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5.1GENERAL

    5.1.1Definitions

    The following definitions are used in this chapter (see Mazzarella, 1972, for more details).Wind velocity is a three-dimensional vector quantity with small-scale random fluctuations in space and time superimposed upon a larger-scale organized flow. It is considered in this form in relation to, for example, airborne pollution and the landing of aircraft. For the purpose of this Guide, however, surface wind will be considered mainly as a two-dimensional vector quantity specified by two numbers representing direction and speed. The extent to which wind is characterized by rapid fluctuations is referred to as gustiness, and single fluctuations are called gusts.Most users of wind data require the averaged horizontal wind, usually expressed in polar coordinates as speed and direction. More and more applications also require information on the variability or gustiness of the wind. For this purpose, three quantities are used, namely the peak gust and the standard deviations of wind speed and direction.Averaged quantities are quantities (for example, horizontal wind speed) that are averaged over a period of 10 to 60 min. This chapter deals mainly with averages over 10 min intervals, as used for forecasting purposes. Climatological statistics usually require averages over each entire hour, day and night. Aeronautical applications often use shorter averaging intervals (see Part II, Chapter 2). Averaging periods shorter than a few minutes do not sufficiently smooth the usually occurring natural turbulent fluctuations of wind; therefore, 1 min “averages” should be described as long gusts.

    Peak gust is the maximum observed wind speed over a specified time interval. With hourly weather reports, the peak gust refers to the wind extreme in the last full hour.

    Gust duration is a measure of the duration of the observed peak gust. The duration is determined by the response of the measuring system. Slowly responding systems smear out the extremes and measure long smooth gusts; fast response systems may indicate sharp wave-front gusts with a short duration.

    For the definition of gust duration an ideal measuring chain is used, namely a single filter that takes a running average over t seconds of the incoming wind signal. Extremes detected behind such a filter are defined as peak 0

    gusts with duration t. Other measuring systems with various filtering elements are said to measure gusts with 0

    duration t when a running average filter with integration time twould have produced an extreme with the same 00

    height (see Beljaars, 1987; WMO, 1987 for further discussion).

    Standard deviation is:

    (5.1)

    where u is a time-dependent signal (for example, horizontal wind speed) with average U and an overbar indicates

    time-averaging over n samples u. The standard deviation is used to characterize the magnitude of the fluctuations in i

    a particular signal.

    Time-constant (of a first-order system) is the time required for a device to detect and indicate about 63 per cent of a step-function change.

    Response length is approximately the passage of wind (in metres) required for the output of a wind-speed sensor to indicate about 63 per cent of a step-function change of the input speed.

    Critical damping (of a sensor such as a wind vane, having a response best described by a second-order differential equation) is the value of damping which gives the most rapid transient response to a step change without overshoot.Damping ratio is the ratio of the actual damping to the critical damping.

    Undamped natural wavelength is the passage of wind that would be required by a vane to go through one period of an oscillation if there were no damping. It is less than the actual “damped” wavelength by a factor if D is the damping

    ratio.

    Variable wind with no mean wind direction is wind where the total variation from the mean wind direction during the previous 10 minutes is 60? or more, and less than 180?, and the wind speed is less than 6 km/h (3 kt), or when the total variation is 180? or more.

5.1.2Units and scales

    –1–1 or in knots (0.515 m s) to the nearest unit, and should Wind speed should be reported to a resolution of 0.5 m s

    represent, for synoptic reports, an average over 10 min. Averages over a shorter period are necessary for certain aeronautical purposes (see Part II, Chapter 2).

    Wind direction should be reported in degrees to the nearest 10?, using a 01 ... 36 code (for example, code 2 means that the wind direction is between 15 and 25?), and should represent an average over 10 min (see Part II, Chapter 2,). Wind direction is defined as the direction from which the wind blows, and is measured clockwise from geographical north, namely, true north.

