July, 1913. 



KNOWLEDGE. 



269 



margins, especially at the north-east corner facing the prevalent 

 wind. The desert appearance of the Long Valley is shown in 

 a series of striking photographs which illustrate the disastrous 

 and desolating effects of the removal of vegetation from an 

 area the climatic conditions of which do not differ from those 

 of the surrounding districts. 



METEOROLOGY. 



By William Marriott, F.R.Met.Soc. 



WINDS IN THE FREE AIR.— At one of the Friday 

 evening discourses at the Royal Institution Mr. C. J. P. Cave 

 dealt with the subject of the " Winds in the Free Air." He 

 has for some years devoted a considerable amount of time 

 and work in carrying on observations in the upper air, first by 

 means of kites and subsequently by small rubber balloons, the 

 movements of the latter being followed by a specially 

 designed theodolite. The lower layers of the atmosphere up 

 to one or two kilometres are the most important to aviators. 

 To meteorologists the higher layers offer problems of greater 

 interest. In considering the winds in the free air it is con- 

 venient to have some datum to which to refer them, and this 

 is known as the gradient wind. Mr. Cave divides the wind 

 structures into five types. In the first three the wind 

 increases above the surface and equals the gradient velocity 

 at a height of half a kilometre or so; above this, in the first 

 class, the wind remains more or less equal to the gradient 

 velocity up to a height of seven or eight kilometres ; in the 

 second class the wind in the upper air greatly exceeds the 

 gradient wind ; and in the third class it falls off again to a 

 lesser value; but in all three classes the direction remains 

 much the same as that of the gradient wind. There are often 

 cases of reversals when the wind in the upper air is very 

 different in direction from that near the surface, and when it 

 bears no relation to the surface-pressure distribution. In 

 cases of reversal it is found that the warm wind flows over the 

 top of the one that comes from a colder region ; there must 

 somewhere be a line where the warm current is rising, where 

 it must be cooled dynamically, and where its moisture may 

 condense into cloud or rain. It is interesting to note that in 

 most cases rain occurs somewhere in the region of the reversal, 

 and in summer thunderstorms are frequent. The last type of 

 wind structure considered was the outflow that seems to take 

 place from the upper layers over a low-pressure system, 

 causing west to north winds in the upper air on the east and 

 south sides of the depression. 



Following on inquiries made by Mr. W. H. Dines on the 

 correlation between the surface pressure and various 

 meteorological elements at a height of nine kilometres, Dr. 

 W. N. Shaw has suggested that the changes of pressure to 

 which our changes of weather are due have their origin, not 

 near the surface of the earth, as hitherto supposed by many 

 meteorologists, but just below the level of the stratosphere at 

 a height of nine kilometres or so above the surface. Mr. Cave 

 says that this view is in accordance with the observed facts of 

 the wind distribution in the different layers of the atmosphere. 



HOURLY OBSERVATIONS OF CLOUD FORMS.— 

 Mr. Spencer C. Russell, who has always been a keen observer 

 of clouds and thunderstorms, gave, at the May Meeting of the 

 Royal Meteorological Society, the results of monthly and hourly 

 cloud-form frequencies at Epsom for the eight years 1903- 

 1910. He had observed during this period, almost single- 

 handed, the amount of cloud and also the various forms of 

 cloud visible at each hour, day and night. This was truly a 

 most remarkable feat of endurance, but it is a valuable 

 addition to this little-studied branch of meteorological science. 

 He arranged the fifteen varieties of clouds into four groups, 

 viz. (1) upper, (2) intermediate, (3) lower, and (4) clouds of 

 diurnal ascending currents. The upper clouds, which include 

 the Cirrus, Cirro-stratus, and Cirro-macula, show a marked 

 prevalence during the summer with minima during the winter. 

