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VOL. III, PP. 41-52, MAY 1, 1891

THE NATIONAL GEOGRAPHIC MAGAZINE




GEOGRAPHY OF THE AIR

ANNUAL REPORT BY VICE-PRESIDENT

A. W. GREELY




WASHINGTON

PUBLISHED BY THE NATIONAL GEOGRAPHIC SOCIETY

Price 25 Cents.


{41}


VOL. III, PP. 41-52, MAY 1, 1891

THE NATIONAL GEOGRAPHIC MAGAZINE




GEOGRAPHY OF THE AIR.

ANNUAL REPORT BY VICE-PRESIDENT

A. W. GREELY.

(_Presented to the Society January 23, 1891._)


In fulfilling the duties growing out of his official position in
connection with this Society, your Vice-President of the Geography of
the Air has been so closely occupied with executive and other official
duties devolving upon him as to preclude his giving that amount of
time and labor to this annual report that the subject merits. Indeed,
no report would be submitted this year had it not seemed better to
insure a continuity of these annual addresses, even if one of them
might not be up to the high standard which should be maintained for
them.

It must have impressed every general reader of scientific journals
that the past year has been marked by the publication of an unusual
number of controversial articles relating entirely or in part to
meteorology. Some of the discussions of this subject appear to be in
the nature of speculation, which, by good authority, is defined to be
"chiefly the work of the imagination, and has little to do with
realities." The status of the meteorological discussion which has been
going on for some time seems to be this: A number of men, applying
themselves to investigation in separate branches or stages of the same
science, are attempting to reconcile their views, which, based as they
are upon entirely different processes of investigation, are not
entirely accordant. Some, at least, of these writers are still
apparently groping in the preliminary, the "natural history" stage of
the {42} science of meteorology, while one alone stands as the
exponent of the "natural philosophy" of meteorology.

To me it seems that it could not have failed to impress any interested
reader who has followed the late publications on the convectional
theory that, in order to clear the ground for definite meteorological
discussion, it is necessary to determine the exact meaning of the
various technical terms employed by the various writers. Whether from
looseness of verbiage originally or from the not infrequent habit of
disputants when worsted to change their ground by claiming to be
misunderstood, we find that some writers are unwilling either to stand
by their first criticisms or to openly abandon them; they prefer to
explain away their defective statements and gradually shift around to
positions almost diametrically opposed to those originally assumed.

The generally accepted theory as to cyclones attributes their
initiatory formation to an unequal distribution of temperature with
resulting mean diminution of pressure, and the movement of the air
from places of high to places of low pressure, the lower air ascending
with a gyratory motion, while air particles moving from opposite
directions form couples which produce rotation. When energetic motions
raise the ascending air to such a height that the temperature, cooled
dynamically in ascending, goes below the dew-point, then the great
store of latent heat thereby set free becomes, it is assumed, the main
source of energy in maintaining the upward convectional movement. The
subsidiary causes are attributed to the diminution of pressure on the
collapse of the vapor, and also to the direct absorption of the sun's
heat at the upper cloud surface.

In anticyclones a slow gyratory descending motion of the air is
assumed. Ferrel considers the cyclone and anticyclone one system, and
believes that air flowing into the cyclone from a "high" at the ground
passes out into the higher atmospheric strata.

Dr. Hann has put forth the hypothesis that the genesis of cyclones and
anticyclones may be sought in the general atmospheric circulation
through a difference of temperature of the air from the equator to the
poles. He speaks of a congestion in the upper or anti-trade winds,
where the air heaps up to a great height, this being the cause of the
anticyclones; and he maintains that the low temperature of the "high"
is due to ground radiation, and that no part of the high pressure is
the result of low temperature.