    “Calm” should be reported when the average wind speed is less than 1 kt. The direction in this case is coded as 00.Wind direction at stations within 1? of the North Pole or 1? of the South Pole should be measured so that the azimuth ring should be aligned with its zero coinciding with the Greenwich 0? meridian.

    There are important differences compared to the synoptic requirement for measuring and reporting wind speed and direction for aeronautical purposes at aerodromes for aircraft take-off and landing (see Part II, Chapter 2). Wind direction should be measured, namely, from the azimuth setting, with respect to true north at all meteorological observing stations. At aerodromes the wind direction must be indicated and reported with respect to magnetic north for aeronautical observations and with an averaging time of 2 min. Where the wind measurements at aerodromes are disseminated beyond the aerodrome as synoptic reports, the direction must be referenced to true north and have an averaging time of 10 min.

    5.1.3Meteorological requirements

    Wind observations or measurements are required for weather monitoring and forecasting, for wind-load climatology, for probability of wind damage and estimation of wind energy, and as part of the estimation of surface fluxes, for example, evaporation for air pollution dispersion and agricultural applications. Performance requirements are given in Part I, –1–1Chapter 1, Annex 1.B. An accuracy for horizontal speed of 0.5 m s below 5 m s and better than 10 per cent above 5 –1m s is usually sufficient. Wind direction should be measured with an accuracy of 5?. Apart from mean wind speed and direction, many applications require standard deviations and extremes (see section 5.8.2). The required accuracy is easily obtained with modern instrumentation. The most difficult aspect of wind measurement is the exposure of the anemometer. Since it is nearly impossible to find a location where the wind speed is representative of a large area, it is recommended that estimates of exposure errors be made (requirements on siting and exposure are provided insee section 5.9 and

    Annex 1.B).

    Many applications require information about the gustiness of the wind. Such applications provide “nowcasts” for aircraft take-off and landing, wind-load climatology, air pollution dispersion problems and exposure correction. Two variables are suitable for routine reading, namely the standard deviation of wind speed and direction and the 3 s peak gust (see Recommendations 3 and 4 (CIMO-X) (WMO, 1990)).

    5.1.4Methods of measurement and observation

    Surface wind is usually measured by a wind vane and cup or propeller anemometer. When the instrumentation is temporarily out of operation or when it is not provided, the direction and force of the wind may be estimated subjectively (the table below provides wind speed equivalents in common use for estimations).

    The instruments and techniques specifically discussed here are only a few of the more convenient ones available and do not comprise a complete list. The references and further reading at the end of this chapter provide a good literature on this subject.

    The sensors briefly described below are cup-rotor and propeller anemometers, and direction vanes. Cup and vane, propeller and vane, and propellers alone are common combinations. Other classic sensors, such as the pitot tube, are less used now for routine measurements but can perform satisfactorily, while new types being developed or currently in use as research tools may become practical for routine measurement with advanced technology.For nearly all applications, it is necessary to measure the averages of wind speed and direction. Many applications also need gustiness data. A wind-measuring system, therefore, consists not only of a sensor, but also of a processing and recording system. The processing takes care of the averaging and the computation of the standard deviations and extremes. In its simplest form, the processing can be done by writing the wind signal with a pen recorder and estimating the mean and extreme by reading the record.

    Wind speed equivalents

    Beaufort scale Wind speed equivalent at a standard heightSpecifications for estimating speed over

    number and of 10 m above open flat groundland

    description

    –1–1–1(kt)(m s)(km h)(mi h)

    Calm; smoke rises vertically0Calm< 10 – 0.2< 1< 1

    Direction of wind shown by smoke-drift but 1Light air1 – 30.3 – 1.51 – 51 – 3not by wind vanes

    Wind felt on face; leaves rustle; ordinary 2Light breeze4 – 61.6 – 3.36 – 114 – 7vanes moved by wind

    Leaves and small twigs in constant motion; 3Gentle breeze7 – 103.4– 5.412 – 198 –12wind extends light flag

    Raises dust and loose paper; small 4Moderate breeze11 – 165.5 – 7.920 – 2813 – 18branches are moved

