terns using a 5 per cent significance level, then we would expect that about five of the most extreme would have probabilities less than 5 per cent, even if none of the patterns were ‘real’. If we consciously look at ten possible patterns, but unconsciously consider 90 others, we may not appreciate this phenomenon.

What this all means is that we should not take the probabilities calculated above too literally as measures of the implausibility of our two null hypotheses. However, having sounded this cautionary note, it must be said that the probabilities are very small, certainly small enough to seriously consider the possibility that (a) and (b) do represent real patterns.

REFERENCES Clarke. G.M. and Cooke. D. (1983) A basic course in srorisrics, 2nd edition. Edward Arnold, London Everitt, B.S. (1977) The analysis ofcontingency rubles. Chapman and Hall, London

THE AUTOMATIC WEATHER STATION NETWORK IN THE BALQUHIDDER CATCHMENTS, SCOTLAND By R.C. JOHNSON’ and T.K.M. SIMP SON^ I Institute of Hydrology, Balquhidder, Lochearnhead, Perthshire rlnstitute of Hydrology, Wallingford, Oxon

HE Institute of Hydrology has been carrying out research at Balquhidder in Highland T Scotland since 1981, looking at the effects of afforestation on water resources. Two catchments have been instrumented, the forested Kirkton catchment (6.85 km2) and the moorland Monachyle catchment (7.70 km2) with different stages in the forest cycle being studied as the land uses change. Blackie (1987) gives a fuller description of the catchments.

Networks of precipitation gauges monitor the input and streamflow gauges the output from the catchments (Johnson etal. 1990; Johnson and Hudson 1987). By usingestimates of the changes in soil moisture and snow storages, the evaporation can be calculated as the residual term in the catchment water balance equation. At selected sites detailed studies are camed out on the processes of evaporation and transpiration from the main vegetation types (Johnson 1990; Wright 1990), and catchment models are being developed using the results from the water balance and process studies.

The main link between all components of the experiment is a network of automatic weather stations (AWSS) positioned at key locations in the catchments. Six AWSS have been used in the Balquhidder catchments to provide detailed radiation, wind, temperature and rainfall data for estimating the Penman potential evapotranspiration (ET) rates. Using the AWS data, it will also be possible to apply the models of water use developed at Balquhidder to other catchments in Scotland and improve regional estimates of evaporation in the Uplands.

OPERATION OF THE AUTOMATIC WEATHER STATIONS

The AWS used at Balquhidder has been developed over the past 25 years by the Institute of Hydrology (Strangeways and Smith 1985). p e stations are designed to operate in remote, mountainous regions where extreme weather conditions can be expected. Seven sensors monitor solar radiation, net radiation, air temperature, wet-bulb temperature, wind run, wind direction and rainfall, at 5-minute intervals (Strangeways 1976). Each station isvisited at least every two weeks for data retrieval and routine maintenance. At the start of the experiment, battery-powered Microdata loggers were used on the AWS, storing the data on c60 cassette tapes. In 1988 these were replaced by the more reliable solar- powered, solid-state Campbell Scientific (CRlO) loggers.

47

The quality control of the AWS data is a multi-stage process involving manual and computerised checking and flagging of data. With the use of solid-state logging and pcF the manual checking is an efficient stage where the operator also develops a better understand- ing of the weather variability in the catchments. The automatic stage is done on a main- frame computer before archiving the data. Routines have been established which compare the data sequentially and identify data which exceed preset limits (Roberts 1981).

THE BALQUHIDDER AUTOMATIC WEATHER STATIONS

The topography at Balquhidder is very rugged; slopes average 15-20 degrees with an altitude range from 250 to 900 metres. The number of AWS in a network is limited by the high operational cost, therefore monitoring sites must be representative of large areas of the catchments. Altitude and vegetation were chosen to be the main distributing criteria, with one high-altitude site and another low-altitude site in each catchment, positioned over grass where possible. One site was selected just out of the Kirkton catchment so that there could be both grass and trees represented for the lower Kirkton.

Figure 1 shows the position of the six AWS sites with the surrounding topographic features; additional details are given in Table 1.