{43} To this hypothesis of Dr. Hann, ascribing the genesis of storms
to the general circulation of the atmosphere, no application of the
laws of dynamics has yet been made with a view of developing it into
an acceptable "theory." If it should be established it does not follow
that it will in any way affect the truth of the commonly accepted
"convectional system," which, founded as it is on the well-known laws
of thermo-dynamics, is not likely to be successfully assailed. There
may be an improved nomenclature for the laws of statics and dynamics
that will express to the mind more clearly the relation of cause and
effect; but until the advance of scientific research modifies the
present formulation of these laws the convectional theory will be
generally accepted as giving the true interpretation of all the
phenomena to which it could be applied.

Professor Russell, in commenting on this subject, expresses the
opinion that the low temperature is due to the convective interchange
of air at a low temperature in the upper strata with air of a high
temperature in lower strata, such convective interchange tending to
make the whole body of air of a temperature coinciding throughout with
the adiabatic rate of upward diminution, with the consequent result of
rendering the air at the surface of the earth cooler than previously
and the upper air warmer. When the upward diminution of temperature is
less than the adiabatic rate, in the forced circulation of air
crossing a mountain ridge, there occurs the dynamic heating which is
observed in the case of the foehn winds. The low temperature near the
earth he does not believe could ever be entirely produced by nocturnal
radiation from the ground. The high pressure, in his opinion, is
largely the result of greater density due to low temperature, as is
very clearly indicated by the fact that the temperature is almost
inversely proportional to the pressure, and that the places of lower
temperature substantially coincide with the places of greatest
pressure.

In advancing hypotheses and inviting discussion the real object is, or
at least should be, to discover the essential cause or causes which
determine the initial formation and subsequent maintenance and
progress of the cyclone. Some real progress in charting lines of equal
density seems to have been made by M. Nils Ekholm following Professor
Abbe's system of "isostaths," one using the term density, the other
buoyancy. Professor Abbe also introduces the factor of the orographic
gradient, but the {44} latter is simply the measure of a resistance.
The objection to this form of determination is this, that it is a
measure of mass only. The density of two masses of air is determined
to be the same; but as the density may result from two entirely
different causes, their physical relations cannot be fully expressed
in units of gravity. The methods of Professor Abbe and of M. Nils
Ekholm undoubtedly give good results, partly from the coincidence that
humidity usually varies directly as the temperature.

The method proposed by Captain James Allen in 1888, which is briefly
described in appendix 24 to the annual report of the Chief Signal
Officer for 1890, appears to afford the means of more clearly
expressing the relations that exist between the mass of the atmosphere
and the forces available for the generation and movement of storms.
Its tentative application at the Signal Office has anticipated and
explained storm movements not indicated or accounted for by the usual
methods.

As pertinent to this matter, there is instanced a study of the
progress of thunder-storms made by Berg, who observes that the line of
storm front in every case investigated made a decidedly conspicuous
bend into the densest part of the lines representing the absolute
humidity.

       *       *       *       *       *

Scientific conditions have so changed that in these later years it
becomes more and more difficult for investigators to publish any work
which may be characterized as _magnum opus_. Under this head, however,
must be classed Buchan's important memoir on the distribution of
atmospheric pressure, temperature, and wind direction over the whole
world; a large quarto volume, which contains much new material. It has
been incorporated with the results of observations during the
Challenger expedition, in which series this work appears. The isobars
and isotherms for each month in the year for the whole earth are
charted on Mercator's projection, and for the northern hemisphere on a
chart constructed on a polar projection. In connection with an
abstruse subject, to which Buchan has paid so much attention, the
diurnal variation of pressure, he opines from the Challenger
observations that the oscillations are due to the heat taken from the
solar rays directly in passing through the air and instantaneously
communicated through the whole mass from top to bottom by heating and
evaporation of water on innumerable dust particles. The afternoon
minimum, he thinks, is caused by upward currents removing a portion of
the lower air. Marked {45} differences exist between the continental
and insular types, since on islands the morning minimum is unusually
large and the afternoon minimum so small as to disappear, while in
continental types the reverse conditions obtain.

