# Climates and Weather

## 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...

## Measurement

There are several ways of determining the barometric pressure. It used to be measured most commonly with a mercury barometer, with a column of mercury (Figure 1.6) replacing a similar device using water, invented by Evangelista Torricelli in 1643. (Mercury is 13.6 times more dense than water and as a result a more convenient column only about 760 mm Figure 1.6 A mercury barometer, in which the atmospheric pressure on the leather washer is balanced by that exerted by the column of mercury....

## Determining The Evaporation Rate

The rate of evaporation from water or land surfaces can be measured in several ways. The following are a few examples. 1 The change of water level is a direct index of evaporation and routinely measured at many weather stations, generally with an American 'Class-A' evaporation pan (or evaporimeter) shown in Figure 4.6 and Plate 4.1. The rate of evaporation from this standard device is known as the pan evaporation rate, Ep. For a lake or reservoir, one may construct a water balance (Chapter 10)....

## 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...

## Variations Of Rainfall

Average figures for rainfall fail to indicate the great differences from one period to the next, which occur in many places. The variability is important a farmer would much prefer a reliable though modest rainfall to an irregular sequence of drought and flood with the same average, and most of the fluctuation of crop yields is due to rainfall variability. Different ways of showing the considerable variability observed in practice are illustrated in Figure 10.8. In Sydney, for instance, the...

The amount of radiant energy received or emitted every second at a surface of one square metre is properly known as the radiance, though we lazily refer to it simply as the 'radiation'. The radiance on a surface facing the Sun, just outside the Earth's atmosphere, is called the solar constant, equal to 1,367 W m2. This is more than the output of a kilowatt electric heater on Figure 2.4 The solar elevation at various times of day and in various months, at Sydney 34 S . For instance, the Sun is...

## 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...

## 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...

## Ocean Temperatures

The sea-surface temperature (SST) was first mapped by Alexander von Humboldt in 1817. Modern maps show long-term values of the SST varying from the sea's freezing point of -1.9 C near polar ice, to over 30 C in the Persian Gulf and the Red Sea in July (Figure 11.2). Temperatures offshore tend to be cooler than inland in January (summer) but warmer in July, around South Africa for instance (Figure 11.3). These differences of temperature cause coastal breezes (Chapter 14). Generally, the...

## Daily Changes

Screen temperatures do not rise and fall over equal periods of the day the cooling period lasts longer Figure 3.11 . This can be explained as follows. Heating of the ground occurs when the net radiation is positive downward, which is the situation between dawn and mid-afternoon Figure 3.12 . Solar radiation declines after noon Figure 2.4 , but the gradual rise of ground temperature leads to a continued increase in the loss of terrestrial radiation Note 2.C . As a result, the net radiation on a...

## Cold Fronts

The basic feature of a cold front is the insinuation of a heavy, cold air mass under a lighter, warm one. The result of the cold air's advance is often the pattern of uplift, subsidence, clouds and rainfall shown in Figure 13.3, the classical Figure 13.2 Conventional depiction of the most common types of fronts and a trough on weather charts. The half-circles and or triangles are plotted on the forward side of the front. Figure 13.2 Conventional depiction of the most common types of fronts and...

## 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...

## Orographic Cloud

Hills deflect winds upwards, so that the air cools, possibly to dewpoint. In that case, clouds form at the Lifting Condensation Level LCL Note 8.B , shown in Figure 8.1. For example, the west coast of the South Island of New Zealand is more cloudy and wet than the east coast Chapter 10 because of the orographic uplift of the mainly westerly winds. A cap cloud, shaped like a contact lens, forms at the crest of an isolated mountain, if winds rise over it sufficiently high to attain the Lifting...

## Rossby Waves

Figure 12.13 shows that the upper westerlies do not flow steadily towards the east, but sway from side to side as Rossby waves, named after the Swedish meteorologist Carl-Gustav Rossby. He explained them in 1939 in terms of the Coriolis effect Note 12.J . The waves are bends in the path along which the winds blow, most evident in the jet stream. If there are six such waves in circling the globe, we say that the wave number is six. The waves travel against the wind direction, at speeds which...

## 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...

## Wind Profile

Synoptic-scale winds are those blowing at least 1 km from the ground, above the PBL (Section 12.2) and beyond the influence of the ground's irregularities. They equal the local gradient winds if there is no acceleration or slowing up, so we will call them 'quasi-gradient winds'. They drag surface air along by transferring energy downwards through turbulent eddies, which are circulations in all directions including the vertical, lasting for seconds or minutes. They are smaller in scale closer to...

