Chapter 14: Atmospheric Pressure and Winds
In 2019, the Arabian Sea did something it had not done in more than a century. In a single year it produced four cyclonic storms — Vayu, Hikka, Kyarr and Maha. Cyclonic storms forming in the Arabian Sea are normal enough. What was unusual was the calendar. Storms in October and November belong, by long habit, to the Bay of Bengal on the other side of the peninsula, and the sea to the west stays comparatively quiet in those months. That year it did not. Behind each of those four names sits one physical event, repeated four times. Over warm tropical water, air grew light and rose. The weight of air pressing down on the sea surface dropped, and air from all around came rushing in to take its place. Because the earth was turning underneath that inrush, the air did not travel straight in. It curved, and wound itself into a spiral thousands of metres deep with a strange patch of calm at the very centre. Nothing in that sequence is exotic. It is the ordinary behaviour of air, running at full strength. Air has weight, that weight is unevenly spread over the earth, and air moves to even the difference out. We call the moving air wind.
Map
Map 14.A1 Caption: The Arabian Sea and the Bay of Bengal, the two seas on either side of peninsular India where tropical cyclones form. Brief: Political-physical outline of South Asia showing the full extent of India as officially defined; label the Arabian Sea, the Bay of Bengal and the Indian Ocean; mark the coastline of peninsular India; draw and label the latitude lines 8° N and 20° N to show the belt in which tropical cyclones of this region originate; no storm tracks, no wind arrows, no dates.
14.1 Atmospheric Pressure
Air is a physical substance. It is a mixture of several gases present in the atmosphere, and like every other substance it has its own weight. Because it has weight, the air standing above any place presses down on that place.
That pressing down is what we measure. The weight of air on a unit area of the earth is called air pressure. Atmospheric pressure is the force per unit area exerted against a surface by the weight of the air above that surface. Standing at sea level, you have the whole depth of the atmosphere resting on your shoulders.
Pressure is expressed in millibars, written mb, and is measured with a mercury barometer. The millibar is a small unit, so ordinary pressures run into four figures: the average atmospheric pressure at sea level is 1013.25 mb, or 760 mm, which is the height of the column of mercury a barometer holds up at sea level. Atmospheric pressure decreases with height.
Pressure And The Weather
Atmospheric pressure is an important factor in producing changes in weather, and it does this through a chain that begins with heat. Contrasts in temperature cause changes in air density, and those changes in density are responsible for the variations in pressure from place to place.
Air will not stay where it is pressed unevenly. These pressure variations cause horizontal movements of air, which we call winds. Winds are therefore not a separate topic from pressure; they are what pressure differences do.
Winds matter to weather in four ways the chapter will return to again and again. (a) They transport heat from one region of the earth to another, warming places that the sun alone would leave cold. (b) They carry moisture from the seas over the land, which is how water reaches places far from any ocean. (c) They help in the occurrence of precipitation, because moist air that is made to rise gives up its water. (d) They affect both the temperature and the humidity of every place they pass over.
Flowchart
Fig. 14.A1 Caption: Uneven heating is the start of the chain, and wind is the result at the end of it. Nodes: sun heats the earth unequally → contrasts in temperature between places → changes in air density → variations in atmospheric pressure → air moves horizontally from higher to lower pressure → wind → heat and moisture carried to new regions → temperature, humidity and precipitation of those regions change
Read that chain in both directions. Forward, it explains why the air is never still. Backwards, it explains why a fall in the barometer is worth noticing: a change in pressure is the earliest visible sign that the air over a place is about to move.
14.2 Measurement Of Pressure
Pressure is normally measured in millibars. The instrument that measures it is the barometer, and the simplest form of it consists of a long narrow tube filled with mercury, standing upside down in a trough of the same liquid.
The tube does not empty into the trough, and that is the whole point of the instrument. Air pressing down on the mercury in the open trough holds the column up inside the tube. At sea level the column it can support stands 760 mm high, so the reading of the height is a reading of the pressure. There are two types of barometers in common use — Fortin’s Barometer and the Aneroid Barometer.
Diagram
Fig. 14.1 Caption: A simple barometer. Brief: Cross-section of a mercury barometer — label the glass tube, the mercury, the trough and the vacuum at the closed top; mark the height of the mercury column at sea level as 29.92 inches or 760 mm or 76 cms, with an arrow spanning from the mercury surface in the trough to the top of the column.
Showing Pressure On A Map
A single reading is of limited use, because weather is made by differences in pressure across a region rather than by the pressure at one station. To see those differences, the variations in pressure are shown on maps by means of isobars, which are lines joining the places having the same barometric pressure.
An isobar works exactly like a contour line on a relief map, except that it joins equal pressure instead of equal height. Read together, a set of isobars turns a scatter of separate barometer readings into a picture of where the air is heavy and where it is light.
14.2.1 Pressure Gradient
Pressure gradient is defined as the decrease in pressure per unit distance in the direction in which the pressure decreases most rapidly. In other words, the rate of change of atmospheric pressure between two points on the earth’s surface is called pressure gradient.
On the weather chart this is indicated by the spacing of the isobars. The gradient is steep if they are close together and gentle if they are far apart. Close spacing of isobars indicates a strong pressure gradient, while wide spacing suggests a weak gradient.
Diagram
Fig. 14.A2 Caption: The same fall in pressure spread over a short distance or a long one gives a steep gradient or a gentle one. Brief: Two side-by-side panels of a weather chart; left panel shows isobars drawn close together and is titled steep gradient, right panel shows isobars widely spaced and is titled gentle gradient; label the isobars only as higher pressure and lower pressure with no numerical values; add a straight arrow across each panel pointing from higher to lower pressure, drawn thick on the left and thin on the right.
The spacing is a slope seen from above. Think of the pressure falling like ground falling away: crowd the drop into a short distance and you get a cliff, spread it over a long distance and you get a gentle incline. Air, like water, moves faster down a steep slope, which is why a forecaster reading closely packed isobars expects strong wind before any wind has been reported.
14.3 Factors Affecting Atmospheric Pressure
Pressure at a place is not fixed. It changes with where the place is, how warm it is, how much moisture the air holds, and the turning of the earth itself. Four factors control it. (a) Altitude decides how much air is stacked above you, and the higher you stand the less there is. (b) Temperature decides how tightly the molecules of that air are packed, because warm air expands and cool air contracts. (c) Water vapour decides how heavy the mixture is, because water molecules are lighter than the molecules of dry air they replace. (d) The rotation of the earth throws air away from the Poles towards the Equator, redistributing it across the latitudes.
