BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 9

Cause of wind It is important at this stage to remember that atmospheric pressure unlike the force of gravity which acts vertically downwards at any point is exerted equally in all dire

CHAPTER NINE

Atmospheric Pressure and Wind

Atmospheric Pressure

Atmospheric pressure at any level (height above the sea) is caused by the weight of air which lies above that level It follows, therefore, that pressure decreases as height increases: for example atmospheric pressure at a height of approximately 5,500 m (18,000 ft) is generally about half its value at ground level

Surface pressure at anyone point varies continually, the average is about 1012 hPa

at sea level

Units of barometric pressure

Pressure may be expressed in "inches" or ”centimetres" being the equivalent to the height of a column of mercury (under certain standard conditions) which is required to balance atmospheric pressure In modern meteorology pressure is expressed in hPa or millibars Atmospheric pressure at sea level is of the order of 1,000 hPa

Isobars

An isobar is a line, drawn on a weather chart, which passes through all points of equal barometric pressure Isobars are spaced at intervals of one or more millibars, depending on the scale of the chart The isobaric patterns which they form enable

us to recognise definite pressure systems such as depressions, anticyclones, ridges, etc., each of which is associated with its own characteristic weather

Cause of wind

It is important at this stage to remember that atmospheric pressure (unlike the force

of gravity which acts vertically downwards) at any point is exerted equally in all directions

Horizontal movement of the air is caused by differences in pressure between one point at that level and another This difference in pressure produces a pressure gradient force, which acts to move the air directly from high pressure to low pressure

Relationship between wind direction and isobars

produces an effect upon the motion of the air which is seen by an observer at the earth's surface The path of the air appears to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere Calculation of this effect is simplified by using an imaginary force, the Coriolis Force, to represent the effect of the earth's rotation

At heights of 600 m or more above the earth's surface the effects of surface friction can be ignored If the isobars are straight and parallel the pressure gradient force is balanced by the Coriolis Force and the Geostrophic Wind blows parallel to the isobars (see Figure 9.2)

Buys Ballot's Law

If, in the northern hemisphere, an observer faces the wind, pressure is lower on his right hand than on his left (see Figure 9.2) whilst the converse is true in the

southern hemisphere

In latitudes within 5° of the equator the Earth's rotation is not effective, the wind flows straight across the isobars and Buys Ballot's Law does not apply

Relationship between pressure gradient and wind speed

The pressure gradient is the change in pressure with distance where the distance

is mea5ured perpendicular to the isobars The greater the pressure gradient the closer the isobars and the stronger the wind Pressure gradient is described a5 steep when the isobars are close together and slack when they are widely spaced

The geostrophic wind speed

This may be found by means of a geostrophic wind scale printed on a synoptic weather chart or else by means of a scale engraved on transparent plastic

The perpendicular distance between two isobars is mea5ured and this distance laid

off from the left hand edge of the scale at the appropriate latitude The geostrophic wind speed is found from the curved lines

The gradient wind

This wind flows parallel to curved isobars A resultant force is needed to allow the air to travel on a curved path, so that now Pressure Gradient Force and Coriolis Force are not exactly equal This results in the gradient wind speed being less than the geostrophic wind speed when circulating around low pressure and more than geostrophic when circulating around high pressure

At the surface the angle between the wind and the isobars varies with the nature

of the surface over which the wind blows; it may generally be taken as about 10° to

15oC over the sea

Diurnal variation of wind speed at the surface is caused by diurnal variation in

convection currents During the day, when convection currents are strongest, the retarding effect of surface friction is diffused through a greater depth of turbulence (see Friction Layer in Appendix 1) Thus the reduction in surface wind force is less

by day At night, when the depth of turbulence is shallow the retarding effect is greater and therefore, the wind force is less The diurnal variation of wind speed over sea area5 is negligible.

