January, 1915. 



KNOWLEDCxE. 



21 



Sussex, in his books on the CHmate of Uckfield and of 

 Crowborough (1898) succinctly described the meteorological 

 character of the several months of the year. With regard 

 to January he said that " previous observations stamp this 

 month as the coldest month of the year, and in severe 

 winters, when there has been little or no frost in the previous 

 December, the cold usually commences during the first or 

 second week, accompanied by a keen north-east wind. 

 Should a change not occur in the course of two or three days, 

 we are nearly certain of ha\'ing a fall of snow and a con- 

 tinuance of the frost. In our very uncertain climate 

 we occasionally find that great mildness prevails during the 

 month. This was especially the case in the years 1846, 

 1851, 1853, 1866, 1877, 1884, and 1890. In 1846 and 

 1866 the weather was often too warm to bear fires with 

 comfort. Very strong south-westerly winds are frequent 

 in such a season, which increase to a hurricane if long con- 

 tinued, and occasion great depression of the barometer. 

 On the other hand, we had very cold weather in the years 

 1850, 1861, 1867, 1871, 1881, 18'9], 1893, 1895, and 1897." 



It may no doubt be of some interest to have a few facts 

 about the weather of the month of January, so the following 

 particulars, which are based on the Greenwich records, may 

 be of some service ; but it must be understood that they 

 are not intended to be taken as a forecast of the weather 

 for the present month. 



The average mean temperature for January is 38° -6, 

 in 1884 it was as high as 43° -9, while in 1881 it was as low 

 as 31°-7. The average maximum temperature is 43°-l ; 

 the highest mean was 48° -5 in 1890, and the lowest 35° -1 

 in 1879. The average minimum temperature is 33° -7 : 

 the highest mean was 39° -4 in 1846, and the lowest 27° -3 

 in 1881. The absolute highest temperature recorded during 

 the period was 57° -0, in 1843, on the 28th, and the absolute 

 lowest 16°-7, in 1841, on the 8th. The average number of 

 days on which the temperature falls to or below the freezing- 

 point is twelve ; and there are on the average two days 

 during which the temperature remains throughout the day 

 continuously below the freezing-point. 



The average rainfall for the month of January is 1 -80 

 inches; the greatest amount was 4-35 inches in 1877, 

 and the least 0-26 inch in 1880. The heaviest fall in one 

 day was 1-61 inches in 1866, on the 11th. The average 

 number of " rain days " {i.e., on which 0-01 inch fell) is 

 14-8, the greatest number of days was twenty- three in 

 1877, while the least number was five in 1858. Snow falls 

 on the average on four days. 



Barometric records for London go back to the year 

 1774, and from these we find that the mean pressure for 

 January is 29-965 inches : the highest mean was 30-387 

 inches in 1779, and the lowest mean was 29-581 inches in 

 1800. Barometric pressure is often subject to great 

 variation, and the extreme readings for the year have 

 occurred in the month of January'. The highest recorded 

 reading in the British Isles was 31-110 inches, at Aberdeen 

 in 1902, on the 31st, and the lowest recorded reading was 

 27-332 inches, at Ochtertyre, Crieff, in 1884, on the 26th. 



" As the day lengthens. 

 So the cold strengthens." 



"If the grass grow in Janiveer, 

 It grows the worse for it aU the year." 



VISIBILITY AS A SIGN OF RAIN.— Letters from 

 several correspondents have appeared in Symons's Meteoro- 

 logical Magazine on the subject of the haziness of the 

 atmosphere with certain u-inds, and also of remarkable 

 clearness as a sign of rain. With regard to \'isibility as 

 a sign of rain, it may be pointed out that this matter was 

 dealt with by the Honourable Ralph Abercromby and 

 W. Jlarriott in their joint paper on " Popular Weather 

 Prognostics " (1882). They showed that in the wedge- 

 shaped area of high Isaronietric pressure, which is frequently 



formed between a retreating and an advancing depression, 

 there is often great \'isibility with a cloudless sky, 

 which occurs during the very fine weather on the east side 

 of the wedge-shaped area. They quoted a number of 

 weather sayings, or prognostics for rain, which they con- 

 sidered were to be explained by this condition of barometric 

 pressure. The reason of " visibility " is uncertain ; the 

 old idea, that it was due to excess of vapour, is no doubt 

 erroneous. The dr^' and wet bulb hygrometer always 

 indicates -a considerable amount of dr^mess when it is 

 remarked. 



There is another distribution of barometric pressure \\-ith 

 which \-isibility as a prognostic of rain occurs, and that 

 Abercromby and Marriott called " straight isobars." 

 Although there is little rain actually with these, the area 

 which they cover to-day will probably be covered by 

 a depression to-morrow, the conditions being favourable 

 for the passage of depressions. With this type there is 

 often great \isibility with a hard overcast sky and moder- 

 ately dry air, in which the stratus seems to play the part 

 of a sunshade, for as soon as the Sun comes out the clearness 

 of distant objects diminishes. This visibility should not 

 be confounded with the visibility with a cloudless sky, 

 which occurs with wedge-shaped isobars. 



VERIFICATION OF THERIMOMETERS.— The testing 

 of meteorological instruments, which was for so many 

 years carried out at the Kew Observatory, has been trans- 

 ferred to the National Physical Laboratory at Teddington. 

 New apparatus has been installed for the testing of thermo- 

 meters, and this has led to a great gain in the rapidity, 

 accuracy, and convenience of testing. The new equipment 

 consists of a water-bath capable of taking vertically seventy- 

 two thermometers, a steam bath for six thermometers, 

 two ice-baths, a horizontal bath, and a low-temperature 

 oil-bath, which latter is in the experimental stage. This 

 oil-bath was designed in \iew of the increasing amount of 

 low- temperature work. It consists of two adjacent compart- 

 ments, the intervening partition being pierced with holes 

 top and bottom. One compartment contains a cooling 

 worm and a propeller stirrer ; the other contains the 

 thermometers under test. The readings are observed 

 through a double window, the device being necessary to 

 prevent condensation of dew on the glass. The cooling 

 worm is connected through a small rotarj' cog pump with 

 a second worm, immersed in a freezing mixture of ice and 

 salt. The circulating liquid is paraffin oil. The design 

 permits temperatures as low as about 7° F. in the testing 

 bath. The liquid chosen for the bath is ordinary paraffin 

 oil. Odourless paraffin oil was first tried, but it exhibited 

 a cumulative increase in \iscosity with use, which effectually 

 prevented its application to the purpose. Already about 

 a thousand thermometers have been tested in this bath, 

 and, with the experience gained, it is proposed to draw up 

 designs for a new- bath to take its place among the permanent 

 equipment. The total number of meteorological and other 

 thermometers tested during the year 1913-14 was eight 

 thousand and sixteen. 



ATMOSPHERIC POTENTIAL GRADIENT.— Mr. 

 W. A. D. Rudge, in a paper to the Royal Society on " Some 

 Sources of Disturbance of the Normal Atmospheric Potential 

 Gradient," states that, besides rain, hail, and snow, there are 

 other factors, such as dust and steam, which may produce 

 sudden variations of the atmospheric potential gradient. 

 From numerous experiments which he has carried out in 

 various places he finds that the presence of dust in the 

 atmosphere has the effect of altering the potential gradient, 

 sometimes increasing, sometimes diminishing, the positive 

 value. The dust from well-used old roads practically always 

 increases the positive potential. Manufacturing operations, 

 w-hich result in the production of dust, disturb locally the 

 potential gradient. The presence of clouds of steam in- 

 creases the positi\'e potential, and the effect persists for 

 some time after the steam has condensed. 



