Surface Winds Of The Globe
Any climate depends on, firstly, local factors and, secondly, advection. The former include radiation, rainfall and evaporation, and the latter the heat and moisture brought by oceans and winds. We have dealt with each of these by now, except the winds, which are the topic of this fourth part of the book. We can distinguish various scales of air movement, just as distinctions were made in Table 1.1 between different scales of climate. There are 'local winds' which will be discussed in Chapter...
Stratiform Clouds
A stratus cloud St may extend horizontally for hundreds of kilometres, but be only 50-500 m thick, with little tendency for vertical growth because the cloud forms by slow uplift within stable air Table 8.2 . The top of the cloud is typically surmounted by an inversion due to radiation cooling of the top of the cloud. Such cloud prevents sunshine reaching the ground in winter at high latitudes, when the Sun is low and therefore the cloud's albedo particularly high see Section 2.5 , and we...
Values Of The Evaporation Rate
The global average of the evaporation rate must equal the annual mean rainfall of about 1,020 Figure 4.10 The change of the actual rate of evaporation Ea from a crop as the soil dries out, showing it as a two-stage process. When the soil is initially fully wetted, Ea is the potential rate E,, which depends primarily on atmospheric conditions i.e. net radiation, temperature, humidity and wind speed , and secondarily on the crop's roughness and albedo. In the first stage of drying, Et is...
Measurement 1
There are several kinds of anemometer for measuring the surface wind. Figure 14.3 shows a simple device used by dinghy sailors, and at some weather stations. The cup anemometer has been commonly used since its invention in 1846 cups on each of three radial arms from a vertical axis are driven by the wind, and their rotations are counted. The number of rotations multiplied by the distance around the cups' circle is proportional to the wind run, the distance that a parcel of air would travel....
Satellite Observations
The wide separation of weather stations where clouds are observed, and their relative absence at sea apart from ship observers , have been major problems, especially in the southern hemisphere where 81 per cent of the area is ocean. Fortunately, total coverage of the world is now provided by satellite observations, developed since the 1960s, notably from satellites described as geostationary Note 8J . These rotate above the equator at the same rate as the Earth, so they appear fixed. Currently...
Models Of The General Circulation
At this point it is useful to summarise the previous sections in terms of a single coherent model of the world's circulation overall. The model must account for the distribution of pressures Figure 1.8 , the meridional transfer of heat Note 5.F and the latitudinal variations of rainfall Figure 10.6 and winds Figure 12.5 . Various models have been suggested in the past. Edmund Halley proposed in 1686 that the easterly Trade winds were following the Sun, flowing towards the part of the Earth that...
Ocean Currents
The winds affect the oceans mostly by influencing surface currents. Let us now consider the pattern of these currents, then an explanation of the pattern and finally the effects on flows beneath the surface. The first maps of the main ocean currents were compiled by Matthew Maury in 1855, using data from the ships' logs of ten countries. A modern map Figure 11.15 shows the huge swirls called 'gyres', which turn anti-clockwise in the southern hemisphere. A complete rotation within each ocean...
Tropical Rainfall
Tropical rainfall often comes from the release of static instability by thunderstorms what is called 'convective rainfall' Chapter 9 . This is true especially over land and over archipelagos Table 7. 1 Pasquill's classification of the stability of the surface atmosphere, according to the degree of insolation and wind speed Table 7. 1 Pasquill's classification of the stability of the surface atmosphere, according to the degree of insolation and wind speed Standard deviation of wind direction...
The Energybalance Equation
So far we have given separate consideration to three major determinants of the climatic environment i the net flux of the radiation energy from the Sun to the ground Section 2.8 , ii its subsequent use in heating both the air Sections 1.6 and 3.2 and the ground Section 3.5 , and iii the evaporation of water Section 4.1 . In later chapters, we shall elaborate on the ways in which these account for the humidity, cloudiness, rainfall and wind, which along with temperature constitute the weather in...
