Note on the mean relative humidity at the royal observatory, Greenwich

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194 QUAETERLY JOURNAL OF THE YETEOBOLO(I1CBL SOCIETY. through which auroral light is seen. I have on nine occasions seen an Aurora through a veil of Cirrus or Cirro-stratus, and on these occasions the pulsations of the Auroral light appeared as if proceeding from the cloud, though their source was in reality far more distant : the effect being very similar to that produced by lightning reflections rendering thin clouds temporarily luminou?. AS for actually phosphorescent clouds, the evidence for their existence 1s undeniably strong, but for my own part, after cloud-gazing nearly every night of my life I have never been favoured with the sight of one, and therefore am driven to believe that the phenomenon is, in our regions of the globe at all events, decidedly exceptional. ~ Mr. WHIPPLE thought that the hmothesis of changes in the form of cirrus cloud in the instance iuoted by Mr. Birber, could be i t once put to the proof by the examination of the records of self-registering magneticnl and electrical instruments for that date. Disturbances of either of tho elements, terrestrial magnetism or atmospheric electricity, were always indicated by these instruments. His own opinion was, that the air was much too dense at the low heights at which cirrus cloud and halo forming crystals floated, to enable the Auroral discharge to take place. H e mas also unable to think with the author, that there could be a sufficiently intense magnetic field of force generated by the action of terrestrial magnetism to alter tile refractive properties of ice crystals, when it is considered how very insignificant the Earths magnetic force at the time of its greatest intensity is, when compared with the intense forces Faraday was compelled to use in order to find out the relation between magnetism and light. An examination of the magnetograph and electrograph curves at the Kew Observatory, shows perfect absence of disturbance of either of the elements between 7 and 9 p.m. on October 23rd, 1877, and therefore it is certain that no aurora occurred on that date. Note, June 21. XI. Note on the Menn Relative Hzhmillity at the Royal Obsmatory, Greenwich. By WILW ELLIS, F.R.A.S. [Received May 9th.--Read June 19th, 1678.1 AT the time when Dr. Tripes paper On the Winter Climate of some English Sea-side Health Resorts* was before the Society for discussion, it occurred to me that it might be useful to determine the mean relative humidity at Greenwich, in each month of the year, for those hours at which the observers under the direction of the Meteorological Society make their observations, 9 a.m. and 9 p.m., and show the difference between the mean values at these hours and the true mean monthly values. Because the deviations at these hours a t Greenwich might serve to indicate, for many of the Societys observing stations, the direction in which the corresponding values a t those stations might be expected to deviate from mean values, and, approximately also, the amount of deviation. The results are contained in the following table. They are derived from the discussion of the photo- graphic records of the *-bulb and wet-bulb thermometers for the twenty years 1849 to 1868, and are deduced from mean values of the dry-bdb and wet-bulb. Complete s a t k t i o n is represented by 100. Quarterly Journal, Vol. IT., p. 111. ELLIS-MEAN R E L A T ~ HUXIDITY AT THE ROYAL OBSERVATORY, GREENWICH. 195 TABLE I.-Mean Monthly Degree of Humidity at the Royal Observatory, Greenwich, as deduced from the photographic records of the Dry-Bulb and Wet-Bulb Thermometers, for the Twenty Years 1849 to 1868. Excess above Mean 1 from 24 hour1y values Mean Monthly Degree of Eurnidity. Month. Mean o 24 hourly values. ___- 87'3 85.2 80'9 76.9 75'4 73'2 72'3 76.2 80-1 86-3 87'3 87'5 Mean if 9 a.m and 9 P.m. values. Of Mean If 9 a.m. and values. 9 P.m. +1'4 +2.5 +2.4 +ZO f 2 . 3 f 2 . I +2.7 +2.3 f 3 ' 6 +2.0 +2.2 + I % At g a.m - 88.6 88.1 8 2 4 75'9 73'0 70'2 70.0 80.0 86.3 89.2 89.0 73'6 At 9 P.m At g a m At 9 P.m - +I'4 +2.1 +3'4 +5'0 +6.9 +7.2 +7'7 +7= +7'4 +40 +Z'S +"7 .___ January .......... February ........ March .......... April .......... May ........... June ........... July ............ August .......... September ...... October .......... November ........ December ........ 88.7 87'3 8 q 3 81.9 82.3 80.4 80.0 83'4 87'5 90' 3 89.8 89'2 88.7 87'7 83.3 78.9 77'7 75'3 75'0 88.3 8 9 1 78'5 83'7 89'5 . t " 3 +z.9 +1-5 -2.4 -3.0 -2.3 -2.6 -1'0 -0.1 0'0 +1.9 f1.j Means ........ 80.; 8 5'4 8 3'0 80.7 -0'2 +47 No. for Reference.. I 2 - 4 5 6 3 Column 5 shows that at Greenwich the 9 a.m. value is smaller than the mean in summer, and larger than the mean in winter : column 6 shows that the 9 p.m. value is larger than the mean throughout the year, but much larger than the mean in summer : column 7 indicates that the mean of the 9 a.m. and 9 p.m. values deviates from the true mean by nearly the same amount in each month of the year, being always too large. The mean monthly values (column 4) change little from April to August, and from October to February ; there is a great decrease between February and April, and a corresponding great increase between Bugusband October, the months of March and September having intermediate and nearly equal values. I t is not proposed that the differences for Greenwich should be generally applied as corrections to determinations made at other places at 9 a.m. and 9 p.m., for the corrections in any month would probably vary with different values of humidity. But the differences indicate, as I haye already said, in what way mean values at these hours at other stations may on the average be expected to deviate from true mean values. As a matter of fact I have examined, though not with any great exactness, the monthly humidity results dsduced from the observations made at the Society'g stations in 1876 and 1876, and find that the differences between the 9 a.m. and 9 p.m. results, in very many cases, agree nearly with those of the preceding table. The values themselves, at many of the stations, appear, however, to be generally greater in these years than the corresponding Greenwich values. 