    Small trees in leaf begin to sway, crested 5Fresh breeze17 – 218.0 – 10.729 – 3819 – 24wavelets form on inland watersLarge branches in motion; whistling heard 6Strong breeze22 – 2710.8 – 13.839 – 4925 – 31in telegraph wires; umbrellas used with

    difficulty

    Whole trees in motion; inconvenience felt 7Near gale28 – 3313.9 – 17.150 – 6132 – 38when walking against the windBreaks twigs off trees; generally impedes 8Gale34 – 4017.2 – 20.762 – 7439 – 46progress

    Slight structural damage occurs (chimney-9Strong gale41 – 4720.8 – 24.475 – 8847 – 54pots and slates removed)

    Seldom experienced inland; trees 10Storm48 – 5524.5 – 28.489 – 10255 – 63uprooted; considerable structural damage

    occurs

    Very rarely experienced; accompanied by 11Violent storm56 – 6328.5 – 32.6103 – 11764 – 72widespread damage

    12Hurricane64 and 32.7 and over118 and 73 and

    overoverover

    Wind speed equivalents for arctic areas and areas where there is no vegetation

    Beaufort scale Wind speed equivalent at a standard heightSpecifications for estimating speed for number and of 10 m above open flat groundarctic areas and areas where there is no descriptionvegetation

    –1–1–1(kt)(m s)(km h)(mi h)

    0Calm< 10 – 0.2< 1< 1

    No noticeable wind. Smoke rises nearly 1Light air1 – 30.3 – 1.51 – 51 – 3vertically.

    Wind felt on face, leaves rustle.2Light breeze4 – 61.6 – 3.36 – 114 – 7

    Hair is disturbed, clothing flaps.3Gentle breeze7 – 103.4 – 5.412 – 198 –12

    Dust and loose paper raised, hair disarranged.4Moderate breeze11 – 165.5 – 7.920 – 2813 – 18

    Force of wind felt on body. Limit of agreeable 5Fresh breeze17 – 218.0 – 10.729 – 3819 – 24wind on land.

    Some inconvenience in walking.6Strong breeze22 – 2710.8 – 13.839 – 4925 – 31

    Difficulty when walking against wind.7Near gale28 – 3313.9 – 17.150 – 6132 – 38

    Difficulty with balance in walking.8Gale34 – 4017.2 – 20.762 – 7439 – 46

    Danger in being blown over.9Strong gale41 – 4720.8 – 24.475 – 8847 – 54

    Trees uprooted, considerable structural damage.10Storm48 – 5524.5 – 28.489 – 10255 – 63

    11Violent storm56 – 6328.5 – 32.6103 – 11764 – 72

    12Hurricane64 and 32.7 and over118 and 73 and

    overoverover

    5.2ESTIMATION OF WIND

    In the absence of equipment for measuring wind, the observations must be made by estimation. The errors in observations made in this way may be large, but, provided that the observations are used with caution, the method may be justified as providing data that would otherwise not be available in any way. If either temporarily or permanently the wind data of some stations are obtained by estimation instead of measurement, this fact should be documented in station records made accessible to data users.

    5.2.1Wind speed

    Estimates are based on the effect of the wind on movable objects. Almost anything which is supported so that it is free to move under the influence of the wind can be used, but the descriptive specifications given in the Beaufort scale of wind force, as reproduced in the table, will be found especially useful.In order to make the estimates, the observer (and the wind-susceptible object) must stand on flat open terrain as far as possible from obstructions. It must always be remembered that even small obstructions cause serious changes in wind speed and deviations in wind direction, especially at their lee side.

    5.2.2Wind direction

    In the case of an absence of instruments, or when the instrumental equipment is unserviceable, the direction should be estimated by observing the drift of smoke from an elevated chimney, the movement of leaves, and so on, in an open situation, or a streamer or pennant fixed to a tall flagstaff. In addition, the wind drogue at an airport may be used when the wind speed is sufficient to move such a device.