14.3.1 Altitude
The atmospheric pressure decreases with height or altitude. The atmospheric pressure is highest at sea level. This is because at higher altitudes the air is thinner or less dense than the air at the sea level.
The maximum air density — the mass of air packed into a given volume — is at the earth’s surface. Air density decreases with height, because the pull of the earth’s gravity is less. The fewer number of gas molecules at higher altitudes means fewer molecular collisions, and so a decrease in air pressure. Since the atmosphere is highly compressible, the overlying layers exert pressure on the low lying layers and squeeze them tight.
What Thin Air Does To People
As the pressure decreases, the amount of oxygen available to breathe also decreases. At high altitudes, atmospheric pressure and available oxygen get so low that people feel breathless. Mountain climbers use bottled oxygen when they ascend very high peaks.
They also take time to get used to the altitude, because quickly moving from higher pressure to lower pressure can cause decompression sickness. Decompression sickness, also called ‘the bends’, is also a problem for scuba divers who come to the surface too quickly. Aircraft create artificial pressure in the cabin so that passengers remain comfortable while flying.
14.3.2 Temperature
Atmospheric pressure decreases with an increase in temperature. This is because when the temperature rises, air expands. The molecules of air move far apart, the air becomes less dense, and it hence exerts less pressure.
On the contrary, with a decrease in temperature the air gets compressed. The space between the molecules decreases, the air becomes more dense, and it exerts more pressure on the region. That is why the Equatorial region has a low pressure belt, whereas the Polar regions have high pressure belts.
Flowchart
Fig. 14.A3 Caption: Heat loosens air and cold packs it, and pressure follows the packing. Nodes: air at a place → is it heated or cooled? heated → air expands → molecules move far apart → air becomes less dense → exerts less pressure → low pressure, as over the Equator; cooled → air is compressed → space between molecules decreases → air becomes more dense → exerts more pressure → high pressure, as over the Poles
14.3.3 Water Vapour
Water vapour concentration affects atmospheric pressure, and it does so in a way that surprises most people. The molecular weight of water is 18 g/mol, while the average molecular weight of air is about 29 g/mol. Molecular weight in grams per mole is simply a way of comparing how heavy the molecules of one gas are against those of another, and by that comparison a water molecule is much the lighter of the two.
When water evaporates and enters the atmosphere as a gas, the water vapour molecules take the place of other gas molecules in the air. So a volume of wet or humid air weighs less than an equal volume of dry air. Therefore, humid air is less dense and exerts less pressure than dry air.
14.3.4 Rotation Of The Earth
Due to the rotation of the earth, the bulk of the air at the Poles is thrown away towards the Equator. The turning earth flings air outward most strongly where its surface moves fastest, which is at the Equator, and least where it barely moves at all, which is at the Poles.
Temperature works alongside this. Since the Equatorial region receives a great amount of heat throughout the year, the air there becomes warm and light and therefore rises, creating low pressure. At the Poles, the cold heavy air sinks down and creates high pressure. In fact, temperature and rotation of the earth together contribute to the formation of world pressure belts.
Diagram
Fig. 14.2 Caption: Rotation affects pressure. Brief: Globe marked with the Equator at 0°, and with 30° N, 60° N, 30° S and 60° S drawn as dashed lines; large curved arrows outside the globe running from Equator towards both Poles; inside, arrows showing deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, with ‘No deflection’ labelled at the Equator and ‘Maximum at poles’ at the top; mark the relative values 0% at the Equator, 50% at 30°, 86.7% at 60° and 100% at the Poles.
Why These Factors Matter
These factors together make air pressure an important parameter in predicting weather. In general, weather becomes stormy when air pressure falls, which happens generally because of warmer, humid air, or because air masses converge at the surface of the earth and cause convection and rising air.
Weather becomes fair when air pressure rises, generally because of drier, colder air, or because air masses diverge. When a low-pressure system moves into an area, it usually leads to cloudiness, wind and precipitation. High-pressure systems usually lead to fair and calm weather.
14.4 World Pressure Belts
The distribution of atmospheric pressure across the latitudes is termed global horizontal distribution of pressure. Its main feature is its zonal character, known as pressure belts — pressure is not scattered at random over the globe but arranged in bands that run east to west, parallel to the Equator.
On the earth’s surface there are in all seven pressure belts. (a) The Equatorial Low, lying astride the Equator, is the single belt that has no partner. (b) The two Sub-tropical Highs lie at about 30°, one in each hemisphere. (c) The two Sub-polar Lows lie polewards of them, again one in each hemisphere. (d) The two Polar Highs sit over the Poles themselves.
Except the Equatorial low, the others form matching pairs in the Northern and Southern Hemispheres. This mirroring is worth pausing on, because it tells you the belts are not accidents of geography. Continents and oceans are arranged quite differently north and south of the Equator, yet the belts still pair off, which means they are made by something that does not care about the map — heat and rotation.
Why The Belts Alternate
There is a pattern of alternate high and low pressure belts over the earth. This is due to the spherical shape of the earth, because different parts of the earth are heated unequally. The Equatorial region receives a great amount of heat throughout the year. Warm air being light, the air at the Equator rises, creating a low pressure.
At the poles the cold heavy air causes high pressure to be formed. It is also due to the rotation of the earth. In the Sub-polar region, around latitudes 60° to 65° North and South of the Equator, the rotation of the earth pushes up the bulk of the air towards the Equator, creating a low pressure belt in this region.
Diagram
Fig. 14.3 Caption: The formation of pressure belts. Brief: Curved cross-section of the earth’s surface with the Equator at the centre and 30° N and 30° S marked; show warm air rising in a broad column at the Equator labelled Low Pressure, and cool air descending on both sides at 30° labelled High Pressure; arrows loop from the rising column outward to the descending limbs and back along the surface towards the Equator, showing the circulation closing.
Flowchart
Fig. 14.A4 Caption: Air that leaves the Equator has to come down somewhere, and where it comes down is where high pressure is made. Nodes: intense heating at the Equator → air expands and rises → Equatorial Low Pressure Belt at the surface → air travels polewards high above the ground → cools and descends at about 30° → Sub-tropical High Pressure Belt → descending air divides into two parts → one part blows towards the Equatorial Low, back to the Equator; the other part blows polewards towards the Circum-polar Low
14.4.1 Equatorial Low Pressure Belts
This low pressure belt extends from 0 to 5° North and South of the Equator. Due to the vertical rays of the sun here, there is intense heating. The air therefore expands and rises as a convection current, causing a low pressure to develop here. A convection current is simply warm light air moving upward while cooler air slides in underneath to take its place.
This low pressure belt is also called the doldrums, because it is a zone of total calm, that is, without any breeze. The calm has a clear cause. Air here is moving almost straight up rather than sideways, and it is sideways movement that we feel as wind.