196 QUARTERLY JOUBNAL OF THE METEOBOLOOIaAL S3CIETY. It may be useful to add for Greenwich the greatest and Ieaat monthly values of humidity, found for each of the twelve months since observations of the dry-bulb and wet-bulb thermometers have been made. For this purpose I have employed the longer series of results deduced from eye observations of the dry-bulb and wet-bulb thermometers taken several times each day, and corrected for diurnal inequaIity in the usual way. This series commences with the year 1841, but as the thermometer stand was, in theyear 1846, shifted from the north to the south side of the Magnetical and Meteorological Observatory, the results for the' thirty years 1847 to 1876 only have been used. The numbers, as extracted from the volumes of Greenwich Observations, are contained in the following table, the mean monthly values for the whole period being added for comparison with those given in Table I. TABLE 11.-Greatest, Least, and Mean Nonthly Degree of Humidity at the Royal Observatory, Greenwich, 88 deduced from eye observations, 1847 to 1876. I Month. ---- I Greatest I Xonthly I Value. 79 76 74 69 2 63 65 72 78 81 78 .. 86'3 8 3'9 80.9 78.0 7 5'3 74-2 74-1 75'2 792 86.3 87'4 87.6 80'7 January ................ February .............. March ................ April .................. May .................. June ................. July .................. August ............. September ............ October ................ November ................ December .............. Mean ............ 92 91 90 86 82 85 8 3 2 93 94 92 -- .. I ---- I The numbers in the preceding table show that the monthly values of humidity at Greenwich vary considerably in amount in every month of the year. The mean values generally closely agree with the mean values derived fiom the photographic records given in Table I. The general mean 80.7 is precisely the same. The dry months in summer appear to be warm months, and the humid months cold, but in winter there seems to be no marked distinction, as regards temperature, between dry and humid months. There is one point in the determination of relative humidity from observa- tions of the dry-bulb and wet-bulb thermometers that should be borne in mind, and that is the effect of small errors in the index corrections. Should the corrections applied be so far uncertain as that, by m combination of errors, the difference between the dry-bulb and wet-bulb thermometers could be erroneous by as much as 0'4, this, at a temperature of 50, may make a difference of as much as 3 in the deduced humidity, more at lower temperatures, but rather less at higher temperatures. ELLIS-MEAN RELATIVE HUMIDITY AT THE ROYAL OBSERVATORY, GREENWICH. 197 DISCUSSION. Mr. DINES said he was glad to corroborate the figures given by Mr. Ellis in his paper. At the time Dr. Tripe's paper was under discussion he had taken out all the returns he could find relating to Greenwich, and on plotting them together with those given in the paper, they agreed SO nearly that they might be considered as one curve ; as far therefore as the figures went they might be taken as data. The only part of Meteorology he proposed to study was that con- nected with vapour, and he frequently took the dew poiut direct, twenty, or even fifty, times a day. In his opinion this part of the question was in the most un- satisfactory condition, and as the Fellows would not meet again for some time, he was glad to have the opportunity of stating his reasons for coming to that conclusion. Six years ago 0 he had remarked lL that whatever tables may be used, the diy and wet bulbs could never be depended upon as giving more than an approximation to the dew point." H e repeated that statemeut to-night, not with doubt or hesitation, but in the strongest language he knew how to use. Again, the figures in Mr. Ellis's tables were worked out from an open stand, and Mr. Cflaisher's tables the same, whereas the Society used a closed stand ; his opinion as to the difference caused by these stands wa3 well known, and he would not repeat i t ; but the discrepancies due to other causes were so serious, that in discussing the question as to whether the stands made a difference of 1" or 2 O , they appeared to him to be straining at a gnat vhile they were at the same time swallowing a camel. He had two sets of dry and wet bulb thermometers precisely similar, by the same maker (Casella). On Monday last these were carefully mounted, and placed side by side in the different stands (4 ft. above the ground). In calculating the dew point from the two stands, they only differed 1O.7, but the difference from the dew points as taken by his hygrometer was 5 O . 