       *       *       *       *       *

Werner Von Siemens, in answering Sprung's criticism on his general air
currents, after repelling certain statements of Sprung, describes his
own theories, which are worthy of restating:

1. All winds are caused by the disturbances of indifferent
equilibrium, and the motion of the air is to restore equilibrium.

2. These disturbances are caused through overheating of the layers of
air near the surface of the earth by insolation, through unsymmetrical
cooling of the higher layers by radiation, and through the heaping up
of air masses caused by obstructions.

3. The disturbances are adjusted by ascending currents, wherein the
particular species of acceleration occurs in which the increase of
velocity is proportioned to the diminution of pressure.

4. The upward currents correspond to equally great descending currents
in which there is a decrease of velocity corresponding to the
acceleration in the upward velocity.

5. If the region of overheating of the air is limited locally, a local
upward current reaching to the highest layers of air arises, and
whirlwinds appear with interior spirally ascending currents and
outside similar spiral descending currents. The result of this is
dispersion of the superfluous heat of the lower air by which the
adiabatic equilibrium is disturbed throughout the whole column of air
taking part in the whirling motion.

6. In case the region of disturbance of the indifferent (or adiabatic)
equilibrium is very extensive, as, for example, the whole of the
tropical zone, the temperature adjustment can no longer be
accomplished by locally ascending whirls, and a whirling current must
then arise involving the whole atmosphere. The same conditions apply
to these as to the local whirls of accelerated upward motion and
retarded descent in such a manner that the velocity at different
altitudes arising from heat converted to work is approximately
proportional to the prevailing pressure at the place.

7. In consequence of the meridional motion produced and maintained by
conversion of heat into work, the whole atmosphere in every latitude
must rotate with approximately the same absolute velocity. Thus the
meridional currents produced by overheating combine with the currents
embracing the whole {46} wind system of the earth, with the result of
disseminating the excess of temperature and humidity of the torrid
zone over the temperate and arctic zones, thereby producing the
prevailing winds.

8. This is accomplished by the production of alternating local
depressions and elevations of barometric pressure by the disturbance
of indifferent equilibrium in the upper layers of the air.

9. "Highs" and "lows" are a consequence of the temperatures and
velocities of the upper currents.

Whence it follows that the most important problem of meteorology is
the investigation of the causes and consequences of the disturbance of
indifferent equilibrium of the atmosphere, and the weightiest problem
in weather prediction is the investigation of the geographical origin
or extraction of air currents pursuing their course above us toward
the pole.

       *       *       *       *       *

In Pomortsew's treatise on synoptic meteorology, published in Russia,
there are full chapters on prediction of weather, whether from
synoptic charts, from observations at a single place, or from
prognostics of great length based on researches on the succession of
warm and cold months. It also contains Pomortsew's investigations on
the types of pressure distribution in eastern Europe, as well as the
average path of cyclones.

       *       *       *       *       *

The favorable opportunities afforded by the Eiffel tower have been
utilized by French meteorologists. M. Angot states that during the
anti-cyclone of November, 1889, the temperature on the tower was
several degrees higher than below. The change of weather set in
earlier, with a strong and warm wind, on the tower, while the air at
the ground was cold and calm. Wind observations on the tower show a
ratio of 3.1 at that height (303 meters) to the velocity at a height
of 21 meters, as determined from 101 days' observations, which,
remarkable at such a small height, discloses the peculiarity of high
mountain stations.

       *       *       *       *       *

Partsch, writing on evidence of climatic changes within historical
times in the Mediterranean region, remarks that too much attention has
been given to changes in crops, the introduction of plants, and the
limits of domestic animals. He states that existing information as to
the harvest time of ancient days indicates an unchanged climate, while
the land-locked lakes in Tunis, which afford the best evidence on
rainfall variation, show absolutely no climatic change.