    Whichever of these aids is used, errors due to perspective are liable to be made unless the observer stands vertically below the indicator. Care should be taken to guard against mistaking local eddies caused by buildings, and the like, for the general drift of the wind.

    In an open location, the surface wind direction can be estimated rather accurately by facing the wind. The direction of the movement of clouds, however low, should not be taken into account.

    5.2.3Wind fluctuations

    No attempt should be made to estimate peak gusts or standard deviations without proper instruments and recording devices.

    5.3SIMPLE INSTRUMENTAL METHODS

    At stations where orthodox anemometers cannot be installed it may be possible to provide some very low-cost, simple instruments that help the observer take measurements that are somewhat more reliable than those obtained by unaided estimation.

    5.3.1Wind speed

    Simple hand-held anemometers, if they are used, should be set up and read in accordance with the maker’s instructions. The measurement should be taken from a point well exposed to the wind,

    and not in the lee of obstructions such as buildings, trees and hillocks. If this is not possible, the measurement point should be a good distance from obstructions, namely at least 10 times the

    obstruction height and upwind or sideways by at least twice the obstruction height.

    5.3.2Wind direction

    Direction may be estimated from a vane (or banner) mounted on a pole that has pointers indicating the principal points of the compass. The vane is observed from below, and wind direction may be estimated to the nearest of the

    16 points of the compass. If the vane oscillates in the wind, the wind direction must be estimated as the average direction about which the oscillations occur.

    5.4CUP AND PROPELLER SENSORS

    Cup and propeller anemometers are commonly used to determine wind speed and consist of two sub-assemblies: the rotor and the signal generator. In well-designed systems, the angular velocity of the cup or propeller rotor is directly proportional to the wind speed, or, more precisely, in the case of the propeller rotor, to the component of the wind speed parallel to the axis of rotation. Also, in well-designed anemometers, the calibration linearity is independent of air density, has good zero and range stability, and is easily reproduced in –1a manufacturing process. Near the starting threshold, say for wind speeds of less than 4 m s, the calibration

    of cup anemometers can deviate substantially from linearity, if the arm connecting the cup to the rotation axis is much longer than the diameter of the cup (Patterson, 1926).

    The nature of the response of the cup and propeller-type wind-speed sensors to changes in wind speed can be characterized by a response length, the magnitude of which is directly proportional to the moment of inertia of the rotor and, in addition, depends on a number of geometric factors (Busch and Kristensen, 1976; Coppin, 1982).

    For almost all cup and propeller-type wind sensors, the response is faster for acceleration than for deceleration, so that the average speed of these rotors overestimates the actual average wind speed. Moreover, vertical velocity fluctuations can cause overspeeding of cup anemometers as a result of reduced cup interference in oblique flow (MacCready, 1966). The total overspeeding can be as much as 10 per cent for some designs and turbulent wind conditions (cup anemometers at 10 m height with a response length of 5 m over very rough terrain; Coppin, 1982). This effect can be minimized by choosing fast-response anemometers, either cup anemometers of a design verified as having a good cosine response or propeller vanes that have virtually no vertical component of overspeeding. In case that performance cannot be investigated in a wind tunnel, operational anemometers can be compared in the field with a calibrated anemometer (Albers, Klug and Westermann, 2000).

    Since both cup and propeller rotors turn with an angular velocity that is directly proportional to speed or to the axial component, they are particularly convenient for driving a wide variety of signal generators. Alternating and

    direct current generators, optical and magnetic pulse generators, and turn-counting dials and registers have been used (WMO, 2001). The choice of signal generator or transducer depends largely on the type of data processor and read-out to be used. Care should be taken to ensure that the bearings and signal generator have low starting and running frictional torques, and that the moment of inertia of the signal generator does not reduce the response too much. In cases of long-distance transmission, voltage signals decrease due to cable resistance losses and are therefore inferior to pulse frequency signals, which are not so affected during transmission.

