220 MONTHLY WEATHER REVIEW.
zine are bein carried on. The chan e has come about,
tion in the Meteorological Office. The cover of t, ic new
publkation gives the portraits of four pioneers of meteor- 010 , all of whom were associated with the Meteorological O&. Of these Aclniiral FitzRoy had charge of thr Office at its initiation, when i t was a branch of t,he Board of Trade, and Mr. Symons was an assistant 60 years ago, but left after a short period and devot.ed himself t,o t.he collection of rainfall returns, from which evolved later tlie British Rainfall Organization. Gens. Sabine and St.ra- chey were, successively, chairmen of the Met,eorological Office when controlled by the Royal Society. Litt,le change has been introduced into t,he st,yle and character of the publication, and it is evidently not intencleil t.o make any radical alteration. In acldition t.o the inter- esting article on “ Weather in the British Isles” fur the preceding month, which has hitherto regularly apl)enred in Symons’s Meteorolo cal Magazine, an tlrt,icle is HOW
valued by readers of the journnl.
f
through the a % sorption of the British !R ainfall Or anizu
given on “Weather d f road” which will tlouhtlcss be
DIFFUSION OF LIGHT BY RAIN, CLOUD, OR FOG.
By A. MALLOCK.
[Abstracted from P-dlngs of the Royal Society, wr. A, vol. 96, pp. 267-272.1
This aper considers the diffusion of light hy particles
length of light. If a source of light is placed within an
indefinitely estended cloud which is composed of pa-fec t,ly transparent small spheres, there is a certain distance l ~e -
yond which none of the direct rays from the sourcc is received. In this case each drop acts RS aseparnte sourci* of li ht and diffuses such light as i t receives uniformly
as the brightness of a perfectly white plane surface which could be substituted, a t the same distance, for the plane in which the perfectly transparent sphericn.1 drops ctre located; for, “since the total quantity of light passing through every spherical surface about, trhe source is cow stant, it is clear that the illumination will vciry inversely
as the square of the distance from the source.”
It is pointed out that the difficulty of seeing things through a stratum of cloud is not due to loss of clefinition, but to diffusion and the consequent lack of cont,rasts between light and shade. This is shown bv t.he perfect, ddnition of the sun through a layer of cloud which really cuts down the light to a great degree by diffusion. A table is prepared showing the effect of a shower or cloud layer in diffusing the li ht from a diste.nt, sour(:(’.
whose f inear diniensions are large +tt.ivc to t.lie wtive
in a K directions. The author defines his “proof plane”
The following teras were tabu 5 atred:
tent in volume as of the cloud being 1/100,000, which tit first was thought to be a reasonable estimate. From the results of the table and certain studies in the ratti of rainfall, it is believed that this value has been overesti-
mated and that, in the case of falling rain. the volume of water can not form anything like 1/100,000 of the totill
volume.
TABLE 1.
N.
n. ui
. urn . M7S
.w .I58 1.25
1U
2). 5
4i. 5
sil
3 1 1.WI
10. ooo
Ir fog an
APRIL, 1920
- .-
Do I
Inches.
4.6
3.65 2. i l 2.32
1. a 1.045
.1R3
.w . a75
.232
. 140 .092
.03!2
.
mist * * *. Init wen whcn ti fog is so tlense t i s to make 11 street Ian$ only just visihle nt ii clistmict. of 10 feet (thr densest whitc fog in niy cspwirnc.e) itr will he found that either thr
tlrops must haw h e n pmch less tluin a t41iousmdtrh of t i n
inch in dicinict,t.r or tlint Q [the volume of water content] niuch lcss than 11)P.” The “ proof plane” referred to :~l)ovr~ wils used in cnlculating i! in tlie table.--C’. L. M. I~i.w/.wh~~.--It iniiy.he of interest in connection with
the ;ihov.r iibstrwt to giv-c. the size of the drop tu he found in vtwious k i d s of precipitation. Further data on this question w~ pu1)laherl in the MONTHLY WEATHER
REVIEW, Octoher 1 , 1919, page 722. Adopting: the same gcnerd classification of type of prcc ipitation PS is pre- sented in that. note, the ilinnieters of the chops aver8 e :h u t tis follows: Fog, 0.01 mni.: mist, 0.1 mm.: clrizz P e,
(1.2 nini.: light rain, 0.45 mm.: moderate rain, 1.0 mm.: hettry rain, 1.5 mni.; escrssive rain, 2.1 mm.: cdoud- l)urst, 3.0 nini. to 5.0 mni. These values show the cor-
rwtiieus of the tiutlior’s wnclusion in spite of the fact that hc was not fami1i:w with tlicse wniputations.- W. J.
H t i mphrc?ys.
LIGHT SCATTERING BY AIR AND THE BLUE COLOR THE SKY. OF
By W. \V. \I’IJ~II), Professor of Esperimeiitd P l i y ~i c ~, John# Hopkina ITiiivemity.
[.4i1111111.’~ wniliinr::.] 1
1. The int.ensity of the light. scathwxl by a given thick-
ness of dust-free nir i n IL t,uhe illuminated by concentrated wiilight. has been cumpared photometrically with the light of tlie sky by.reilucing tlie intensity of the latter unt,il a match was secured. The rat,io of the two inten- sities w:is compared wit.h the calculated ratio, making certain assumptions in the ctwe of the light of the sky and a fair agreement found.
2. The intensity of t,he light scattered by dust-free air nearly in the dirertk)n of the incident light h s been ex- amined a.1~1 fouiid to he not, very different from the inten- sity scatbared in R perpendicular direction. I t is theo- retically twice as bright, but, the conditions of the experi- nient did not permit of the determination of a difference of this amount. This indicates that, the enornious. in- crease in t,he intensity of the slip close t.0 the sun’s limb (over twentyfold) results from diffraction by motes in t.he air,, and would ?)e wholly absent if the atmosphere were pei-fectJy clean.
3. The scattering power of the air near the ground on tlie clearest days in tlie country has been found to be about, 2.6 times t,he average scattering power of the atmosphke. .. _. . - .-- -
I Reprinted from the I.aindm, 15rli1il~urgh~ and 1)uhlin Phil. Mag., London, Aprll,
1Y”D. sixth swics, vol. 30, 34,. 3 2 . 11. 433.