14.4.2 Sub-tropical High Pressure Belts
At about 30° North and South of the Equator lies the area where the ascending Equatorial air currents descend. This area is thus an area of high pressure. It is also called the Horse latitude.
Winds always blow from high pressure to low pressure, and this belt has low pressure on both sides of it. So the winds from the sub-tropical region blow towards the Equator as Trade winds, and towards the Sub-Polar Low-Pressure as Westerlies. Both of these are studied in detail later in this chapter.
14.4.3 Circum-polar Low Pressure Belts
These belts, located between 60° and 70° in each hemisphere, are known as Circum-polar Low Pressure Belts. In the Sub-tropical region the descending air gets divided into two parts. One part blows towards the Equatorial Low Pressure Belt. The other part blows towards the Circum-polar Low Pressure Belt.
This zone is marked by the ascent of warm Sub-tropical air over cold polar air blowing from the poles. Warm air is lighter, so where the two meet, the warm air is forced to climb over the cold, and rising air leaves low pressure behind it.
Due to the earth’s rotation, the winds surrounding the Polar region blow towards the Equator. Centrifugal forces operating in this region — the outward push felt by anything moving in a circle — create the low pressure belt appropriately called the Circum-polar Low Pressure Belt. This region is marked by violent storms in winter.
14.4.4 Polar High Pressure Areas
At the North and South Poles, between 70° to 90° North and South, the temperatures are always extremely low. The cold descending air gives rise to high pressures over the Poles.
These areas of Polar high pressure are known as the Polar Highs. These regions are characterised by permanent Ice Caps. The ice is part of the reason the high pressure lasts: a white ice surface throws back much of the sunlight that reaches it, so the air above stays cold, stays dense, and keeps sinking.
14.5 Shifting Of Pressure Belts
If the earth had not been inclined towards the sun, the pressure belts, as described above, would have been as they are — fixed bands, staying at the same latitudes all year. But it is not so, because the earth is inclined 23½° towards the sun.
On account of this inclination, differences in the heating of the continents, the oceans and the pressure conditions in January and July vary greatly. January represents the winter season and July the summer season in the Northern Hemisphere. Opposite conditions prevail in the Southern Hemisphere.
Diagram
Fig. 14.4 Caption: Pressure Belts. Brief: Globe showing the pressure and wind belts together — label Equatorial Low (Doldrums) at 0° with rising warm moist air and low pressure, Subtropical High (Horse Latitudes) at 30° N and 30° S with descending cool dry air and high pressure, Circum Polar Low at 60° N and 60° S, and Polar High at both Poles; between the belts draw and name the Northeast Trade Winds, Southeast Trade Winds, Westerlies in both hemispheres, and Polar Easterlies, with arrows curving in the direction each wind blows.
The Belts Follow The Overhead Sun
The belts are made by heat, so they move to wherever the heat is strongest. When the sun is overhead on the Tropic of Cancer, on 21 June, the pressure belts shift 5° northward. When it shines vertically overhead on the Tropic of Capricorn, on 22 December, they shift 5° southward from their original position.
When the sun shines vertically over the Equator on 21st March and 23rd September, the two Equinoxes, the pressure belts remain balanced in both the hemispheres. Neither hemisphere is favoured on those days, so the belts sit where they would if the earth had no tilt at all.
Diagram
Fig. 14.5 Caption: Shifting of the Pressure Belts. Brief: Three panels side by side headed 21 JUNE, 21 MARCH & 23 SEPTEMBER, and 23 DECEMBER; each panel shows the same set of horizontal pressure belts as coloured bands with the position of the SUN marked at the side; in the first panel the whole set of bands is drawn shifted northward, in the second it is balanced about the centre, and in the third it is shifted southward; small arrows within each band show wind direction.
Flowchart
Fig. 14.A5 Caption: The belts swing north and south with the sun and return to where they began each year. Nodes: 21 March, sun overhead at the Equator → belts balanced → 21 June, sun overhead on the Tropic of Cancer → belts shift 5° northward → 23 September, sun overhead at the Equator → belts balanced again → 22 December, sun overhead on the Tropic of Capricorn → belts shift 5° southward → back to 21 March
What The Shift Does To Climate
The shifting of the pressure belts causes seasonal changes in the climate, especially between latitudes 30° and 40° in both hemispheres. This band is the doorstep between two belts, so a 5° swing is enough to put it under one belt for half the year and the next belt for the other half.
In this region the Mediterranean type of climate is experienced because of the shifting of the permanent belts southwards and northwards with the overhead position of the sun. During winters the Westerlies prevail and cause rain. During summers dry Trade Winds blow offshore and are unable to give rainfall in these regions.
14.6 Winds
Air flows from areas of high pressure to areas of low pressure. The horizontal movement of the air is called wind. It is nature’s attempt to balance inequalities in air pressure, and it goes on ceaselessly because the sun keeps creating fresh inequalities faster than the air can flatten them out.
Air also moves up and down, and that movement has a different name. The vertical or nearly vertical movement of air is referred to as an air current. The two are not rivals but partners: an air current lifts air away from the surface at one place, wind carries air along the surface to replace it, and the air that rose has to come down somewhere else. Winds and air currents together manage a system of circulation in the atmosphere.
14.6.1 Factors Affecting Direction And Velocity Of Wind
The direction and speed of wind are controlled by a combination of factors. These are the pressure gradient, the Coriolis force, altitude, latitude, rotation of the earth, the centripetal acceleration and friction. Two of them matter most, and the chapter takes them in turn.
Pressure Gradient
We have already studied the pressure gradient. The greater the difference in pressure between two points, the steeper is the pressure gradient and the higher is the wind speed.
Since winds blow from a higher to a lower pressure area, and perpendicular to the isobars, these wind blows are parallel to the gradient and at right angles to the isobars. In other words, air takes the shortest route downhill in pressure, cutting straight across the lines rather than running along them.
Diagram
Fig. 14.7 Caption: Winds blow from high pressure to low pressure. Brief: A set of four curved isobars drawn roughly parallel down the frame and labelled ISOBARS 1031, 1018, 1015 and 1012 m b; a single long straight arrow labelled WIND DIRECTION crossing all four isobars at right angles, pointing from the 1031 mb isobar towards the 1012 mb isobar.
Coriolis Effect
The earth rotates on its inclined axis. If it did not, winds would follow the direction of the pressure gradient and travel in a straight line from high pressure to low. But the rotation produces another force other than the pressure force. It is called the ‘Coriolis Effect’ or ‘Coriolis Force’, which deflects the air.