       *       *       *       *       *

{47} Van Bebber, in writing on weather types, claims that a line drawn
from the center of a cyclone perpendicularly in the direction of the
heaviest gradients will in general be perpendicular to the subsequent
path of the "low," and that these lows leave high temperature on the
right hand.

       *       *       *       *       *

Hill, in describing hail-stones and tornadoes in India, explains them
on the principle of the great diminution of temperature upwards in the
air, but a critic, in combating this theory, objects to the high and
low stations selected to show temperatures.

       *       *       *       *       *

The so-called "weather plant" of the tropics has passed through the
process of investigation with the usual result. It appears surprising
that in these days it should be believed that any plant or animal can
foretell weather 48 hours in advance, particularly after considering
the vast amount of proof as to the enormous rapidity with which
weather-changes progress from day to day.

       *       *       *       *       *

Hugo Meyer, in treating the precipitation of central Germany for the
ten years ending in 1885, pertinently remarks that the same
significance does not attach to the same rainfall for all places and
different times of the year, for this average value is not the amount
most likely to fall in any particular interval of time, since there is
a limit to the extent of the negative deviations on one side--that is,
0 or no rainfall, while on the positive side there is no limit. The
most probable depth of rainfall, therefore, is less than the mean
value, the preponderance of negative over positive deviations being
about 10 per cent. and sometimes as great as 20 per cent.

       *       *       *       *       *

Professor W. M. Davis wrote an interesting review of Professor
Ferrel's popular treatise on the winds, published a year ago.
Commenting on the review, the editor of _Meteorologische Zeltschrift_,
Vienna, remarks on a very important omission in the treatise, namely,
the absence of all reference to the diurnal variation of the wind and
the many interesting relations it bears to other phenomena, a notable
omission in a treatise specially devoted to winds. The treatment of
the monsoon wind and its relation to the general circulation is highly
commended by the editor, and indicated as being all new.

       *       *       *       *       *

Your Vice-President has elsewhere expressed his opinion that monsoon
winds, applying the term by liberal construction to signify winds
which recur with returning seasons, cannot with {48} any degree of
correctness be asserted to prevail in the United States. It is true
that the prevailing surface winds of the greater part of the United
States come from the western quadrants--that is, between southwest and
northwest--and so are in substantial harmony with the general
atmospheric circulation as shown by the upper-wind currents of Mount
Washington (from the northwest) and Pike's peak (from the southwest).
But, apart from the easterly and northeasterly trades on the Florida
coast, it appears from the records that in no case for any
considerable section of the country do 50 per cent. of the winds blow,
for any consecutive number of months, either from any single point or
from two neighboring points of the compass. Occasionally, however, the
local configuration of the country is such that winds are drawn up or
down valleys, and, being diverted from their free and proper
direction, the wind in such cases follows the trend of the valley or
depression.

       *       *       *       *       *

In general your Vice-President would feel inclined to refer only
casually to the work proceeding from the Bureau over which he has the
honor to preside, but this year has been marked by special researches
and investigations of general interest. As the work of investigation
has been entrusted to the professors of the Signal Service, due credit
should not be refused them from their own official chief.

Special reference should be made to the work of Professor Charles F.
Marvin, whose successful experiments on wind pressures and velocities
have attracted the attention of experts both in Europe and in this
country. Unfortunately there was available only a small sum (about one
hundred dollars) for the expense of experiments, but with this petty
sum, supplemented by his ingenuity, Professor Marvin has very
satisfactorily determined the coëfficients of the various forms of the
Robinson anemometer, with which instrument the velocity of the wind is
very generally determined. Following these investigations, the Royal
Meteorological Society of England reopened the question, which, after
a costly set of experiments with results widely differing from those
of Professor Marvin, had been considered closed.