Circulations Within The Troposphere
So far we have been considering winds near the suface, but they are only half the story. There is another but related pattern of winds aloft in the troposphere. Different winds at various levels are shown by movements of cirrus cloud in directions quite distinct from those of low-level clouds. We shall consider winds at two levels especially, where the pressures are around 850 hPa at about 1,500 m and about 300 hPa at about 9 km , respectively. The first represents conditions in the lower...
Sunspots
A sunspot is a relatively dark area on the Sun, at temperatures of only about 3,000 Kelvin, occupying up to 0.2 per cent of the Sun's visible area Figure 2.7 . Each spot appears to drift eastwards showing that the Sun rotates once every 27 days and towards the Sun's equator, before disappearing. Sunspots alter the solar constant by less than 0.3 per cent, but are associated with very slight increases of ultra-violet radiation and of solar wind, which consists of high-speed electrons and nuclear...
Lows
A low or depression is a pattern of reduced pressure, leading to rings of isobars about the point of lowest pressure. The circular pattern was recognised by Heinrich Dove in 1828 and led to the name 'cyclone', which comes from the Greek for a coiled snake. We will discuss various kinds of low in this section, at the risk of seeming to imply that they are distinct bodies of air. They are not. Air masses and winds are real, but a low is simply part of a pattern. The movement of a low in the...
Values Lvq
Typical radiation conditions are shown in Table 2.5 for a forest and meadow. Even though the solar radiation onto the meadow happened to be more i.e. 336 instead of 292 W m2 , the net radiation onto the forest was greater, on account of a lower albedo. The radiation efficiency i.e. the ratio Rn Rs was 64 per cent for the forest and 51 per cent for the meadow at the time of the measurements. The ratio is quite different at the South Pole, where Rn is negative Table 2.6 . The net radiation at the...
Thermometers
The first thermometer was invented in 1592 by Galileo, who measured the expansion of air and found that its volume increases in proportion to its temperature if the pressure is kept constant Note 1.M . His air thermometer tube was bent round, and so temperatures were described in 'degrees' around the circle. In 1714, Fahrenheit used the expansion of mercury instead of air to obtain a less bulky method of temperature measurement. Mercury thermometers are widely used nowadays, though they have...
The Coriolis Effect And The Oceans
So far we have considered ocean temperature and salinity, two of the factors controlling currents in the sea, which in turn govern coastal climates. A third factor is the 'Coriolis effect', the 'apparent deflection of moving objects, due to the observer being on a rotating Earth', named after Gaspard de Coriolis 1792-1843 . Unfortunately, it is not easy to understand immediately, being different from commonsense observation, and so various explanations are offered in what follows. As a...
Cloud Electricity
The precipitation from cumulonimbus cloud is often accompanied by lightning and thunder, which will now be considered. Figure 9.12 Variation with season of the distribution of thunderdays in Africa. Figure 9.12 Variation with season of the distribution of thunderdays in Africa. Lucretius, a Roman poet of the first century BC, thought that lightning consists of sparks from the collision of large clouds. But it is usually explained nowadays as due to charge separation within the updraught of a...
Frontal Inversion
Where adjacent air masses of different temperature intrude on each other there is a 'frontal zone', discussed in Chapter 13. A 'cold front' occurs when a cold air mass wedges underneath a warmer one. So a temperature sounding through the wedge shows a slanting boundary between colder air below and warmer air above, i.e. there is an inversion at the boundary. In this case, the inversion indicates the inclement weather associated with a front, whereas a subsidence inversion implies calm and clear...
Nonlocal Instability
So far we have assumed constant values of the environmental lapse rate ELR i.e. straight lines in Figs 7.3 and 7.5 , to be compared with the SALR or DALR, according to whether the air is saturated or not. Such a comparison indicates the local static stability. However, the ELR is rarely constant in practice, so measurements do not yield a straight line but a curving line as in Figure 7.1 and Figure 7.6. The practical consequence of this is that the occurrence of convection does not depend on...