The deflection is the least at the Equator and greatest at the Poles. This tends to turn the flow of air by changing its direction from its original straight path. The wind starts deflecting to its right in the Northern Hemisphere. In the Southern Hemisphere it starts deflecting to its left from its original path.
The result is that winds are named for a direction they did not start out with. Thus a wind blowing from the north becomes north-easterly in the Northern Hemisphere, and a wind blowing from the south becomes south-easterly in the Southern Hemisphere — the North East Trade winds are the standing example. This is due to maximum speed of rotation at the Equator, hence the deflection is less there.
Diagram
Fig. 14.6 Caption: Coriolis Force. The dotted arrows show the actual direction of winds, the arrows show the deflected winds. Brief: Globe with dashed latitude lines at 30° N, 60° N, the Equator, 30° S and 60° S; on each side show dotted straight arrows for the original path of the wind and solid curved arrows for the deflected path, curving right in the Northern Hemisphere and left in the Southern; label the relative Coriolis effect along the rim as 0% at the Equator, 50% at 30°, 86% at 60° and 100% at both Poles.
14.7 Types Of Winds
Not every wind behaves in the same way. Some blow steadily for a lifetime, some turn right around with the seasons, and some are felt only in one valley or one plain. Winds are classified by that behaviour, and the chapter deals with three chief types. (a) The winds which blow throughout the year from one latitude to the other, in response to the latitudinal differences in air pressure, are known as Permanent or Prevailing or Planetary Winds. (b) Certain winds reverse their direction periodically with the season and are called Periodic Winds. (c) There are certain winds in different parts of the world which flow in a comparatively small area and have special characteristics. These are called Local Winds.
The three types differ in the scale they work at, and that is the key to keeping them apart. A permanent wind answers to the planet’s own pressure belts, so it stretches across whole oceans and does not change its mind. A periodic wind answers to the seasonal difference in heating between a landmass and the sea beside it, so it lasts as long as that difference lasts and then reverses. A local wind answers to the shape of one stretch of country — a mountain wall, a desert, a valley floor — and it dies out a short distance beyond it.
Flowchart
Fig. 14.A6 Caption: The three chief types of wind, separated by how long they last and how far they reach. Nodes: winds → does the wind blow the same way all year? yes → Permanent, Prevailing or Planetary Winds, blowing from one latitude to another across the globe; no → does it reverse with the season? yes → Periodic Winds, following the seasonal change in heating; no → Local Winds, confined to a small area and shaped by the country they cross
14.8 Permanent Winds
Permanent winds are the planet’s own circulation, running between the pressure belts described earlier. There are three of them in each hemisphere, and each one simply fills the gap between a high pressure belt and the low pressure belt next door.
14.8.1 The Trade Winds
The winds blowing from the Sub-Tropical High Pressure area, at 30° N and S, towards the Equatorial Low Pressure belt are the extremely steady winds known as the trade winds. The Trade Winds are a result of a pressure gradient from the Sub-Tropical Belt of High Pressure to the Equatorial Belt of Low Pressure.
The name has nothing to do with buying and selling. It comes from the German word trade, meaning ‘track’. ‘To blow trade’ means ‘to blow steadily in the same direction and in a constant course’, and steadiness is exactly what these winds are known for.
North and South of the Equatorial Belt are the Trade Winds in the zone lying between 5° and 30° North and South. In other words, they cover almost the entire area between 30°N and 30°S latitudes on both sides of the Equator.
Why They Are Called North-East And South-East
In the Northern Hemisphere, the wind moving towards the Equator is deflected by the earth’s rotation to flow south-westward. Thus the prevailing wind there is from the North-East, and it has been named the ‘North-East Trade’. In the Southern Hemisphere, deflection of the wind is towards the left, and this causes the ‘South-East Trades’.
Remember that a wind is always named for the direction it comes from, not the direction it is heading. A wind arriving from the north-east and travelling towards the south-west is a north-easterly.
Flowchart
Fig. 14.A7 Caption: One wind, two coasts, two entirely different results. Nodes: Sub-tropical High Pressure Belt at 30° → air flows towards the Equatorial Low → Coriolis deflection → in the Northern Hemisphere, deflected right, giving the North-East Trades; in the Southern Hemisphere, deflected left, giving the South-East Trades → the wind crosses warm ocean and picks up moisture → does it blow on-shore or off-shore? on-shore on the eastern coasts of continents → heavy rainfall; off-shore on the western coasts → no rainfall → aridity
Wet Coasts And Dry Coasts
Trade winds bring heavy rainfall to the eastern coasts of continents lying within the Tropics. On the western coasts of continents, these winds do not bring any rainfall. It is because here they are ‘off-shore’ winds, or winds blowing just parallel to the shores.
The difference is simply which way the wind is facing when it meets the coast. An on-shore wind has crossed open sea before it arrives, so it is loaded with moisture and gives up its water over the land. An off-shore wind has crossed dry land before it reaches the coast, and it leaves with the sea rather than arriving from it. Therefore the western areas within the tropics suffer from aridity, which is a long-term shortage of rainfall.
The proof of this is written across the world map. The great deserts of the Sahara in northern Africa, the Kalahari in southern Africa, the Atacama along the western coast of South America, and the Great Australian Deserts, all lie on the western margins of the continents, within the tropical latitudes.
Map
Map 14.A2 Caption: The great tropical deserts named in this chapter, all of them on the western margins of their continents. Brief: World outline map with continents and the Equator, Tropic of Cancer and Tropic of Capricorn drawn and labelled; shade and name the Sahara, the Kalahari, the Atacama and the Great Australian Deserts; add arrows on the eastern coasts within the tropics showing the Trade Winds blowing on-shore, and arrows on the western margins showing them blowing off-shore; key to distinguish desert shading from wind arrows; no other natural regions and no rainfall figures shown.
Characteristics Of Trade Winds
They blow from the Sub-tropical High Pressure to the Equatorial Low Pressure, and four features follow from that. (a) Since they are warm winds, they pick up moisture and are responsible for heavy rainfall on the eastern sides of tropical lands. (b) They are called North-East Trades in the Northern Hemisphere and South East Trades in the Southern Hemisphere, and the winds and pressure belts move a few degrees north and south along with the movement of the overhead sun. (c) They have fixed velocity and are regular. (d) They are permanent or prevailing winds.
14.8.2 The Westerlies
The Westerlies, or the Prevailing Westerly Winds, blow between 35° and 60° North and South latitudes, from the Sub-Tropical High Pressure Belts towards the Sub-Polar Low-Pressure Belts. They begin in the same high pressure belt as the Trades, but travel away from the Equator instead of towards it.