EFFECT OFTEMPERATURE ON SURFACE PRESSURE

A knowledge of the following will greatly assist in the understanding and memo rising of the prevailing and seasonal winds of the world

Unequal heating of the Earth's surface

During the course of a year sea surface temperatures change very little by comparison with land surfaces (especially in the interior of the continents) In the middle latitudes in summer the land becomes warmer than the surrounding sea, whereas in winter land temperatures are generally below that of the adjacent sea This is because the specific heat capacity of water is higher than that of land

If an area, such as a large land mass, is subjected to a long period of surface heating the air column above the area attains a mean temperature greater than that

of its environment Pressure at an upper level within the column then becomes higher than in the surrounding air at the same upper level

At this upper level air tends to flow away from the high pressure area towards cooler regions

This reduces the total quantity of air over the warm area and so causes pressure

to fall at the surface (less air, less weight and therefore less pressure) Similarly an inflow of air into an upper level of a cold region will cause surface pressure to rise (See Figure 9.5)

The continent of Asia shows a very marked example of the above Surface pressure is low over north-west India in summer and very high over Siberia in winter (See Monsoon)

THE PLANETARY SYSTEM OF PRESSURE AND WINDS

Within the tropics the Sun's rays are nearly vertical throughout the whole of the year At the polar caps they are nearly horizontal during the half-year that the sun is above the horizon Thus surface heating is strong in equatorial latitudes and very weak in polar regions (See Figure 9.6)

Idealised pressure distribution and wind circulation on a uniform globe

If the surface of the earth were uniform eg completely covered by water, belts of high pressure would develop in some latitudes such as polar regions and belts of low pressure would develop in others such as equatorial latitudes The air moving towards the areas of lower pressure would be deflected due to the earth's rotation The "idealised" pressure distribution and surface wind flow is shown in the diagram

Wind circulation on the Earth

The idealised wind pattern illustrated is modified due to the presence of the continental land masses since there are large seasonal temperature variations over the continents The modification is more significant in the northern hemisphere The southern hemisphere has a small total land area in comparison to the great expanse of ocean and the wind circulation more nearly conforms to the ideal pattern

WORLD PRESSURE DISTRIBUTION AND PREVAILING WINDS

Figures 9.8 and 9.9 show the mean distribution of Mean Sea Level (M.S.L.) pressure for the months of January and July The subtropical high pressure belts, though much broken up in the summer by land masses, are still clearly recognisable in both figures with the highs lying towards the eastern sides of the oceans These oceanic highs move north and south a little, following the annual movement of the Sun Take special note of the seasonal changes over Asia and compare the general flow of isobars in the North Indian Ocean with the North

Atlantic and North Pacific Bear in mind that Figures 9.8 and 9.9 show mean pressures for their respective months and that the pressure distribution locally on any particular day may show isobaric patterns very different to those illustrated

THE PREVAILING WINDS OFTHE OCEANS

The prevailing winds of the oceans conform to the main flow of isobars for the season and follow Buys Ballot's Law The winds, especially in the southern hemisphere, show a similarity to those described for the uniform globe They are, however, only mean winds and considerable variations can be expected locally from time to time Ignoring, for the moment, the prevailing winds of the Indian Ocean, there is a definite clockwise circulation round the highs of the North Pacific and North Atlantic, and an anticlokwise circulation in the South Pacific and South Atlantic The surface outflow of air from these highs produces the N E Trades and the S E Trades on their equatorial sides; westerly winds prevail on the poleward sides In the central areas of these anticyclones light variable winds and calms with fine, clear weather generally persist Vessels which are dependent only on sail for their propulsion can be delayed for long periods in these regions

Trade Winds

The Trade Winds blow more or less constantly (except when monsoons prevail) throughout all seasons at a mean speed of around 14 knots and are generally strongest in the late winter They extend from about latitude 30° towards the equator and change their direction gradually with the curvature of the isobars The Trade Wind areas follow slightly the annual movement of the Sun Note, however, that in the South Atlantic the S E Trades blow right up to and across the equator throughout the whole year

Winds of the temperate zones

Westerly winds predominate on the poleward sides of the oceanic highs, but the winds of the temperate zones are subject to considerable variation in direction and force, because they are in the very disturbed region of travelling depressions and anticyclones which generally travel from west to east In the southern hemisphere the westerlies blow right round the world with great consistency and frequently attain gale force which gives them the name of Roaring Forties