The general results of these researches, which are believed to be
sufficiently definite for general questions, are not only prized by
the scientist, but they are of value to the engineer and the builder.
Indeed, to all interested in costly structures or extended works
liable to harm from wind pressures, the factor of safety is {49} a
matter of no small pecuniary importance. These experiments show that,
as was formerly believed to be the case, the wind pressure varies as
the square of the velocity of the wind, expressed in miles per hour;
but a most important fact has developed, namely, that the pressure in
pounds per square foot is equal to the miles of hourly velocity
multiplied by 0.004 instead of 0.005, as was formerly assumed.

Professor Marvin was not content with one system of experiments, but
he further attacked the problem in a direct manner by a method which
checked and verified his experiments with the whirling machine. On the
summit of Mount Washington, at an elevation of 6,300 feet, he obtained
simultaneously and under the same conditions, by automatic and
electrical apparatus, continuous registration of the pressure of the
wind in pounds per square foot and of the velocity in miles per hour.

The results thus verified can be considered as conclusive from a
general standpoint. The corrections for the Robinson anemometer thus
determined from these experiments are comparatively unimportant at low
velocities, say from 10 to 15 miles per hour, being only a fraction of
a mile per hour. The uncorrected velocities, however, are in all cases
too large, and by greater and greater amounts the higher the velocity.
At 60 miles per hour the observed velocities are about 12 miles per
hour too high, and for an indicated velocity of 90 miles the
experiments show that the actual velocity is but a fraction over 69
miles per hour.

The anemometer formula found to satisfy most closely the entire range
of experiments has the following form for velocities in miles per
hour:

  Log. _V_ = 0.509 + 0.9012 log. _v_.

This difference indicated by the formula may seem small and
insignificant, as it is in the case of light winds, but at very high
velocities the differences are very great. For instance, an actual
velocity of 60 miles per hour may occur at some time in almost any
locality of the United States for a few minutes, and even greater
velocities are occasionally reported, apart from severe tornadoes.
Under the old coëfficients for the Robinson anemometer an actual
velocity of 60 miles per hour would have been reported as 77 miles per
hour, which under the old factor of 0.005 would mean a pressure of
29.6 pounds per square foot; but when considered with reference to the
true velocity of 60 miles, under {50} the new factor of 0.004, the
pressure would only be 14.4 pounds per square foot--a reduction of
over 50 per cent. from the pressure-values formerly accepted.

Professor Marvin has undertaken to verify, and also to extend to even
lower temperatures, the observations of Regnault as to the pressure of
aqueous vapor at low temperatures, especial attention being given to
temperature conditions from 0° centigrade to -50° centigrade. These
observations disclose, below 0° centigrade, small but constant
differences from the values assigned by Regnault.

In all this work Professor Marvin has shown such ingenuity of
resource, such skill in adapting means to the end, and such deftness
in improvising and manufacturing the requisite instruments as have
elicited commendation from all who have seen his work and followed his
methods. Your Vice-President alludes to this not only to give that
credit rightfully due to Professor Marvin, but to illustrate this as a
type of the highly important work which is being done in all branches
of science here in Washington by young men sometimes illy equipped as
to means, and still more illy paid. Men engaged in work of original
investigation should receive higher pay than clerks in charge of
routine duties; but unfortunately the majority of them do not.

       *       *       *       *       *

The work of Professor Hazen in charting tornadoes and in determining
their relative frequency and severity is directly in the line of the
Geography of the Air.