In the Northern Hemisphere, the Westerlies generally blow from the south-west to the north-east, and in the Southern Hemisphere from the north-west to the south-east. These are on shore winds on the west coasts and off-shore winds on their east coasts. The on-shore winds bring rainfall while the off-shore winds do not bring rainfall.
Notice that this is the reverse of the pattern the Trades produce. In the trade wind belt the eastern coasts are wet; in the westerly belt it is the western coasts that are wet, because that is the side the wind arrives from.
Characteristics Of Westerlies
Four features mark these winds out. (a) They blow from the Sub-tropical High Pressure to the Sub-polar low. (b) They are very strong winds, and often blow from the western side of a landmass. (c) They are interspersed by cyclones and cause light drizzle. (d) They are stronger in the southern hemisphere, as there is an absence of landmass.
14.8.3 The Polar Easterlies
The winds that originate in the North and South Polar regions and blow towards the Circum-polar Low Pressure Zone are known as Polar Winds. They start from the Polar High Pressure Zone, and originate from the ice-capped landmass in the Arctic and Antarctic latitudes.
In the Northern Hemisphere, they blow from the north-east, and are called the North-East Polar Winds. In the Southern Hemisphere, they blow from the south-east and are called the South-East Polar Winds. They are also deflected to the west in both hemispheres, and hence are known as Polar Easterlies.
Characteristics Of Polar Winds
Three features are worth holding on to. (a) They are very cold winds, because they begin over permanently frozen surfaces. (b) They are also referred to as Polar Easterlies from the direction in which they blow. (c) When they blow over oceans they become warm.
14.9 Periodic Winds
Periodic winds blow at regular intervals or in regular cycles. They are winds that result from localised differences in pressure and temperature, rather than from the world pressure belts. For example, land and sea breezes and the seasonal winds.
The word localised matters. A permanent wind is set up by a difference between one latitude and another. A periodic wind is set up by a difference between two surfaces lying side by side — land and water — which heat and cool at different rates.
14.9.1 Land And Sea Breezes
Land and sea breezes are caused by the differential rate of heating of the land and the sea. Land takes in heat quickly and loses it quickly, while water warms slowly and holds its warmth long after the sun has gone.
During the day the land gets heated faster than the adjoining sea. This creates a low pressure zone on the land and a high pressure zone over the sea. Thus the winds blow from the sea to the land, and are called Sea Breezes.
At night the reverse of this happens. The land loses its heat faster than the sea, so the sea is now the warmer surface with lower pressure above it, and the winds blow from land to sea. These are called Land Breezes.
Diagram
Fig. 14.8 Caption: (A) Cool Sea Breeze, (B) Warm Land Breeze. Brief: Two block diagrams of the same stretch of coast. In panel A, label Cooler Sea and Warmer Land, show warm air ascending over the land with wavy upward arrows, cool air descending over the sea, and horizontal arrows crossing from sea to land labelled Sea Breeze. In panel B, label Warmer Sea and Cooler Land, show warm air ascending over the sea, cool air descending over the land, and horizontal arrows crossing from land to sea labelled Land Breeze.
Flowchart
Fig. 14.A8 Caption: The breeze turns right around every twelve hours because the land changes temperature faster than the sea. Nodes: day → land heats faster than the sea → low pressure over land, high pressure over sea → air moves from sea to land → sea breeze → night → land cools faster than the sea → high pressure over land, low pressure over sea → air moves from land to sea → land breeze → back to day
The Monsoon Winds
Monsoons are periodic seasonal winds blowing in the regions of South East Asia and Northern Australia. The word monsoon is derived from the Arabic word Mausim, meaning ‘season’.
They develop because of differences in the heating conditions of the continent and the oceans. They are divided into two wind systems — the Summer Monsoon and the Winter Monsoon.
14.9.2 Summer Monsoons
In summer the land gets more heated than the sea. Hence there develops a centre of low pressure on the land. Over the adjoining sea, the air is comparatively cool, and a high pressure develops there. This causes the winds to blow from the sea to the land. It is the ‘Summer Monsoon’.
In May, June and July, the plains of India and China are heated by the vertical rays of the sun. The intense heat develops a continental low pressure. During these months, over the Indian Ocean, a high pressure area develops.
So the winds blow from the Indian Ocean northward and north-westward into Asia. As they blow from the sea to the land, they bring heavy rainfall to South-East Asia. The summer monsoon winds blow south-west, so they are known as the ‘South-West Summer Monsoon’.
Map
Fig. 14.9 (a) Caption: Summer Monsoon Winds. Brief: Map of southern and eastern Asia headed SUMMER MONSOON (JULY), showing the full extent of India as officially defined; label LOW PRESSURE over the landmass and HIGH PRESSURE over the ocean to the south; draw bold arrows running from the Indian Ocean northward and north-westward across the coastline into the continent; no rainfall figures and no isobar values.
14.9.3 Winter Monsoons
During the winter season, the conditions are just the reverse of those of summer. A high pressure develops over a big landmass stretching from Central Asia up to the north-west Indian plain. At the same time a low pressure zone develops in the Indian Ocean.
As the winds now blow from the land to the sea, they bring cold dry weather. They are incapable of producing rain, because they begin over a dry continental interior with no water to pick up.
Map
Fig. 14.9 (b) Caption: Winter Monsoon Winds. Brief: Map of southern and eastern Asia headed WINTER MONSOON (JANUARY), showing the full extent of India as officially defined; label HIGH PRESSURE over the landmass stretching from Central Asia to the north-west Indian plain, and LOW PRESSURE over the ocean to the south; draw bold arrows running from the interior of the continent outward across the coastline towards the sea; no rainfall figures and no isobar values.
Where The Winter Monsoon Does Bring Rain
There is an exception, and it depends entirely on how much sea a wind has crossed. When these winds blow over seas and pass over the adjoining land, they bring some rainfall.
The Southern Coromandel Coast of Tamil Nadu, on the eastern edge of the southern Indian peninsula, gets rain from the winter monsoons. So do the Vietnamese Coast and the west coast of Japan. The winter monsoon winds blow north-east, so this monsoon is known as the ‘North-East Winter Monsoons’.
14.10 Local Winds
Local winds are restricted to a certain place only. They may be warm or cold depending upon the area from which they blow, because a wind takes its character from the country it has just crossed. A wind off a desert arrives hot and dry; a wind off a snowfield arrives cold. For example, the Harmattan is a hot local wind of the Sahara desert.
The Loo
In the plains of northern India and Pakistan, sometimes a very hot and dry wind blows from the west in summer in the afternoons. It is known as the loo. Its temperature invariably ranges between 45°C and 50°C.