    The required and achievable characteristics for wind-speed sensors are included in Part I, Chapter 1, Annex 1.B.5.5WIND-DIRECTION VANES

    For the purpose of obtaining a satisfactory measurement, a wind vane will be suitable if it is well balanced so as not to have a preferred position in case the axis is not vertical. Multiple vane fins should preferably be parallel to the vane axis, because a vane with two fins at angles > 10? to its axis has two equilibrium positions which each differ significantly from the real wind direction (Wieringa and van Lindert, 1971).

    The response of the usual underdamped wind vane to a sudden change in wind direction is normally characterized by overshoot and oscillation about its true position, with the amplitude decreasing approximately exponentially. Two variables are used to define this response: the “undamped natural frequency” or “wavelength” and the “damping ratio”, the ratio of the actual damping to the critical damping (MacCready, 1966; Mazzarella, 1972). A damping ratio between 0.3 and 0.7 is considered to be good and as having not too much overshoot, and a reasonably fast response (Wieringa, 1967). Where a relatively long period average is to be computed from data captured at short intervals, it is self-evident that lower damping ratios may be acceptable.

    The signal generator is essentially a shaft-angle transducer, and many varieties have been employed. Potentiometers, alternating and direct current synchros, digital angle-encoder discs, direct reading dials and rotary switches have been used to advantage. The choice of signal generator is largely a matter of the type of data processor and read-out used. Care should be taken to ensure that the bearings and signal generator have low starting and running frictional torques. The simplest recording method is to have a sheet mounted around a cylinder rotating with the vane axis, on which a writing instrument slowly travels downward.The absolute accuracy of direction measurement also depends on the care with which the instrument has been aligned to true north. The required and achievable characteristics for wind-direction vanes are included in Part I, Chapter 1, Annex 1.B.

    5.6OTHER WIND SENSORS

    Many physical principles can be used to measure wind speed and direction, all of which have their own merits and problems. New systems often have been developed for specific purposes, such as small-scale fluctuations and air pollution studies (see for example, Smith (1980)). The following are other types of sensors:(a)Pitot tube anemometers, which measure the overpressure in a tube that is kept aligned with the wind vector by

    means of a direction vane (see Gold (1936) and WMO (1984a) for a description of the Dines anemometer).

    The Dines linearizing recording system deals with the speed averaging problem caused by the quadratic

    relation between wind speed and pressure, and it also provides useful gustiness records without requiring

    electrical power;

    (b)Sonic anemometers, which measure the time between emission and reception of an

    ultrasonic pulse travelling over a fixed distance (Kaimal, 1980). Because sonic anemometers have no

    moving parts owing to their principle, they have high durability and little accuracy deterioration. (c)Hot-disc anemometers are recently developed solid-state instruments which measure the temperature

    gradient across a chip arrangement. This provides both wind speed and direction at accuracies within the

    specification of Part I, Chapter 1, Annex 1.B (Van Oudheusden and Huijsing, 1991; Makinwa, Huijsing and

    Hagedoorn, 2001). They are sturdy, and steady in calibration, but operational experience is limited so far;(d)Hot-wire anemometers measure the cooling of thin heated wires. Operationally they are rather unreliable,

    both because of excessive fragility and because their calibration changes rather fast in unclean or wet

    surroundings. They are not recommended for use in precipitation;

    (e)Antique swinging-plate vanes are a little better than no instrument at all;

    (f)Remote wind-sensing techniques with sound (sodar), light (LIDAR) or electromagnetic waves (radar) are

    uncommon in routine meteorological networks and will not be discussed in this Guide. Details are

    provided in Lenschow (1986).

5.7SENSORS AND SENSOR COMBINATIONS FOR COMPONENT RESOLUTION

    Propellers which respond only to the wind speed component that is parallel to the axis of rotation of the rotor can be mounted orthogonally to produce two read-outs which are directly proportional to the components in the axis directions. Other sensors, such as twin-axis sonic anemometers, perform the same function at the expense of more sophisticated electronic adjuncts. Orthogonal propellers have the disadvantage that exact cosine response (namely, pure component sensitivity) is difficult to attain. A cup anemometer/vane combination or a propeller vane can also be used as a component device when the velocity components are computed from the measured wind speed and direction.