Intertropical Convergence Zone ITCZ
Figure 12.1 shows that winds between the tropics converge on a line which we call the Intertropical Convergence Zone i.e. ITCZ or equatorial trough. This line of convergence near the equator is also discernible in a map of streamlines Figure 12.2 Note 12.A . It is actually a band a few hundred kilometres wide, enclosing places where winds flow inwards are 'confluent' and subsequently rise convectively. It is the latitude of the highest air temperature and vapour pressure near the surface, and...
Changes of Cloud Form
Figure 8.3 shows how a change in surface conditions across a coastline can induce certain clouds, depending on whether the wind is onshore or offshore, and whether the land is warmer in summer or colder in winter than the sea. Convective clouds may form if the lower troposphere is unstable, e.g. when humid air from a warm ocean flows over even warmer land in summer upper left of Figure 8.3 or when offshore cold air blows over a warmer sea upper right , especially as the sea increases the...
Observing Cloudiness
Clouds are observed regularly at weather stations. The extent to which the sky appears covered at the time of observation is expressed in terms of oktas, representing eighths of the sky seen by the observer. Thus 8 oktas of cloud represents a totally overcast sky Figure 8.9 . A cross is drawn through the cloud circle under foggy conditions. The inexperienced often underestimate cloudiness by ignoring the cirrus, or else they overestimate the amount of cumulus because it tends to cover much of...
Subsidence Inversion
A subsiding layer becomes compressed as it sinks to levels at higher pressures, so the top descends further than the bottom of the layer does i.e. warms more , and consequently the lapse rate within the layer changes in a way that results in greater stability. This is the converse of the instability created within a rising layer Section 7.4 . More importantly, the inversion that commonly occurs above the lowest few hundred metres is due to the air against the ground being unable to descend...
Weather Forecasting
Forecasters compress the information from a weather station by means of an internationally agreed code, which allows data to be sent rapidly by cable or radio, and shared with other weather bureaux. The code is a series of five-figure numbers in a standard sequence agreed in 1982. The information can subsequently be displayed in a standard fashion Figure 15.2 on the corresponding point on a map. Such a map, with figures from many places, is called a synoptic chart, providing a snapshot of the...
Other Instruments
Temperatures can also be measured in the following ways 1 Two sheets of different metals are bonded together by rolling, to form a bimetal combination which bends when warmed because the metals expand differently. The bending controls a pointer which indicates the temperature on a suitably calibrated scale. This is the usual basis for an instrument called a thermograph which records temperatures on moving graph paper. 2 The electrical resistance of a wire increases with higher temperatures and...
Ground or Radiation Inversion
The ground begins to cool from about the midafternoon Figure 3.12 , increasingly lowering the temperature of the air above and producing a shallow inversion. For instance, one set of soundings in Sydney showed a surface inversion 10 m deep at 5.30 p.m., but 100 m by 8.45 p.m. and 150 m by dawn. Thereafter the ground is warmed by the Sun, and air rising from it establishes a dry adiabatic lapse rate from the surface. Figure 7.1 shows such lifting of the base of the inversion to 100 m by 8.40...
Cloud Due to Frontal Uplift
The low-pressure region and the uplift just mentioned are associated with a well-defined boundary between polar cold air and subtropical warmer air, called a 'front', discussed in Chapter 13. A cold front involves cold air underrunning warmer air, just as with a small-scale density current, and results in the uplift of the warmer air. This creates long bands of cloud over the front and behind it. But sometimes the warm air advances over the cooler air, creating a warm front and then the cloud...
Composition Of Air
Air is a mixture of various gases added together. It also contains water vapour, dust and droplets, in quantities which vary with time, location and altitude. Samples collected by balloon as early as 1784 showed the uniformity of air's composition up to 3 km. Later measurements have confirmed that the air up to 80 km or so consists chiefly of nitrogen and oxygen in almost constant proportions, forming a well-mixed layer called the homosphere, within which only the amount of water vapour and...