This is not a distant curiosity but the wind that empties streets and markets across northern India every May. It may cause sunstroke to people, which is why work outdoors stops through the middle of the day while it blows.
Foehn And Chinook
A strong warm wind develops on the leeward side of the Alps, the mountain chain of south-central Europe. The leeward side is the sheltered side, away from the direction the wind is coming from. Due to a regional pressure gradient, air is forced to cross the barrier, and the crossing changes it completely.
As the air ascends the southern slopes of the Alps, it expands and cools. Condensation takes place when the air is saturated — that is, when it is holding all the water vapour it can carry — and this causes rain and snowfall on the higher slopes.
However, on descending the northern slopes, the wind experiences an increase in pressure and temperature. Due to this the air is compressed and warmed. Most of its moisture is lost, and it reaches the valley bottom as a dry, hot wind, called the Foehn. The temperature of this wind is from 15°C to 20°C.
Flowchart
Fig. 14.A9 Caption: A wind that climbs a mountain wet and cool comes down the other side dry and warm. Nodes: pressure gradient forces air to cross the mountain barrier → air ascends the slope → it expands and cools → air becomes saturated → condensation → rain and snowfall on the higher slopes → air crosses the crest, now with most of its moisture lost → air descends the far slope → pressure and temperature increase → air is compressed and warmed → dry hot wind reaches the valley bottom, the Foehn or the Chinook
The Foehn is of use for melting snow, and it hastens the ripening of grapes in the valleys it warms. A similar kind of wind in the USA and Canada moves down the west slopes of the Rockies, the mountain chain of western North America, and these are known as chinooks. The word chinook literally means ‘snow eater’. It is beneficial to ranches east of the Rockies, as it keeps the grasslands clear from snow so that cattle can go on grazing through the winter.
Map
Fig. 14.10 Caption: Chinooks. Brief: Outline map of the USA and southern Canada; mark a low pressure centre inland with curved arrows circling it; draw and label the warm front and the cold front extending from it in the conventional symbols; show arrows for the chinook moving down the eastern side of the Rockies over the grasslands; no temperature or pressure values printed.
The Mistral
During winter, areas adjacent to highlands may experience a local cold wind which originates over the snowcapped mountains or highlands and blows down the valley. These winds have been given local names in the different regions where they occur.
The most famous is the mistral, which blows from the Alps over France towards the Mediterranean Sea. Even though the skies are clear, the mistral brings down the temperature below freezing point. Clear skies usually promise mild weather, so a cold snap under an open sky is the signature of a wind of this kind.
These local winds often have a considerable effect on climatic conditions, notably on the temperature of a place. None of them lasts long or travels far, yet a single afternoon of the loo or a night of the mistral can decide what a season feels like to the people living through it.
14.11 Variable Winds
These winds are related to pressure systems and blow in small areas. They are called variable because they do not blow in any definite direction, and their direction varies with the movement of the pressure system itself. A permanent wind can be predicted from a map of the pressure belts; a variable wind has to be tracked, because it travels and takes its winds with it.
Their speed also depends on the intensity of the depression. A depression is simply an area where pressure is low, and the deeper it is, the harder the surrounding air rushes in. These winds last only for a few days. There are two chief types of variable winds — cyclones and anticyclones.
14.11.1 Cyclones
In low latitudes, an intense depression with a low pressure centre is known as a Tropical cyclone in the Indian Ocean area, as hurricanes in the Caribbean, typhoons in China and willy-willies in Australia. The storm is the same thing in each case; only the local name changes.
A cyclone is a portion of the atmosphere in which the pressure is lowest in the centre. The winds blow inward in the opposite direction, that is, from the south or south-west in the front of the cyclone, and from the north or north-west in its rear.
When a cyclone approaches, the reading on a barometer falls because of the low pressure, but the thermometer rises due to the warm south or south-west winds. Those two instruments moving in opposite directions are the earliest warning a station has. The rising moisture-laden air of such a cyclone results in heavy rain in the centre of the depression.
Cyclones, on account of the Coriolis Force, blow in an anti-clockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere. This is the same deflection studied earlier, now bending air that is spiralling inward rather than travelling across the globe.
Tropical Cyclones
Cyclones are associated with turbulent weather conditions, thick cloud cover, strong winds and rainfall. Tropical cyclones thus cause heavy damage to property and loss of human lives.
The cyclones generally originate in the tropical region between 8° and 20° N and S. They are more frequent in summer because of the movement of the Doldrum belt away from the Equator. Most often they originate in the South China Sea, off the coast of South East Asia, and cause a lot of damage to life and property in the countries bordering that region. In the Bay of Bengal and the Arabian Sea, on either side of peninsular India, they cause great damage, as they are strong winds even though their range is small.
Inside The Storm
The central part of a cyclone is a calm region and is known as the eye of the cyclone, surrounded by a turbulent vortex. The vortex is the whirling wall of winds that rings the centre, and it is there that the storm does its damage. The force of the winds depends on the intensity of the low pressure centre and the surrounding high pressure. The greater the difference, the stronger is the wind.
Diagram
Fig. 14.11 Caption: A cross section of a tropical cyclone. Brief: Vertical cross-section through a tropical cyclone, labelled INNER ZONE at the centre and OUTER ZONE on both sides; show rapidly rising moist air on either side of the centre with upward arrows, descending air in the middle with downward dashed arrows, and label the base as vortex, eye, vortex from left to right.
The weather conditions associated with a tropical cyclone follow one another in a fixed order as the storm passes over a place. (a) When there is a tropical cyclone the air is still, but the temperature and the humidity are high. There is a sudden drop in air pressure. (b) When the front of the vortex arrives, there are strong winds and thick clouds, then the winds become violent with great speed. Dense clouds and heavy rain reduce visibility. (c) The eye of the cyclone heralds a calm condition. (d) When the rear of the vortex arrives, there are violent winds, thick clouds and heavy rain. The wind blows in the opposite direction.
Flowchart
Fig. 14.A10 Caption: The calm in the middle is not the end of the storm but the halfway mark. Nodes: still air, high temperature and humidity, sudden fall in pressure → front of the vortex arrives → strong winds, thick clouds, violent speed, heavy rain, poor visibility → eye of the cyclone passes overhead → calm conditions → rear of the vortex arrives → violent winds again, now blowing in the opposite direction, with thick clouds and heavy rain → storm moves away
That reversal in point (d) is the most dangerous moment of the whole sequence. People who step outside during the calm of the eye, believing the storm to be over, are caught by winds that return from the other side.
Tornadoes And The Other Names
A tornado is a tropical cyclone which occurs over land. Its diameter is hardly 300 or 400 metres, which is smaller than many school grounds, and this makes it a very concentrated storm rather than a weak one. It looks like a dark, funnel-shaped cloud extending downward from the base of the thunderstorm.
