o for the strong Hawaiian high latitude in the north-eastern Pacific.
Winds in the high pressure cells are distributed around a considerable range of compass directions. Calms prevail as much as a quarter of the time. The cells have generally fair, clear weather, with a strong tendency to dryness. Most of the world’s great deserts lie is this zone and in the adjacent trade wind belt.
Belt of Westerlies
Between latitudes 40o and 60o north and south, is the belt of westerlies or prevailing westerly winds. Within the westerly wind belt, winds blow from any direction of the compass, but the westerly components are definitely predominant. Storm winds are common in this belt, as are frequent cloudy days with continued precipitation. Weather is highly changeable.
In the Northern Hemisphere, land masses cause considerable disruption of the westerly wind belt, but in the Southern Hemisphere, between the latitude 40o and 60o south, there is an almost unbroken belt of ocean. Here the westerlies gain great strength and persistence.
The belt was extensively used for selling vessels travelling eastwards from the south Atlantic Ocean to Australia, Tasmania, New Zealand and the Southern Pacific Islands. Although the westerly wind belts no longer exert a strong influence over the routes of modern ocean vessels, they are important in long distance flying. Transoceanic and transcontinental flights in the easterly direction require less fuel and shorter time. On westward flights, strong headwinds may eat dangerously into the fuel supply on the plane and in any event necessitate reduced payload.
A wind system termed polar easterlies has been described as the characteristic of the Arctic and the polar zones. The concept is greatly over simplified, if not actually erroneous, for winds in these regions take a variety of directions, as dictated by local weather disturbances. Perhaps in Antarctica, where an ice-capped land mass rests squarely upon the pole and is surrounded by a vast oceanic expense, the outward spiraling flow of polar easterlies is a valid concept. Deflected to the left in the Southern Hemisphere, the radial winds would spiral counter clockwise, producing a system of south-easterly winds.