Sea Breezes
These coastal winds are due to sea-surface temperatures SST varying each day by only a degree or so, whilst surface air temperatures onshore change by around ten times as much Section 3.4 . The result is that daytime temperatures inland are appreciably warmer than the SST, and the warming spreads throughout the planetary boundary layer. Also, the onshore warmth leads to thermals which ascend to the top of the PBL and gradually extend it upwards Figure 7.1 . The warming of a column of 1 km by 5...
Circulation of the Wind
Another factor affects any global wind that takes Figure 12.8 The apparent deflection of a parcel of air moving from a belt of high pressure in the southern hemisphere, e.g. from the band of subtropical high pressures. The parcel is assumed stationary initially. As soon as it starts to move, it suffers a sideways Coriolis force, increasing in proportion to its acceleration. The force deflects the parcel until it is travelling along an isobar, with a constant speed such that the Coriolis force...
The Polar Cell
Winds at the highest latitudes tend to be easterly Figure 12.1, Figure 12.5 and Figure 12.10 , which can be explained by a circulation like that of the Hadley cell. Air over the pole continually cools, becoming more dense and therefore subsides, eventually sliding down the dome of Antarctic ice. This flow is deflected westward by the powerful Coriolis effect of high latitudes, forming the south-easterlies which help form the Antarctic front between 60-65 S, where the MSLP is lower than anywhere...
N N N N N N
Figure 14.10 Prevailing surface winds at Brisbane. The numbers indicate the mean speed km h . Daytime winds from the north-east, east and south-east are sea breezes, which are fostered at Brisbane in summer by easterly gradient winds resulting from the shift southwards of the highs across Australia Section 13.6 . period October-March are sufficiently strong to distort trees growing 16 km inland. The sea breeze on the desert west coast of South Africa typically reaches 10 m s. A sea breeze...
Hadley Cells
There are great differences between surface and upper winds at the tropical latitudes Figure 12.10 , with easterly Trade winds surmounted by westerlies, notably in winter. The explanation is as follows. The Trades tend towards the ITCZ Figure 12.1 , where there is a chain of centres of convergence associated with convective storms and these lift air into the upper atmosphere. The raised air increases the upper-level pressure locally, which creates winds poleward as irregular 'anti-Trades'....
Natural Resources In Snow Climates
Snow falls instead of rain at places of high elevation and high latitude Figure 3.6 . For instance, there is snow every year above about 1,500 m in south-east Australia, and there were nine brief occasions between 1900 and 1979 of snow on the inland edge of Sydney, which is beside the sea at 34 S. But there is no permanent snow even on Australia's highest mountain Mt Kosciusko at 2,228 m, 37 S because it is too low Figure 3.6 . Likewise, there is no permanent snow even on the several peaks over...
Frontal Movement
About a hundred cold fronts track along the southern coasts of South Africa and Australia annually, i.e. about two a week on average. Most derive from low pressures at about 60 S, extending into the troughs between subtropical highs Figure 12.1 and Figure 12.7 , as in Figure 13.1 for a particular day. They are typically oriented north-west south-east and tend to Figure 13.5 The correspondence of cloud seen from a satellite and the position of a front, both at noon GMT on 8 November 1995. The...
Various Evaporation Rates
So far we have mainly considered the simplest case, that of evaporation from an extensive water surface, i.e. the lake evaporation rateEo. It is useful to consider this as the surface has a more or less standard roughness, albedo and wetness, and reservoirs are important to us. But it is difficult to measure Eo accurately. Also, there is the complication that evaporation from the upwind edge of a large lake moistens the air, so that evaporation downwind is reduced. Evaporation from a choppy sea...
Hygrograph
The current RH is shown directly by a hygrometer, whilst a hygrograph records the value on a chart. An example is a hair hygrograph, which depends on the extension of fibres resulting from their increased absorption of moisture in high humidities Horace de Saussure 1740-99 found that human hair extends by about 2.5 per cent between extremes of dryness and wetness. But the length changes are not quite proportional to moisture variations, so the RH scale is irregular on the recording paper. By...