It has low pressure in the centre, with very strong winds blowing inwards. The speed of the winds may be as high as 500 to 800 km per hour. Tornadoes are generally experienced over tropical waters in North America.
Diagram
Fig. 14.12 Caption: Tornado. Brief: Photograph of a tornado over open country — a dark funnel-shaped cloud reaching from the base of a heavy thunderstorm down towards the ground, with the horizon and a few trees visible for scale.
Other forms of Tropical cyclones are hurricanes in the south-east Caribbean region, typhoons in eastern China, Japan and the Philippines islands, and cyclones in the Bay of Bengal and the Arabian sea.
Temperate Cyclones
Temperate cyclones are active over the mid-latitudinal region between 35° latitude and 65° latitude in both hemispheres. They are also known as extra-tropical or wave cyclones.
The two kinds of cyclone differ in almost every respect. Tropical cyclones are produced and developed mainly over the sea, while temperate cyclones are produced both on land and on sea. Tropical cyclones are limited to a small area and usually travel from east to west, whereas temperate cyclones occupy areas measuring a couple of thousands of kilometres and move from west to east. In a temperate cyclone, wind speed is low and all the sectors of the cyclone have different temperatures, and the rainfall is light and continues for many days.
The centre also behaves differently. The centre of a tropical cyclone is known as the eye, and the wind is calm at the centre with no rainfall. In a temperate cyclone, there is not a single place where winds and rains are inactive, and the direction of winds rapidly changes at the front.
The temperate cyclones are associated with anticyclones which precede and succeed a cyclone. The approach of a temperate cyclone is marked by a fall in temperature, a fall in the mercury level, wind shifts, and a halo around the sun and the moon — a ring of light seen when high thin cloud runs ahead of the storm. A light drizzle follows, which turns into a heavy downpour.
14.11.2 Anticyclones
An anticyclone is a fine atmospheric condition, and it is the cyclone turned inside out. There is High Pressure in the core or centre and Low Pressure around it, so the winds blow gently outwards instead of rushing inwards.
These winds are clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere — again the opposite of the cyclone in the same hemisphere, because the Coriolis Force is now bending air that is moving outward. Since they are centres of high pressure, they are found in regions of descending air currents, and descending air brings the clear calm weather these systems are known for.
Diagram
Fig. 14.13 Caption: The direction of anticyclones. Brief: Two oval pressure systems labelled (a) Northern Hemisphere and (b) Southern Hemisphere; in each, mark the core as High pressure with concentric dashed isobars labelled 1032 mb and 1028 mb; draw arrows spiralling outwards from the centre, clockwise in panel (a) and anticlockwise in panel (b).
14.12 Jet Streams
Jet streams refer to the concentrated bands of rapid air movement found at the tropopause and the stratosphere, located at 10–15 kilometres above the surface of the Earth. The tropopause is the ceiling of the lowest layer of the atmosphere, and 10 to 15 km is roughly the height at which a long-distance passenger aircraft cruises.
They are formed near the boundaries of adjacent air masses with significant differences in temperature, such as the Polar region and the warmer air to the south. A sharp temperature difference across a short distance means a sharp pressure difference, and a sharp pressure difference means fast-moving air.
They are associated with latitudes where the poleward temperature gradient is particularly strong. Two such zones occur in each hemisphere. The sub-tropical jet stream occurs at about 30° latitude, and the other, the polar front jet stream, is associated with the polar front zone in each hemisphere. The major jet streams are westerly winds, flowing west to east, in the Northern Hemisphere.
During the summer, easterly jets are formed in tropical regions, typically in a region where dry air encounters more humid air at high altitudes. Low level jets can form wherever low level winds are squeezed together, typically between an oncoming front and a high pressure cell.
Flowchart
Fig. 14.A11 Caption: Two air masses of very different temperature, meeting high above the ground, make a river of fast-moving air. Nodes: cold polar air lies beside warmer air to the south → a strong poleward temperature gradient forms at the boundary → a steep pressure difference develops at 10–15 km, near the tropopause → air accelerates into a narrow concentrated band → jet stream → in the Northern Hemisphere it flows west to east as a westerly → two zones form in each hemisphere: the sub-tropical jet at about 30° latitude, and the polar front jet at the polar front
14.12.1 Importance Of Jet Streams
Jet streams matter to ordinary life far more than their height suggests, and the chapter gives three reasons. (a) Most weather systems do not just sit over an area; they are instead moved forward with the jet stream, so the position and strength of the jet stream then helps meteorologists forecast future weather events. (b) The airline industry consistently uses the jet stream for its flights, because by flying commercial aircraft within the jet stream the flight time gets reduced, and the reduced flight time together with the aid of the strong winds leads to a reduction in fuel consumption. (c) Precipitation usually increases in California, on the western coast of the United States, because the polar jet stream moves farther south and brings more storms with it. In addition, precipitation often increases in Europe, because the jet stream is stronger in the Northern Atlantic.
Whatever the position of the jet stream, it has a significant impact on the world’s weather patterns and on severe weather events like floods and droughts. It is therefore essential that meteorologists understand as much as possible about the jet streams, and continue to track their movement, in order to monitor weather conditions around the world.
14.13 Summary
- Air is a physical substance with weight, and the weight of air on a unit area of the earth is called air pressure.
- Atmospheric pressure is the force per unit area exerted by the weight of the air above a surface.
- It is measured with a mercury barometer and expressed in millibars.
- The average pressure at sea level is 1013.25 mb, or 760 mm of mercury.
- Pressure decreases with height.
- Isobars are lines joining places of equal barometric pressure, and their spacing on a weather chart shows the pressure gradient.
- Pressure gradient is the decrease in pressure per unit distance in the direction in which pressure falls most rapidly.
- Closely spaced isobars mean a strong gradient; widely spaced isobars mean a weak one.
- Four factors control atmospheric pressure at a place.
- Altitude — air is thinner and less dense with height, so pressure is highest at sea level.
- Temperature — warm air expands, becomes less dense and exerts less pressure, while cold air is compressed and exerts more.
- Water vapour — water has a molecular weight of 18 g/mol against about 29 g/mol for air, so humid air is less dense than dry air.
- Rotation of the earth — air at the Poles is thrown towards the Equator.
- The distribution of pressure across the latitudes is termed global horizontal distribution of pressure, and it is arranged in seven belts.
- The Equatorial Low, from 0 to 5° N and S, is also called the doldrums and is a zone of total calm.
- The two Sub-tropical Highs at about 30° are the Horse latitudes, where the ascending Equatorial air descends.