## 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...

## 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...

## Zonal Winds

Figure 12.10 shows the average zonal winds at all levels. The bottom of the diagram indicates surface equatorial easterlies, then a belt of westerlies in each hemisphere, with easterlies again at higher latitudes. There are westerlies of over 30 m s (i.e. above 108 km h) in the subtropics at a height of about 12 km, just below the temperature minimum of the tropopause (Figure 1.9). These strong winds result from steep north-south gradients of temperature at that level. Note 12.E explains why...

## 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...

## 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...

## 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...

## 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...

## 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...

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'....

## 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...

## 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...

## Climate Classification

Surface data collected as described in Section 15.1 allow us to summarise the climate of a place in terms of average conditions and deviations from them. Particular attention is usually paid to monthly mean values of the daily-mean temperatures and daily extremes, along with the monthly rainfall. The next step in condensing the flood of numbers that arise in continuously measuring the weather everywhere is to group homoclimes, places with similar climates. This imposes orderliness on 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...

## Wind Energy

Winds can be harnessed to provide power, though extreme winds may damage structures (Note 14.B), and some of the wind's energy goes into eroding the land, which is the other aspect we will consider in this section. The power of wind at velocity V (m s) through an area of a square metre equals .V3 2 watts (Note 14.F), where is the air's density (about 1.25 kg m3 at sea-level). For instance, a breeze of 5 m s blowing onto the rotating blades of 3 m radius of a wind turbine contains 78 W m2 of...

## Altering The Energy Balance

As examples of changing an energy balance, we will consider the effects of preventing evaporation at the surface, altering the ground's albedo, and differences due to the orientation of sloping ground. A layer of oil on a lake greatly reduces evaporation, and a clear-plastic cover has the Figure 5.9 Comparisons of the available energy Rn with the amounts used in evaporation L.E, heating of the ground G and heating of the air H from a wheat crop in Canberra, Australia. At any moment, Rn equals...

## 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...

## Elevation

Mountains force winds to rise, creating orographic cloud and rain (Figure 10.4 and Figure 10.5) in a complicated way. First, any moist and stable airstream is forced by the wind to rise over a mountain chain, and will lose some of its moisture upon ascent to the Table 10.1 Effect of latitude and season on rainfall Rainfall (mm) during three months, averaged over ten degrees crest (Figure 7.2). This mechanism is important at high latitudes. Second, orographic uplift may destabilise any...

## Urban Temperatures

Temperatures measured at street level in large cities can be several degrees higher than those in the surrounding countryside Figure 3.17 . Isotherms on a map of an isolated city and its environs look like the height contours of an island, which leads to the clumsy notion of an 'urban heat-island effect' a simpler term is urban heating. It is evident in some cities during the day but is particularly apparent at night, being often greatest at about 9 p.m., because city surfaces cool more slowly...

## 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...

## Occluded Fronts

More common is an occluded front, where cold and warm fronts overlap, so that the lower-level cold front shuts off (i.e. 'occludes') the upper warm front. This situation is illustrated in Figure 13.9. It is triggered by a poleward kink in the roughly east-west polar front (Figure 12.12), the boundary between a relatively warm northwesterly moving at 7 m s, say, on the equatorward side, and colder southwesterlies on the poleward side blowing at 13 m s, for instance (Section 12.3). The two...

## Composition Of

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...

## Water Balances

The change of moisture in an area equals the difference between (i) the gain (as precipitation and inflow) and (ii) the loss by evaporation and outflow. This equality is the 'water balance' (Section 4.4). It may be considered on any scale of space or time. The 'change of moisture' might involve an alteration of level in a reservoir, or wetter soil, for instance. Runoff occurs once the storage is full (Figure 10.13). Water-balance estimates are often made in agriculture to check the need for...

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...

## Lapse Rates

The change of temperature with height is called the lapse rate. A positive lapse rate represents the normal condition with cooler air above, the temperature falling with increased height. The opposite, a negative lapse rate (ie the temperature increases with height) is called an inversion (Chapter 7). Temperature profiles show the actual lapse rate (or environmental lapse rate) at each level, i.e. the tangent to the profile at that level. Typical profiles in Figure 1.9 show positive lapse rates...

## 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...

## 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...

## Vapour Pressure And Evaporation

The concept of vapour pressure is needed in order to explain why a liquid is sometimes depleted by evaporation and sometimes augmented by condensation. We first consider gas molecules in a box, which move to and fro, continually colliding with each other and with the walls of the box. Everything is at the same temperature. Such a model of colliding molecules is the basis of the Kinetic Theory of Gases outlined by Rudolf Clausius in 1857. The molecules' impacts press the walls outwards, with a...