9 . Then as to the degree of Humidity (saturation = 100) the two stands differed only 47 per cent., but if compared with that given by the dew point taken direct the difference was 14.3 per cent. On the day he had named more than ordinary care had been used, but it must not be supposed he had given them an exceptional case ; he had many other days like i t in his books, and only the previous day, after a heavy shower, with a difference of 5" between the dry and wet bulbs, the wet bulb was of the same tem erature as the dew point. He would now give an instance of what he callex a good day, when all worked smoothly together :-On June l l th , 1877, with a difference of 12" between the dry and wet bulbs, the dew points only differed OO.2, and the Humidity only 1.4 per cent. Supposing the means for determining the dew point correctly at hand, he was sorry to say their difficulties were not over; on some days it was almost impossible to say what the dew point reall was ; every LIE of wind seemed to come charged with a different quantity oTmoisture, a n f h e had frequently seen the dew come and go on his hygrometer, like a flash of lightning, 5O or 6 O above what would generally be taken as the dew point. Again as to elevation, generally speaking the dew point was lo to 2" higher near the ground than a t 4 ft. above it, and at 50 fi. high it was from 2" to 6" lower than near the ground. After these remarks he need scarcely say that he agreed with their late President (Mr. Baton) in doubting the advisability of separating the tension of d q air &om that of vapour, and in giving the latter to three places of decimals, the figures of which really meant that there was sufficient vapour above us to cover the ground with water to a. depth of 5 inches, whereas he thought (and he believed he was not alone in his opinion) that one-third or one-half of that amount would be nearer the truth ; but perhaps it was best to say they did not know much about it. H e had forgotten to mention that on one evening in the last month there was a difference of 33 per cent. in the degrees of Humidity a t 4 ft. and at 50 ft. above the ground ; and on calculating the tension of vapour at the two elevations, and taking the difference, he was called upon to believe that within 50 ft. of the earth there was vapour enough to cover the earth with water to the depth of an inch, enough to saturate the air one hundred times over ; t o those who believed in the correctness of the method now adopted, there was no escape from this dilemma. H e feared the whole thing was rotten to the core. British Association Report for 1872, p. 59. 198 QTiaBTEaLY JOURNAL OF THE METEOROLOGICAL SOCIETY. Dr. TRIPE agreed with Mr. Dines that the indications of the dry and met bulb thermometers should not be too greatly relied upon, and for this reason the Society does not now work out the hygrometric deductions for every day from the observations at their stations, but calculates them from the monthly means. As the have the daily observations, they can in after ears have the observations Mr. STRACHAN said that there was one thing that might be said in favour of the hygrometrical results obtained by the Tables at present in use, and that was that there was a marked agreement between the results for different adjacent stations. Mr. Syafozrs thought that the results of the dry and met bulb thermometers were very doubtful. Mr. ELLIS remarked, through the Secretary, that it might be mentioned that, although the factors used in determining the dew-point from observations of the dry and wet bulb thermometers will not, in every individual case, give the same dew- point as direct hygrometrical observation, they appear fairly to do so on the average ; indeed, the factors are based on comparisons of the dry and wet bulb thermo- meter indications with direct observations. Whatever, therefore, may be its deficiencies, the wet-bulb thermometer, being in nearly universal use, should not be hastily discarded, at least until some instrument as ready and efficient, and as likely to be as generally used, is devised. The instrument gives us useful information which we do not get, and which, a t present, we are not likely to get, in any other way. worke CT out correctly, when more accurate tables can ge formed. XII. On a Method of sometimes determining the amount of the Diurnal By the Hon. Pariatwn of the Barometer on any particular day. ~ P H A.BEBCBOMBY, F.M.S. peceived April 24th.-Read June 19th, 1878.1 THE method which has hitherto been universally adopted, in treating of the diurnal variations of the barometer, has been to take the mean vdue of a large number of observations at each hour of the day, and to draw deduc- tions from the average amount of variation thus determined. In a former paper, the author has shown that the dinrnal barometer variation is seen, from the inspection of barograms, to be always as it were superimposed on the larger fluctuations of pressure due to the passage of cyclones, &c. Also, that the actual amount differs from day to day, and that it is from observation of the kind of weather which is associated with large or small variation that the cause of the variations will probably be discovered. Hence the desirability of being able to measure, as in the barographic trace for July 2nd, 1877, shown in fig. 1, the actual amount of variation on any particular day. When the non-diurnal fluctnations of the barometer are curved, it is manifestly impossible to separate them from the diurnal variation, but when, as is often the case, the generd motion of the barometer, as seen on a barogram, seems to be a straight line, either rising or falling or steady for ~ my 24 consecutive hours, as in the barographic trace for July 2nd, 1877,* shown in fig. 1, the amount of diurnal variation which overrides it can be determined very approximately b J the following considerations. * this fig. the lettris M1 M2, X1 X2, refer to the 1st and 2nd minima and maxima respeotioely .