The Polar Front and the Ferrel Cell
Not all the air subsiding around 30 latitude spirals out from the surface highs to become the Trades. The winds on the poleward side of each high in the southern hemisphere tend to emerge as midlatitude northwesterlies Figure 12.1 , carrying relatively warm air towards the pole. These winds encounter cold southwesterlies at around 45 S, and the convergence brings together air masses of different temperatures. The highly irregular interface is called a 'front', in this case the polar front...
Rainfall Intensity
Amounts of rainfall are measured in terms of the depth of the layer created by spreading the water on a horizontal surface. It is now expressed in millimetres depth, and the rate or intensity of precipitation during a given period is the total collected divided by the duration, usually expressed in millimetres per hour. It can be measured for periods longer than a few minutes by means of a pluviograph, an instrument for recording the times between refillings of a small cup into which the...
SeaBreeze Front
The leading edge of the sea breeze is called the 'sea-breeze front', which propagates inland Figure 14.8 The growth of a sea-breeze cell during the day. Figure 14.8 The growth of a sea-breeze cell during the day. Figure 14.9 The diurnal variation of the winds at Jakarta 6 S at different elevations. Wind speed is shown in units of km h. The diagram shows onshore surface winds solid lines from about 9 a.m. till 9 p.m., though the strongest winds occur around 4 p.m. at about 200 m. The return flow...
Stratospheric Inversion
This was considered in Section 1.8. It suppresses the vertical motions within turbulent air, creating the calmness desirable in air travel, for instance, since long-distance aircraft fly at about 10 km. Also, thunderstorms Chapter 9 and weather systems Chapter 13 are confined to the troposphere by the inversion, and its impermeability allows quite different chemical concentrations above and below. For instance, ozone is much more abundant above the tropopause at the bottom of the inversion.
Mountain Winds
Mountains have several effects on winds. Firstly, there is generally an increase of velocity at higher levels, towards the speeds characteristic of the upper troposphere Figure 12.10 . Secondly, a strong wind perpendicular to a high and long range will undulate on the lee side if there happens to be a slightly higher inversion layer to bounce against. These undulations called lee waves yield lines of cloud fixed parallel and downwind of the mountains wherever the upper part of an undulation is...
The Upper Westerlies
Strong westerly winds extend over most of the upper troposphere just below the tropopause Figure 12.10 . They are esentially thermal winds Note 12.F , due to the meridional gradient of near-surface temperatures at midlatitudes. The westerlies are strongest near the polar front Figure 12.12 , where the temperature gradient is steepest. They are not found higher than the tropopause because the latitudinal gradient of temperature is actally reversed at such elevations, i.e. it is colder at lower...
Processes
The formation of upper-atmosphere ozone involves photo-dissociation of normal oxygen. The molecules are split into pairs of separate single atoms by the impact of UV Figure 1.4 , as explained by Sydney Chapman in 1930. These single atoms can subsequently collide and combine with other normal oxygen molecules O2, to create ozone O3. The interaction of the UV and oxygen happens most around 40 km, because there is too little photo-dissociation at lower levels, where the UV has been attenuated by...
Saturated Air
So far we have assumed that the rising parcel does not become saturated as it cools. If it does become saturated, then any further adiabatic cooling is partly offset by the resulting condensation, which releases latent heat Note 7.C . The consequent lapse rate of the saturated parcel is referred to as the saturated adiabatic lapse rate SALR . This is the second of the two theoretical lapse rates mentioned above for comparison with the measured lapse rate, the ELR. Clearly, offsetting some of...
The Hydrologic Cycle
Now we turn from the flows of energy about the Earth to deal with the resulting movements of water. These include the process of evaporation discussed in Chapter 4, resulting in water vapour in the atmosphere. Water vapour is an important greenhouse gas Section 2.7 and a key component of the hydrologic cycle symbolised in Figure 6.1. The hydrologic cycle consists of the circulation of water from land and ocean to atmosphere, then condensation, normally into cloud, followed by precipitation back...