SNOWFALL LOSSES AT MOUNTAIN RAINGAUGES By GEOFFREY REYNOLDS North of Scotland Hydro-Electric Board

HE standard raingauge is not a very efficient collector of snowfall. It has T been specially developed with liquid precipitation in mind and when snow is falling several sources of error arise.

(I) Wind eddying around the gauge causes a serious loss of catch as owing to their relatively large size and low terminal velocity snowflakes are more affected by wind than raindrops.

(2) The snowflakes, particularly if approaching the gauge under windy conditions with a large angle of incidence, tend to adhere to the rim and ultim- ately cap over the entire orifice. Even if capping does not take place the rim intercepts a large percentage of the snowflakes owing to their relatively large size. With a wind of 10 knots almost 20 per cent of the flakes which should enter the gauge come into contact with the gauge rim (Kurtyka 1953). Some loss must be anticipated from these interceptions.

(3) It is standard and admirable practice to expose a raingauge in as sheltered a site as possible. This will probably be the first place to be buried in deep snowdrifts. Once this has occurred the catch in the raingauge is mean- ingless.

Snowfall measurements with well-attended daily gauges are difficult enough. With inaccessible gauges, visited weekly or monthly, they become impossible. Automatic recorders have, so far, proved unreliable even with a small heater inside unless visited at least daily.

Hudleston (1934), after years of exhaustive study and experiment with raingauges, reached the conclusion:

' Whenever precipitation takes the form of snow every kind of raingauge, whether it be bare or sheltered by artificial means, becomes utterly unreliable . . . During the interval of time between the commencement of a snow storm and the moment when a gauge gets completely buried, its record for catch is hope- lessly small if it happens to be normally exposed, and hopelessly large if it happens to be within a turf-walled enclosure . . . After a raingauge is com- pletely buried by snow it will catch approximately its proper share of melted water in the superincumbent snow as the thaw comes on . . .'

Nothing that has happened in the ensuing 25 years has changed in the least the validity or force of this quotation.

Accepting this, we have to devise a method of estimating winter pre- cipitation (which includes snow) on mountain sites. A scrutiny of isomeric rainfall maps, expressing average monthly rainfall totals as a percentage of the annual average, shows that the isomeric values vary reasonably smoothly over the British Isles and this gives us a lead on a possible method to develop. The construction and interpretation of such maps are discussed by Salter (1921).

If these isomeric maps have any real meaning, and the fact that good local agreement is found in the values month by month proves their validity,

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it is clear that ratios between the catches of two adjacent ideally-sited gauges, only a few miles apart, should remain nearly constant throughout the year. Therefore this ratio can be computed for the snow-free part of the year and used to estimate the precipitation at the high level or inaccessible gauge from the daily readings of the low level and well-attended gauge during the winter months. The isomers are based on yj-year averages and close correlation between catches in any individual month is not always to be expected, never- theless in view of the difficulties of direct measurement mentioned previously, the method is worthy of investigation.

Such an investigation has recently been carried out using the last five years' records (1954-58) of the North of Scotland Hydro-Electric Board's gauges in and around the catchment area of the River Gany in Inverness-shire. (see Fig. I).

The gauge at Kingie Camp was selected as the base gauge as it is the farthest up the glen which is read daily. All the gauge sites have been inspected by a Meteorological Office inspector and deemed reasonably satisfactory, although some minor moves have been recommended and carried out.

One of the indications of over-exposure of a raingauge is that it should catch relatively less in winter (the windier part of the year) than in summer compared with a nearby and better-sheltered gauge. Before the investigation proper began the Kingie Camp record was tested against the two gauges lower down the valley, Inchlaggan and Craigard. The month by month mean ratios are plotted in Fig. 2.

As each monthly ratio was based on only five values there was some irregularity, which would not arise had a longer series of data been available for analysis. The monthly ratios were therefore smoothed before plotting by overlapping means using the formula

where a, b and c are consecutive crude monthly ratios and b' is the smoothed ratio for the central month.

On every occasion that a monthly mean ratio appeared out of accord with the monthly ratios on either side of it, a check was made of the five individual ratios which it comprised to ascertain whether the discordant ratio was due to one erroneous or unusual reading biasing the whole. This never proved to be the case and the mean ratios are all well substantiated by the data.

It will be observed from the Inchlaggan and Craigard graphs that Kingie Camp catches a greater ratio of rain in winter than summer than either of the others. This confirms that the Kingie exposure is at least as good as the other two. J t is felt that the explanation of the small seasonal ratio change is due to a combination of two factors, the closer proximity of Kingie to the peaks so that the depression rains of winter would be augmented by orographic effects, and the eastern gauges receiving a greater share of summer inland convectional and thunderstorm rain. The test is deemed to show that the site of the Kingie Camp gauge is adequate to be used as a base for this investigation.

b'=(~+zb+~) /4 (1)

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Turning to the main investigation and proceeding up glen from Kingie, the Lochan na Sgud ratio displays little seasonal variation. The small difference between the catches (on average only 4 per cent-about 3) inches) is rather surprising in view of the Meteorological Office isohyets which show a difference of 12 or 13 inches in annual average rainfall but there is little difference in alti- tude between the two gauges and no reason to doubt the ratio. The exposure here is evidently adequate.