Fig. I The Balquhidder automatic weather station network

TABLE 1 Balquhidder AWS site details

KIRKTON KIRKTON KIRKTON TULLOCH LOWER UPPER FOREST (1) FOREST (2) HIGH FARM MONACHYLE MONACHYLE

Start date 30 July '86 7 June '89 1 July '81 31 Mar. '82 1 July '81 20 Dec. '83 Finish date 2 Nov. '89 - - Grid ref. 525236 530223 513229 520204 475231 481249 Altitude(m) 380 270 670 135 310 470 Vegetation spruce clear fell grass grass gr= heather

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- - -

The low-altitude sites are greatly influenced by the surrounding topography while the high-altitude sites are very well exposed. Tulloch Farm is in an eat-west oriented glen, while the Lower Monachyle and Kirkton Forest stations are in north-south glens. Wind directions are dominated by the orientation of the glens and shading by the surrounding hills reduces the receipt of direct solar radiation, especially in the winter.

The vegetation at each site is different in some aspects. Of the three grass sites the Tulloch Farm grass is kept short, Lower Monachyle is ungrazed, long and coarse, whereas Kirkton High is grazed into coarse tussocks. At Upper Monachyle the heather is deep ( M a n ) . Kirkton Forest (1) was in the centre of the Kirkton Forest on a 22-metre high tower with the instruments some 3 metres above the tree canopy. This site was dismantled in 1989 when clear felling approached the site. The replacement site, Kirkton Forest (2), is lower down the glen in a clear felled area.

SUMMARY OF AWS DATA, 1983-89 Table 2 presents a summary of the annual mean values for four stations for the period

1 W . Many problems occurred at Kirkton Forest (l), so complete years of data are rare. This station h a been omitted from Table 2. There were less frequent problems at the other stations although in some years, mainly due to logger problems, the data capture was less than 90 per cent. The validity of presenting mean annual values and 1983-89 mean values using incomplete data is dubious and detailed intercomparisons should be treated with caution.

The topographic influence can be seen in the results presented in Table 2. The incoming solar radiation for the shaded, low-altitude sites is low, especially from Lower Monachyle, and the wind direction at Lower Monachyle and Tulloch Farm was dominated by the orientation of the glens. The effect of altitude differences is shown in the tempera- ture depression data, with Kirkton High most often in or near to the cloud base. Bearing in mind the limited nature of the data the relationship of temperature and wind speed with altitude is remarkably consistent. For temperature the mean lapse rate was 0.90degC per lOOm altitude (coefficient of determination, rz = 0.94), which is similar to that previously reported (Johnson 1985) when fewer data were used. The increase in wind speed with altitude was 0.64ms-1 per lOOm (rz = 0.96).

CONCLUSIONS

This paper has described the AWSS in the Balquhidder catchments and given some general results from the data. This network of stations is, as far as the authors know, the most dense network in Britain and includes the highest-altitude station which is continu- ously monitored through the year. The harsh nature of the climate in Highland Scotland has made it difficult to achieve a data capture of 90 per cent although modem logging systems have improved this situation and future years’ data should be more complete.

The summary of the annual mean data has shown how topography can dominate weather in mountainous areas. Wind direction and solar radiation are influenced by the orientation of the surrounding hills while wind speed and temperature are influenced by the increasing exposure at higher altitudes. The lapse rate in temperature for the period 1983- 89 was found to be 0.90 degC per lOOm and the increase in wind speed with altitude was 0.64m s-1 per 1OOm.

REFERENCES

Blackie, J.R. (1987) The Balquhidder catchments, Scotland: the first four years. Trans. R. Soc.

Johnson, R.C. (1985) Mountain and glen climatic contrasts at Balquhidder. J. Mereorof., 10, pp.105-

- (1990) The interception, throughfall and stemflow in a forest in Highland Scotland and the

Edinburgh, 78, pp.227-239

108

comparison with other upland forests in the UK. J. Hydrof., ll8, pp.281-288

49

TABLE 2 Annual mean values for the Balauhidder AWS sites 1983 1984 1985 1986 1987 1988

Solar radiation ( W m- 2, Tulloch Farm 84 * Lower Monachylc 79 Y2 Upper Monachyle * Y7 Kirkton High 83 *

Net radiation (W m-2) Tulloch Farm 28 * Lower Monachyle 38 46 Upper Monachyle * 50 Kirkton High 40 *

Temperature depression (deg C ) Tulloch Farm 1.5 * Lower Monachylc I .3 1.4 Upper Monachyle * I .2 Kirkton High 0.9 *

Temperature (“C) Tulloch Farm Y.4 * Lower Monachylc 7.5 X. 1 Upper Monachylc * 5.5 Kirkton High 4.4 * Wind speed (m SKI)