- The two Circum-polar Lows lie between 60° and 70° and are marked by violent storms in winter.
- The two Polar Highs lie between 70° and 90° and are characterised by permanent Ice Caps.
- Because the earth is inclined 23½°, the belts shift with the overhead sun.
- They move 5° northward on 21 June and 5° southward on 22 December.
- They stay balanced at the Equinoxes, on 21 March and 23 September.
- This shifting gives the Mediterranean type of climate between 30° and 40°, with rain from the winter Westerlies and dry offshore Trade Winds in summer.
- Wind is the horizontal movement of air from high pressure to low pressure, while the vertical movement of air is called an air current.
- Wind direction and velocity are controlled by the pressure gradient, the Coriolis force, altitude, latitude, rotation of the earth, centripetal acceleration and friction.
- A steeper pressure gradient gives a higher wind speed, and winds cross the isobars at right angles.
- The Coriolis Effect deflects winds to the right in the Northern Hemisphere and to the left in the Southern, least at the Equator and greatest at the Poles.
- Permanent or planetary winds blow all year between the pressure belts.
- The Trade Winds blow from the Sub-tropical High to the Equatorial Low, as North-East Trades and South-East Trades, bringing heavy rain to eastern tropical coasts and leaving the western margins arid.
- The Westerlies blow between 35° and 60° from the Sub-tropical High to the Sub-polar Low, and are stronger in the Southern Hemisphere because landmass is absent.
- The Polar Easterlies blow from the Polar High to the Circum-polar Low and are very cold.
- Periodic winds reverse with the season, because land and sea heat and cool at different rates.
- Sea breezes blow from sea to land by day; land breezes blow from land to sea by night.
- The South-West Summer Monsoon blows from the Indian Ocean into Asia and brings heavy rainfall.
- The North-East Winter Monsoon blows from land to sea and is dry, except where it crosses a sea, as on the Coromandel Coast.
- Local winds are restricted to a small area and take their character from the land they cross.
- The loo is a hot dry summer wind of the northern plains, ranging between 45°C and 50°C.
- The Foehn and the chinook are warm dry winds descending the leeward slopes of the Alps and the Rockies.
- The mistral is a cold wind blowing from the Alps over France towards the Mediterranean Sea.
- Variable winds are tied to travelling pressure systems and last only a few days.
- A cyclone has lowest pressure at the centre, with winds spiralling inwards, anti-clockwise in the Northern Hemisphere and clockwise in the Southern.
- Tropical cyclones form between 8° and 20° N and S, have a calm eye ringed by a turbulent vortex, and are called hurricanes, typhoons and willy-willies elsewhere.
- A tornado is a tropical cyclone over land, only 300 or 400 metres across, with winds of 500 to 800 km per hour.
- Temperate cyclones form between 35° and 65°, cover thousands of kilometres, move west to east, and bring light rainfall over many days.
- An anticyclone has high pressure at the core, with winds blowing gently outwards, and brings fine weather.
- Jet streams are concentrated bands of rapid air movement at 10–15 kilometres above the surface, forming where the temperature gradient between air masses is strong.
- The sub-tropical jet occurs at about 30° latitude and the polar front jet at the polar front, two zones in each hemisphere.
- They steer weather systems, shorten flight times and reduce fuel use, and shift precipitation patterns when they move.
Flowchart
Fig. 14.A12 Caption: The whole chapter in one chain — unequal heating makes pressure differences, pressure differences make winds, and the earth’s turning decides which way they go. Nodes: unequal heating of a spherical earth → contrasts in temperature and air density → seven world pressure belts → pressure gradient between belts → air moves from high to low pressure → Coriolis deflection, right in the north and left in the south → winds → permanent winds, from the belts, blowing all year as Trades, Westerlies and Polar Easterlies; periodic winds, from land and sea heating unequally, reversing as breezes and monsoons; local winds, from the country they cross, as the loo, Foehn, chinook and mistral; variable winds, from travelling pressure systems, as cyclones and anticyclones → weather at a place: temperature, humidity and precipitation → the belts shift 5° with the overhead sun → back to unequal heating in the next season
What This Chapter Cannot Tell You
The seven belts are drawn as neat unbroken bands, but the real atmosphere is not so tidy. The belts are broken up by continents and oceans, they wander from year to year, and a single day’s weather chart may show nothing resembling them.
Nor does the chapter give the data behind its own claims. It states that pressure falls with height and that cyclones form between 8° and 20°, but it prints no measurements from which those limits could be checked, and no record of how often the limits are exceeded.
Above all, this chapter explains why a wind blows, not what it will do to a particular place on a particular day. That gap between the general pattern and the local forecast is the work of the meteorologist, and it is why storms must be tracked individually even when the physics behind them is fully understood.
14.14 Check Your Concepts
Look at Fig. 14.7, in which the isobars are marked 1031, 1018, 1015 and 1012 mb.
- Calculate the total fall in pressure, in millibars, from the first isobar to the last.
- Explain why the wind direction arrow crosses the isobars at right angles rather than running along them.
A ship reports a storm forming over the sea at 12° N in the month of May.
- Using Table 14.1, state which two rows of the table help you decide whether this is a tropical or a temperate cyclone.
- Name the type of cyclone it is, and give the reason for your answer.
Air is forced to cross the Alps and descends the far side as the Foehn. Trace the process in order, from the moment the air begins to climb the slope to the moment it reaches the valley bottom, and explain at which stage the air loses its moisture and at which stage it gains its warmth.
The Sahara, the Kalahari, the Atacama and the Great Australian Deserts all lie on the western margins of their continents within the tropics. Explain why the Trade Winds leave these margins arid while bringing heavy rainfall to the eastern coasts of the same continents.
Distinguish between cyclones and anticyclones.
Give a geographical reason for each of the following.
- The Westerlies in the Southern Hemisphere blow with greater force than those in the Northern Hemisphere.
- There is a seasonal shifting of the pressure belts.
- As we go higher, the atmospheric pressure decreases.
What is the Coriolis Effect? How does it affect the planetary winds?
Explain the weather conditions associated with tropical and temperate cyclones.
14.15 Objective Questions
- Study the direction of the wind in Fig. 14.13. The anticyclone shown in panel (a) is occurring in the
- Northern Hemisphere
- Southern Hemisphere
- Equatorial Region
- Polar Region
- Your uncle was puzzled because his flight from Delhi to London took more than 9½ hours, but his return flight from London took about 8½ hours. As a student of geography, how would you explain this to him?
- The pressure gradient supported the return flight.
- Flying along a jet stream reduces flight time.
- The Coriolis Force deflects air, reducing flight time.
- None of the above.