Great attention had previously been given to this subject by
Lieutenant John P. Finley, who, with indefatigable industry, had
accumulated an enormous mass of data relative to these violent
outbursts of nature's forces. The figures and deductions previously
put forth under the authority of the Signal Service having been
questioned, the Chief Signal Officer felt obliged, in view of the
growing practical importance of the question, as indicated by the
great sums annually paid out in the Ohio valley and in the
trans-Mississippi region for protection against tornadoes, to reöpen
the subject. Instructions of the most conservative character were
given to Professor Hazen to determine carefully the prevalence and
number of tornadoes in the United States, the areas devastated by
them, and the number of lives lost annually. This work was carefully
scrutinized during its progress to see that it should be devoid of
theory and rest on the solid basis of fact. The results are most
assuring to every {51} one, and must serve to allay the unreasonable
fears of the inhabitants of the so-called "tornado districts." It
appears that there is no part of the United States in which annually
more than one square mile of devastation or severe destruction can be
expected for each 185,000 square miles, although cases of _limited
destruction_ may occur annually for about every 5,000 square miles of
area. In no state may destructive tornadoes be expected, on an
average, more than once in two years; and the area over which total
destruction can be expected is, as shown by the foregoing figures,
exceedingly small, even in localities most liable to these violent
storms. The annual death casualties from tornadoes have averaged, in
the last 18 years, 102 annually; but it is believed that the death
rate from lightning is greater than that from tornadoes, since during
March to August, 1890, the names of 110 are on record who have lost
their lives by lightning, although the data are incomplete, especially
as regards the southern states. These statistics cannot be passed by
lightly, however, and it is doubtful if in the main they are much in
error. By them it appears from five years' record that the average
annual death rate by lightning in the United States is 3.8 per million
of inhabitants, or 0.2 above the average. In Sweden, for sixty years,
the average has been 3.0; in France, for forty-nine years, 3.1; in
Baden, for seventeen years, 3.8; and in Prussia, for fifteen years,
4.4 per million.

Other figures, given by a life-insurance agent in St. Louis, which the
author claims to have compiled with great care, place the average
annual rate of death from lightning in the United States at 206, being
more than double the deaths from tornadoes. It must be understood that
these figures are not vouched for, and must be very cautiously
received, as originating with companies interested pecuniarily in the
statistics.

On the whole, therefore, it may be safely assumed that tornadoes are
not so destructive to life as thunder-storms.

       *       *       *       *       *

Professor Thomas Russell has formulated a method for prediction of
cold waves. They always occur after "lows" and before "highs," and
different cold waves vary in extent from three "units" to sixty. A
"unit" of temperature-fall is taken as a fall of twenty degrees over
an area of 50,000 square miles.

The temperature-fall curves in the United States are approximately
elliptical in shape. Perfect ellipses represent actual temperature
falls with an error not exceeding six degrees in {52} most cases.
These fall lines are intersections of planes with a cone which
graphically represents the totality of temperature-fall, the contents
of the cone being equal to the area of its base multiplied by its
altitude, which is the greatest fall in temperature at the center of
the cold wave.

A formula has been devised, based on 127 special cases, representing
the amount of fall in terms of the amount of barometric depression in
a "low," and the amount of excess if a "high," and the density of the
isothermal lines in the region.

From proper consideration of the type of low area, shape of isobars,
and position of the long axis, definite conclusions can be drawn as to
the subsequent shape of the elliptical twenty-degree temperature-fall
area and its position.

A method has been devised, also by Professor Russell, for determining
the maximum fall of temperature at the center of the cold wave. The
maximum fall and extent of fall being known, from suitably prepared
tables, the area of twenty-degree fall can be derived. Previously
prepared pieces of card-board are laid in the proper position on a map
of suitable scale, and lines drawn around them. Between the line
representing the twenty-degree fall and the center, the other falls of
thirty degrees, forty degrees, etc., are sketched in.

       *       *       *       *       *

The foregoing sketch of the geography of the air may appear too
superficial and limited for the purposes of this Society, but its
further elaboration was impracticable. Indeed, the subject of
meteorology could hardly have been touched upon this year had it not
been for the courtesy of Professor Russell in placing at my disposal
notes upon translations from foreign publications, especially from the
German; which publications I have been unable to examine save in a
casual way.

The address, as it is, is submitted only in the hope that it may
serve, if no other purpose, at least to indicate the great interest
which now obtains in the geography of the air, and which manifests
itself in the production of meteorological pamphlets and publications
too numerous to permit any one charged with important executive duties
to examine them all, even in a non-critical way.