    5.8DATA-PROCESSING METHODS

    Signals from anemometer/vane combinations can be processed and averaged in many different ways. Before considering the aspects of the entire wind-measuring chain (exposure, sensing, transmission, filtering, recording and processing), it is useful to discuss the problem of averaging. This Guide deals with the following outputs: averaged horizontal wind (components or speed/direction), standard deviations and peak gust.

    5.8.1Averaging

    The averaging of wind vectors or their components is straightforward in principle, but there are a few problems associated with it. The first is that the mean vector speed in the average wind direction U is less than the

    average of all instantaneous wind speeds by a small amount, generally a few per cent (MacCready, 1966; Wieringa 1980a). If necessary, this may be corrected if the standard deviation of wind direction s is measured; d

    for the ratio of U, and the averaged instantaneous wind speeds is (Frenkiel, 1951):

    (5.2)

    This effect of crosswind turbulence is often confused with the overestimation (overspeeding), causing distortion in the standard deviation s (see u

    section 5.4).

    The second problem is the discontinuity of the wind direction between 0 and 360?. This problem can be solved either by recording on a cylinder or by extending the recorder range (for example to 540? with an automatic device switching the range from 0 to 360 and from 540 to 180), or by a computer algorithm that makes successive samples continuous by adding or subtracting 360? when necessary. The fact that the first-order response of a cup anemometer and the second-order response of a vane cannot be fully matched is a problem of minor importance, because the response differences are reflected only in the high-frequency part of the fluctuations.

    From the fundamental point of view, component averaging is preferable over the independent averaging of speed and direction. However, the differences are very small and, for most applications, component averages can easily be derived from average speed and direction. This also applies to the corresponding standard deviations. From the technical point of view, the independent treatment of speed and direction is preferable for a number of reasons. First of all, the processing of the signal for speed and direction is independent, which implies that the operation of one instrument can continue even when the other drops out. Secondly, this data reduction is simpler than in those cases where components have to be computed. Lastly, the independent treatment of speed and direction is compatible with common usage (including SYNOP and SHIP coding).

    The averages of horizontal wind speed can be obtained with a number of both mechanical and electrical devices. Perhaps the simplest example is a mechanical rotation-counting register on a cup anemometer commonly used to measure the passage of wind during a chosen averaging time interval. At the other end of the complexity spectrum, electrical pulse generators drive special-

    purpose digital processors, which can easily calculate averages, peak gusts and standard deviations. If wind speed and direction are recorded as continuous graphs, an observer can estimate 10 min averages fairly accurately from a pen recording. The recorded wind trace can also be used to read peak gusts. The reading of dials or meters gives a feel for the wind speed and its variability, but is subject to large errors when averages are needed. Instantaneous read-outs are, therefore, less suitable to obtain

    10 min averages for standard weather reports.

    5.8.2Peak gusts and standard deviations

    The computation or recording of wind fluctuations is extremely sensitive to the dynamic response of all the elements of the measuring chain, including response length and damping ratio of the sensors. Additionally, the dynamic response of the system as a whole determines the duration of peak gusts, as defined in section 5.1.1. Slowly responding systems spread out the extremes and indicate wide gusts with small amplitude, whereas fast-response

    systems record high and narrow peaks (gusts of short duration). It is clear that the dynamic response of wind systems has to be carefully designed to obtain gusts or standard deviations that are accurate, reliable and compatible between stations.