 Morning and evening maxima, with a midday decline, are 

 common to all these varieties. The intermediate clouds, 

 which include Cirro-cumulus, Alto-stratus, Alto-cumulus, 



and Cumulo-stratus, are also more prevalent in the summer 

 than in the winter. The lower forms, which include Strato- 

 cumulus, Nimbus, Fracto-nimbus, Fracto-cumulus, Stratos, 

 and Fog, attain their maxima in the winter months, their 

 minimum frequency being in the summer. The clouds of 

 diurnal ascending currents, Cumulus and Cumulo-nimbus, are 

 independent of any seasonal variation in hourly frequency, 

 the maxima, at noon and 3 p.m. respectively, taking place at 

 these hours in every month of the year. 



The total number of individual records made by Mr. Russell 

 approximate close upon 100,000. The Cumulus cloud yielded 

 the greatest number of daily values (1,622), the Stratus coming 

 next (1,155), but the Stratus had the greatest number of 

 hourly records (13,497), the Cumulus being next (11,414). 



METEOROLOGY AND MAGNETISM AT THE 

 ROYAL OBSERVATORY, GREEN WICH.— In his Report to 

 the Board of Visitors of the Royal Observatory at the Annual 

 Visitation on June 7th the Astronomer-Royal gave particulars 

 respecting the meteorological and magnetic work carried on 

 at Greenwich. The chief results for the twelve months ended 

 April 30th, 1913, were: The highest temperature in the shade 

 (recorded on the open stand) was 90°-0 on July 12th. On 

 twelve days the highest temperature equalled or exceeded 

 80° -0, but none of these days occurred after July, maximum 

 readings of 70° and upwards occurring only seven times in 

 August, and not at all after. The lowest temperature during 

 the same period was 24° -2 on February 23rd. There were 

 twenty-seven days during the winter on which the temperature 

 fell as low as 32°, or less than half the average. The mean 

 temperature for the twelve months was 49°- 8. The mean 

 daily horizontal movement of the air was three hundred and 

 ten miles, which is twenty-six miles above the average of the 

 previous forty-five years. The greatest recorded daily move- 

 ment was eight hundred and forty-five miles on March 19th, 

 and the least fifty-six miles on October 11th. The total rain- 

 fall was 25-61 inches, or 1-49 inches greater than the average. 

 The number of hours of bright sunshine was one thousand 

 three hundred and thirteen out of a possible four thousand 

 four hundred and fifty-seven hours, giving a mean proportion 

 of 0-295, constant sunshine being represented by one. As 

 compared with the previous twelvemonth the deficiency of 

 .more than five hundred hours is fully accounted for by the 

 fact that in July, August, September, and April the duration of 

 sunshine was only about half what it was in the corresponding 

 months. 



The magnetic elements for 1912 determined from obser- 

 vations in the Magnetic Pavilion are as follows : — 



Mean declination 15° 24'-3 West 



Mean horizontal force ... 0-18528 (in C.G.S. units) 

 Mean dip 66° 51' 46" 



The Astronomer- Royal stated tnat in the new magnetic 

 observatory shortly to be erected provision is made for the 

 continuation of the long series of Greenwich observations of 

 the variations of the magnetic elements. This series is 

 unique as regards the length of time during which observations 

 have been made on the same site. The care which has been 

 taken to guard the observatory from all artificial electro- 

 magnetic disturbances which could affect the accuracy of the 

 observations has preserved the suitability of the site for such 

 work. The present wooden structure, however, is old, and 

 needs extensive repairs or renewal. The latter course was 

 considered preferable, and designs have been prepared for a 

 new building in the Magnetic Pavilion enclosure in the Park, 

 where the absolute observations are made. The main feature 

 of the building is the provision by insulation and heating for 

 the reduction of the daily range of temperature. Also, to 

 avoid damp, it will be above ground. The building will 

 therefore be adapted to house a set of modern instruments. 

 Although the change of site is small, the new and the old 

 instruments will be run concurrently for a period sufficient to 

 give a good comparison between the two. 



THE ATMOSPHERIC TURBIDITY OF 1912.— The 

 great deficiency of bright sunshine referred to above was not 

 confined to Greenwich only, and it was no doubt connected with 