Tulloch Farm 2.2 * Lower Mondchyk 3.3 3.0 Uppcr Monachylc * 3.5 Kirkton High * *

Wind direcrron (degrees) Tulloch Farm 332 * Lower Monachyle 146 1 1 0 Upper Monachylc * 24 I Kirkton High 228 *

81 * *

85

* * * 42

* * *

0 . X

* * * 2.0

* * *

6.0

* * * 207

88

96 *

*

30

56 *

*

I .5

1 . 1 *

*

7.7

5.3 *

*

32 I

223 *

*

90 82 90 89

33 31 58 42

I .4 1 . 1

0.6 *

7.7 Y .4 5.4 2.0

I .7 2.7 3.2 5.2

33Y I62 24x 214

80 74 82 *

31 .30 51 *

1.4 1.2 * *

8.2 7.3 6. I *

2.0 3. I * *

328 I 6Y 200 *

1989 MEAN

93 86 83 82 96 Y2 % 88

46 34 36 36 53 .s4 44 42

1.3 1.4 I .4 1.3 1.2 1.2 1 .o 0.8

8.3 8.3 7.3 7.9 6.3 5.7 4.9 3.3

2.2 2.0 3.3 3.1 4.1 3.6 5.6 5.6

312 Ihh 234 219

* Lcss than YO per cent data retrieval in year

Johnson. R.C. and Hudson. J.A. (19x7) Flow gauging in two .ScotriS/7 upland catchments. Proc. BHS National Hydrology Symposium. Univcrsity of Hull. September 1987. pp.35.1-35.2

Johnson, R.C.. Blackie. J.R.. and Hudson. J.A. (IYYO) Methods of estimating precipitation inputs to the Balquhidder expcrimental basin. Scotland. In: Hydrology in mountainous regiom (Proc. Lausanne Symposium. August IW)) IAHS Publ. No. lY3, pp.7-14

Roberts, G. (19x1) The processing of hydrological data. lnstiture of Hydrology Repon No. 70 Strangeways. 1 .C. (1976) The long-term performance of a network of automatic weatherstatiom and rhe

factors affecting this. Proc. Tcchnical Confcrencc on Automatic Weather Stations, Reading University. September 1Y76. pp.24-45

Strangcways. I.C. and Smith S.W. (19x5) Development and use of automatic weather stations. Weather. 40. pp.277-285

Wright. I.R. (1W) A lysimetcr for thc mcasurcmcnt o f evaporation from high altitude grass. In: Hydro/og.y in mountainous regions (Proc. Lausannc Symposium. August 1990) IAHS Publ. No. 193. pp.7Y-86

50

CUMULONIMBUS OVER THE ALPS

From a colour transparency by Willi Schmid An isolated hailstorm from this cumulonimbus over the Swiss Alps occurred on 29 July 1979; the lOcm radar in the foreground was used for hail measurements during the Grossversuch-IV hail suppression experiment (1977-82) by the Swiss Federal Institute of Technology (ETH), Zurich

A NOTE ON THE REMARKABLE WINTER OF 1988/89 OVER THE UNITED KINGDOM

By R. MURRAY Wokingham, Berkshire

E seasonal rainfall, temperature and sunshine of winter 1YW89 and many notewor- T” thy weather events during the winter have been admirably presented by Northcott (1989). This note tries to put an unusual winter in perspective, and also amends the somewhat misleading statement in the Summary of Northcott’s article. The combination of very high mean temperature over central England (CET) and below average rainfall in England and Wales (EWR) is fairly uncommon, but separately neither EWR nor CET broke known records, although the mean temperature was only marginally below the highest on record in winter 1868/6Y (Manley 1974).

CIRCULATION CHARACTERISTICS

The predominant circulation over the British Isles can be assessed with the help of the PSCM indices (see Murray and Lewis 1%; Murray and Benwelll970). Some 25 years ago computer facilities and machinable data for use in long-range forecasting were very limited. My aim in producing the PSCM indices from the daily synoptic types over the British Isles (Lamb 1972) was to distil the dominant synoptic features over a season (or month) from the rather confusing minutiae of the daily surface charts. In several papers (e.g. Murray and Benwelll972, Figs. 1,2 and 3) it has been demonstrated that the indices in quint form can usefully specify the large-scale anomalous circulation shown on surface pressure anomaly charts. Nowadays the facilities available to the professional meteorologist enable much

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