    Before specifying the appropriate response characteristics of wind-measuring systems, it is necessary to define the gust duration as required by the application. Wind extremes are mainly used for warning purposes and for the climatology of extreme loads on buildings, constructions and aircraft. It is important to realize that the shortest gusts have neither the time nor the horizontal extent to exert their full damaging effect on large constructions. WMO (1987) concludes that a gust duration of about 3 s accommodates most potential users. Gusts that persist for about 3 s correspond to a “wind run” (duration multiplied by the average wind speed) of the order of 50 to 100 m in strong wind conditions. This is sufficient to engulf structures of ordinary suburban/urban size and to expose them to the full load of a potentially damaging gust.

    The standard deviation of wind direction and wind speed can easily be computed with microcomputer-based equipment by taking samples of the signals at intervals of about 1 s. Sampling frequencies should not be too great, because the sensor itself provides smoothing over a multiple of its response distance (Wieringa, 1980b). A sampling

    frequency of 0.25 Hz is suitable in most cases, but depends on the response distance of the sensor and the wind speed. Part III, Chapter 2, includes a detailed discussion of the theory of sampling sensor signals.Simultaneous computation of the standard deviation of the horizontal wind speed over 10 min together with the detection of gusts with a duration of a few seconds gives interesting requirements for electronic filters. The gusts are most critical with regard to filtering, so in practice the system is optimized for them. Any low-pass filter used for the detection of peak gusts measured by fast anemometers, smoothing over a few seconds, may reduce the standard deviation by up to 10 per cent. This can be corrected if the filtering variables in the measuring chain are well documented. Often, in practice, the reduction is less because the standard deviation increases if the average wind speed shows a positive or negative trend. Alternatively, the unfiltered signal can be recorded separately for the purpose of measuring an unbiased standard deviation. In the next section, recommendations are made for wind-measuring systems with exact values for the filter variables.

    In order to determine peak gusts accurately, it is desirable to sample the filtered wind signal every 0.25 s (frequency 4 Hz). Lower sampling frequencies can be used, but it should be realized that the estimate of the extreme will generally be lower as the extreme in the filtered signal may occur between samples.

    Apart from the wind vane inertial damping, any further filtering should be avoided for wind direction. This means that the standard deviation of wind direction can be determined within 2 per cent with most wind vanes.Accurate computation of the standard deviation of wind direction requires a minimum resolution of the digitization process, which is often done on the shaft of the vane by means of a digital encoder. A 7 bit resolution is quite sufficient here because then a 5? unit for the standard deviation can still be measured with an accuracy of 1 per cent (WMO, 1987).

    15.8.3Recommendations for the design of wind-measuring systems

    Wind-measuring systems can be designed in many different ways; it is impossible to cover all design options in this Guide. Two common examples are given here, one with mainly analogue signal treatment and the other with digital signal processing (WMO, 1987).

    The first system consists of an anemometer with a response length of 5 m, a pulse generator that generates pulses at a frequency proportional to the rotation rate of the anemometer (preferably several pulses per rotation), a counting device that counts the pulses at intervals of 0.25 s, and a microprocessor that computes averages and standard deviation over 10 min intervals on the basis of 0.25 s samples. The extreme has to be determined from 3 s averages, namely, by averaging over the last 12 samples. This averaging has to be done every 0.25 s (namely, overlapping 3 s averages every 0.25 s). The wind direction is measured with a vane that has an undamped wavelength of 5 m, a damping ratio of 0.3, and a 7 bit digital encoder that is sampled every second. Averages and standard deviations are computed over 10 min intervals, where successive samples are checked for continuity. If two successive samples differ by more than 180?, the difference is decreased by adding or subtracting 360? from the second sample. With response lengths of 5 m for the anemometer and the wind vane (damping ratio 0.3, undamped wavelength 10 m), the standard deviations of wind speed and wind direction are reduced by about 7 and 2 per cent, respectively. The gust duration corresponding to the entire measuring chain (as defined in section 5.1.1) is about 3 s.

