VISIBILITY IN METEOROLOGY 95 
All telephotometers suffer from a serious contraction 
of their scales as the visual range becomes greater. It 
has also been shown by Middleton [387] that the beam 
must be made extremely narrow if very large errors are 
to be avoided in some kinds of weather. 
Telephotometers have been devised which measure 
the ratio of the luminances of a distant object and the 
horizon sky just above it, as for example that of Lohle 
[31]. Difficulties of eliminating stray light, at least in 
any simple routine of observation, limit the precision 
of such instruments. Somewhat more practical, particu- 
larly with modern photoelectric circuits, is the measure- 
ment of the apparent luminance of a nearby deep black 
box, specially constructed for the purpose. 
Scattering meters are not numerous. The “polar 
nephelometer”’ of Waldram [48, 49] which measures the 
light scattered by a sample of air in almost any direc- 
tion is an excellent example. The ‘“Loofah” [1] measures 
the scattering at 150°, which has been found to approxi- 
mate a mean value under many conditions, especially 
at sea. All such meters fail under urban conditions 
because they take no account of absorption. 
Finally there are any number of empirical instruments 
(Jones [28], Wigand [51] and others) which operate 
by the addition of “‘veiling glare” to the distant scene. 
Of these, the type most sound in theory was devised 
independently by Shallenberger and Little [42] in the 
United States, and by Waldram [50] in England. It is 
called the ‘disappearance range gauge” by Waldram, 
and presents the observer with two images of the hori- 
zon one above the other, of which the fainter may be 
made to disappear by turning a control. It is at least 
a valuable aid to visual estimation. 
In spite of the very large number of instruments which 
have been devised, the problem remains open for solu- 
tion; and perhaps it is largely an economic problem. 
The writer hesitates to predict the advent of a useful 
instrument at such a cost that meteorological services 
will give it wide distribution. 
The Visual Range in Practice 
In the absence of imstruments, meteorological ob- 
servers estimate “‘the visibility”? by observing whatever 
objects (or lights atnight) may be available around their 
stations. It is officially recommended [27] that dark- 
colored marks should be used in the daytime, and that 
they should be visible against the horizon sky whenever 
possible; also that they should be of reasonable angular 
dimensions. The Conference of Directors at Washington 
in 1947 also recommended a table [27, p. 118] which 
relates the visibility of lights to the daytime visibility 
code. This table was prepared on the basis of equations 
(10) and (12), using three different and highly arbitrary 
values of #, , “‘pending the results of further investiga- 
tion.” We shall return to this matter in a moment, but 
first we wish to refer to resolution 169 of the same con- 
ference [27, p. 220], which sets forth the ‘‘Table for 
VV (Visibility).’? This table proceeds in steps of 20 
meters from 20 to 200 m, which is probably useful; but 
then it continues in steps of 200 m up to 16 km! It is 
unfortunate that the framers of the code omitted to 
provide instructions which would tell the unhappy 
observer how he is to distinguish between a visibility 
of 15.6 km and one of 15.8 km. Among the workers in 
this field it would be difficult to find one optimist who 
would expect even the most elaborate instrument to 
perform such a task. 
In the absence of absurd requirements such as the 
above, the estimation of ‘‘visibility” in the daytime is a 
fairly satisfactory procedure if a reasonable series of 
suitable marks is available—not necessarily one for 
every distance in the code. It is otherwise at night. 
The central difficulty of night observations les in the 
unknown and variable adaptation level of the observer’s 
eye, accentuated by the fact that the observer has 
recently come from a brightly lighted room. In the 
absence of an instrument the observation is often the 
merest guess, and a suitable objective meter is greatly 
to be desired for use at night. 
We shall close with a brief remark on the treatment 
of visibility observations as climatological data. The 
usual procedure is to count the number of occurrences of 
each code number, but it has been pointed out by 
Poulter [40] and by Wright [53] that this gives an 
undue prominence to the greater visual ranges. The 
expedients adopted by these two workers to improve 
the situation are very different. Poulter multiplied each 
frequency by the reciprocal of the range of distances 
embraced by the corresponding code number; Wright 
calculated what amounts to the mean extinction coeffi- 
cient. Neither of these procedures involves much extra 
work and they should be given consideration by the 
climatologists. 
Possibilities for Progress 
It will be evident from what has been set down above 
that any further progress is likely to be made in the 
direction of experiment rather than theory. Our most 
hampering uncertainty concerns the value, or range of 
values, of the threshold of contrast actually entermg 
into meteorological observations of “visibility,” and 
some serious effort should be made to clear this mat- 
ter up. The questions of attention and search are also 
in this domain of psychophysics, and could be the 
subjects of an immense amount of work. However, they 
are of military rather than meteorological interest. 
On the physical side, the elegant researches of Des- 
sens should be extended, and the question of the origin 
of the nuclei needs further expert attention. 
There is also a need for new instruments: a simple, 
inexpensive one for use at many stations mainly, or 
even exclusively, at night; and a more elaborate tele- 
photometer or other such “visibility meter” for impor- 
tant stations like aircraft carriers and large airports. 
In order to be of any use at all, an instrument of the 
latter sort would have to be very accurate, sensitive, 
and stable. 
Finally, the applause of all meteorologists and avia- 
tors is certain to greet anyone who devises a really 
practical method of measuring the transparency of the 
atmosphere as a function of height for at least a few 