The Allt Beithe, Glen Quoich Lodge and Kinlochhourn curves are similar to each other and indicative of slight over-exposure at these sites. The ratios are highest in the least windy spring and early summer months. After flatten- ing out from August to November each ratio falls over the winter months. This variation is due partly to loss of catch when snow is occurring and partly

3! a w 1.80 3 w u 5 V 6 0 Y

e Y 2

3

1.40

a W

1.20

I c z 0 1.00 I

b .eo

0 a t 4

I. 80

1.60

1 .40

1.20

1.00

4 0

-60

Fig. 2. Smoothed monthly mean ratios which the other Gany gauges bear to the catch at Kingie Camp

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to increased loss of rain on occasions of strong wind. It would seem reasonable to estimate the winter precipitation under these circumstances by applying the August to October average ratio to the Kingie Camp catch for those months when the direct ratio is lower.

This procedure indicates an average winter loss of catch of:

Glen Quoich Lodge . . . . . . .. 1'0 ,, Kinlochhourn .. . . . . .. .. 0.8 ,, AM Beithe .. .. .. .. . . 4-2 inches

At the remaining three sites, Kinlochquoich, Cruadach and Coire Shubh Beag, there has been a shift of the maximum ratio from summer to spring. It is suggested that this is due to the peaks triggering off convective storms travelling down in a north-west airstream, prevalent at that time of year, between Skye and the mainland. Adopting the procedure of the previous paragraphs, the following estimations of winter losses are reached :

Kinlochquoich . . .. .. . . . . 4.3 inches Cruadach .. .. .. .. . . .. 4.6 ,,

The graph at Coire Shubh Beag is so irregular that nothing could be done with it. The site is an unusual one, the gauge being perched on a rocky spur sticking out from the south side of the gleh. On its own it would be open from west through north to east, but it is protected by a crescent of much higher ground from south-east through south to north-west and by the steep north side of the glen only a short distance away. The general air flow is very turbulent in the area and often entirely contrary to that prevailing in the open.

Five years' records are insufficient to determine reliable long term averages for the gauges, but the area at the head of this glen is of special interest as it contains the wettest area of appreciable size in the British Isles, only a small area around the summit of Snowdon being wetter.

In recent years, owing to changes in top water-level caused by increased impounding of Loch Quoich to provide added storage for the hydro-electric scheme, the gauge site has been moved a short distance. Tests with an overlap gauge show there is no significant difference between the catches of the two sites. The ratios used here refer to the new site.

Rainfall at Glen Quoich Lodge has been measured since 1876.

The 196-50 averages for Glen Quoich Lodge are :

Jan. Feb. Mar. Apr. May June July Aug. 9 t . Oct. Nov. Dec. Year in. in. in. in. in. in. in. in. in. in. in. in. in.

14.25 9.91 7-60 7.14 5.15 6-39 6.92 7.68 10.36 13-31 11-72 12.84 113-27

These could be used by those interested to determine approximate means for the other gauges but the computation of means is not the purpose of this paper. Here we have tried to demonstrate a method of estimating winter precipitation, and to compute the average error involved in present methods. The winter loss of approximately 4 inches at the high-level gauges is only an average, and individual winters may differ considerably from it. .

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Accepting Hudleston’s proposition that once a gauge is buried it collects a reasonably true catch of additional snow, it would appear that these four inches represent the snow required to bury the gauge. With intermittent thaws this may represent several separate snow coverings. This conclusion should not be taken further, however, for there is no evidence to show that the gauge sites, selected to catch a representative sample of rain, are covered by a representative snow cover. If they are good rainfall sites, and thus sheltered from over much wind, they probably catch more than their share of deep snow cover.

REFERENCES HUDLESTON, F. 1934 British Rainfall 1933. p. 274 KURTYKA, J. C. 1953 Precipitation Measurements Study, Repovt of Investigation

No. 20. State of Illinois SALTER, M. de C. S. Biitish Rainfall 1920, p. 254 1921

MR. WURZELL’S WEATHER WISDOM SHORT WARNIN’, SOON PAST

After a fine morning, I was surprised to be caught without my raincoat as I walked home to lunch. I t is true the school barometer had fallen quite a lot during the morning, but I had hardly expected the weather to break so suddenly. So heavy was the rain that I was glad to duck into the hospitably open door of Mr. Wurzell’s cottage, and borrow an old coat from the shepherd.

“ Look’s as if we shan’t get any practice a t the net this evening,” I remarked. We were due for our annual match with Upper Wortley the coming Sunday, and we wanted to get in all the practice we could.

“ ’Twon’t pelt like it long,” the sage foretold. “ Fine an’ broight by ev’nin’ ’twill be, I be a-tellin’ ee.”

Not greatly heartened by this cheerful forecast, I ran home with lowered head against the driving rain. There was no sign of any improvement as I skeltered back to the school, pausing long enough to return the borrowed coat, and voice some doubt as to the evening’s prospects.

” Fine an’ cool ’twill be ‘sev’nin’,” the old man persisted. Sure enough, a blustering wind blew the rain away during the afternoon.

“ Right again, Mr. Wurzell,” I commented. “ Looked hopeless to me.” “ Short warnin’, soon past, my 01’ gran’fer uster say,” Mr. Wurzell replied,

What come in fast do be amovin’ so quick a5 it be

When we arrived at the Green for practice, the old man was there before us.

“ an’ ‘e baht far wrong ! gone a’mos’ afore it get ’ere. ’Ere an’ gone in no time ! ”

And that seems to be fairly true of any fast-moving trough. A. J. WHITEN

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