    The second system consists of an anemometer with a response length of 5 m, a voltage generator producing a voltage proportional to the rotation rate of the anemometer, analogue-to-digital conversion every second, and the digital processing of samples. The wind-direction part consists of a vane with an undamped wavelength of 5 m and a damping ratio of 0.3, followed by analogue-to-digital conversion every second and digital computation of averages and standard deviations. To determine peak gusts the voltage is filtered with a first-order filter with a time-constant of 1 s and analogue-to-digital conversion every 0.25 s. With regard to filtering, this system is slightly 1Recommended by the Commission for Instruments and Methods of Observation at its tenth session (1989).

    different from the first one in that standard deviations of wind speed and direction are filtered by 12 per cent and 2 per cent, respectively, while again the gust duration is about 3 s. This system can also be operated with a pen recorder connected to the analogue output instead of the analogue-to-digital converter. Only averages and extremes can be read now, and the gust duration is about 3 s, unless the pen recorder responds more slowly than the first-order filter.

    The signal-processing procedure, as described above, is in accordance with Recommendation 3 (CIMO-X) (WMO, 1990) and guarantees optimal accuracy. The procedure, however, is fairly complicated and demanding as it involves overlapping averages and a relatively high sampling frequency. For many applications, it is quite acceptable to reduce the sampling rate down to one sample every 3 s, provided that the wind signal has been

    averaged over 3s intervals (namely, non-overlapping averaging intervals). The resulting gust duration is about 5 s and the reduction in standard deviation is 12 per cent (Beljaars, 1987; WMO, 1987).

    5.9EXPOSURE OF WIND INSTRUMENTS

    5.9.1General problems

    Wind speed increases considerably with height, particularly over rough terrain.For this reason, a standard height of

    10 m above open terrain is specified for the exposure of wind instruments. For wind direction, the corresponding shift over such a height interval is relatively small and can be ignored in surface wind measurements. An optimum wind observation location is one where the observed wind is representative of the wind over an area of at least a few kilometres, or can easily be corrected to make it representative.

    For terrain that is uneven, contains obstacles, or is non-homogeneous in surface cover, both wind speed and direction can be affected considerably. Corrections are often possible, and the tools to compute such corrections are becoming available. To improve the applicability of wind data, essential information to perform such corrections should be transmitted to the users in addition to the direct measurements.

    5.9.2Anemometers over land

    The standard exposure of wind instruments over level, open terrain is 10 m above the ground. Open terrain is defined as an area

    where the distance between the anemometer and any obstruction is at least 10 times the height of the obstruction. Wind measurements that are taken in the direct wake of tree rows, buildings or any other obstacle are of little value and contain little

    information about the unperturbed wind. Since wakes can easily extend downwind to 12 or

    15 times the obstacle height, the requirement of 10 obstruction heights is an absolute minimum. In practice, it is often difficult to

    find a good or even acceptable location for a wind station. The importance of optimizing the location can hardly be overstressed;

    nonetheless, it is difficult to give universal guidelines. In some cases the data can principally be corrected for influences by

    obstructions, as follows:

     Obstacles distant by more than 30 times their heights: no correction need to be applied to the wind data

     Obstacles distant by more than 20 times their heights: correction can be applied.

     Obstacles distant by more than 10 times their heights: correction may be applied taking special care, in some

    situations.

    It should be noted that in case of distances below 20 times the height of the obstacle, the measured value before correction can

    be erroneous by up to 25%; in case of a distance around 10 times the height of obstacles, the measured value can in some cases

    even be of opposite direction.

    Detailed information on the exposure correction is provided below in paragraph 5.9.4.

    In the table below, the classification of wind observing sites based on their siting and exposure is summarized. Full details on

    the Siting Classification for Surface Observing Stations on Land, which provides additional guidance on the selection

    can be found in Part I, of a site and the location of a wind sensor within a site, to optimize its representativeness

    Annex 1.B. of this Guide)

    Clasdistance of mast to distance of sensors to narrow roughness class ignore single

    ssurrounding obstacles* obstacles** (with height > 8m, index***obstacles below

    (with height h)width w)x m

    130 h15 w2 – 4 (roughness x = 4? ?

    length ? 0.1 m)

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