The Intertropical Convergence Zone (ITCZ)

This band of convergence is due to the meeting of air from the northern and southern hemispheres This fluctuates seasonally, its range of movement being small in some areas of the ocean and very large in monsoon areas The belt of separation between the NE and SE Trades has its maximum width on the eastern sides of the Atlantic and Pacific Oceans, and these regions of light and variable winds and calms are known as the doldrums They are further characterised by very heavy convectional rain and thunderstorms These stormy areas are easily identified on satellite images The doldrums of the North Atlantic remain north of the equator throughout all seasons Towards the western sides of the oceans the Trade Winds tend to flow nearly parallel to one another and finally become easterly

in direction

MONSOONS

Large land masses become heated in summer and, as explained earlier in the chapter pressure becomes low over the land and high over the sea The reverse takes place in winter The resulting wind circulations tend to persist throughout their particular seasons and are called monsoons The most developed monsoons occur over southern and eastern Asia They occur to a lesser degree in West Africa, America and Australia In general, the monsoons of summer being heavily moisture laden from a long sea passage, are associated with much convection or orographic rain on reaching the coast A winter monsoon is cool and dry with mainly fine weather, unless it has a long path over the sea

Monsoons of the Indian Ocean and China Sea

In northern summer the wind circulates anticlockwise round an extensive low centred over North West India The pressure gradient extends beyond the North Indian Ocean into the southern hemisphere, so that the S E Trades of the South Indian Ocean cross the equator and, due to the rotational effect of the Earth, veer to

S W as they are drawn into the monsoonal circulation In the North Indian Ocean and western part of the North Pacific the Trade Winds disappear completely during the period of the south-west monsoon (See Figure 9.11)

The south-west monsoon season is from June to September (inclusive) In the Indian Ocean it blows as a strong wind reaching gale force at times During its long passage over the warm sea it picks up a vast quantity of moisture and gives very heavy orographic rain on the windward coasts of India Tropical cyclones occur in the Indian Ocean and Bay of Bengal, especially at the beginning and end of the south-west monsoon

In the China Sea this summer monsoon is less strong than in the Indian _ Ocean and the rainfall is comparatively slight More often than not it flows between south and east rather than south west Typhoons occur frequently, particularly in October

In northern winter a large anticyclone is situated over Siberia and the north-east monsoon, which blows from October to March (with much less force than the summer monsoon) extends over the North Indian Ocean and China Sea, crosses the equator gradually backing to the N.W: and reaches Australia as the "northwest monsoon" In the North Indian Ocean this monsoon is dry and usually brings fine and clear weather Along the coast of China and Indo-China the pressure gradient

is steep and the winds stronger Between January and April, in the China Sea and along the South China coast, periods of overcast drizzly weather with mist or fog occur From February to April such periods may persist for over a week The local name for these periods is Crachin (See Figure 9.10)

Examples of other monsoons

When reading the following refer to Figures 9.8 and 9.9 which show world pressure distribution for January and July, respectively:

Northern Australia and Indonesia Winds are south-easterly in winter and

northwesterly in summer

West coast of Africa-Gulf of Guinea A south-west monsoon blows from June to

September The effect extends from latitude about 8°N to about 20oS

South-eastern part of USA (North of the Gulf of Mexico) The prevailing winds are north-westerly in winter and south-westerly in summer They are disturbed by travelling depressions

East coast of Brazil A north-east monsoon blows from September to March when pressure over Brazil is low

LAND AND SEA BREEZES

The principal wind systems of the world undergo local modifications for various reasons but due mainly to the unequal heating and cooling of land and sea Land and sea breezes (a diurnal effect) occur most frequently and are more pronounced

in countries where solar heating is powerful They are experienced in temperate latitudes during warm summer weather but rarely exceed Force 3 and may extend

10 to 15 miles on either side of the coastline In the tropics they sometimes reach Force 5 and may be felt 20 miles from the coast

The most favourable conditions for land and sea breezes are anticyclonic, that is with clear skies and very light winds Under such conditions in summer months the land heats up rapidly during the day whilst the sea remains cool The warm air over the land rises and is replaced by air flowing in from over the sea This sea breeze