Johann Heinrich Jacob Müller.

Principles of physics and meteorology online

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very different climate, and we may thus easily see that theoretical
considerations do not suffice as data, from whence to draw conclu-
sions regarding climatic relations ; the true distribution of heat
over the earth's surface can only be satisfactorily ascertained by
means of observations conducted for a protracted term of years.
Humboldt was the first who entered here with success upon the
course of induction, the sole and only path that leads to truth in
all physical sciences. On his voyages and travels in both hemi-
spheres, he collected, with unwearied zeal, facts which, by his
excellent mode of combining them, have first laid the foundation
of scientific meteorology.

Observation of the Thermometer. In order to be able to ob-
serve accurately the temperature of the air at different places, we
must place a good thermometer in the open air, upon the north
side of a building, and 3 or 4 decimetres removed from the wall,
so that it may not receive the sun's rays ; we must, likewise, be
careful that there is no white wall in the neighborhood, from which
rays of heat may be reflected towards the thermometer. If the
thermometer should be moistened by rain, we must carefully dry
the bulb five minutes before we use it, for the suspended drops of
water would, by their evaporation, lower the temperature of the
mercury in the bulb.

It is often of the greatest importance to meteorology to learn
the highest and lowest temperature that may have prevailed during
any interval, without it being absolutely necessary to observe the
exact moment in which this maximum or minimum occurs. This
may be effected by the thermometrograph, represented in Fig.



560 OBSERVATION OF THE THERMOMETER.

517, which consists of two thermometers, the tubes of which are
placed horizontally, and of which one is a mercurial, and the

Fig. 517.




other a spirit thermometer. In the tube of the former lies a steel
pin, which is pushed through the column of mercury when the
mercury in the bulb expands ; when, however, the thermometer
is re-cooled, the mercurial column recedes, while the steel pin
remains in the position to which it was pushed at the highest
stand of the thermometer ; such a thermometer, consequently,
yields the maximum of the temperature that may have prevailed
within a certain period.

Within the tube of the spirit thermometer, is a very fine glass
rod, somewhat thicker at its extremities, as may be plainly seen
in Fig. 517; this glass rod lies within the column of spirits, and
on the spirit cooling in the bulb, and the fluid retreating in the
tube to the first knob of this rod, the latter will be carried away
with the retreating fluid column, when any further sinking of the
temperature occurs, owing to the adhesion between the spirit and
the glass; if, however, the fluid in the bulb be again warmed, it
will, on the rising of the thermometer, pass by the rod without
carrying it with it; this index, which must be made of some darkly
stained glass, in order to be made more apparent, remains, conse-
quently, lying in the place corresponding to the minimum of the
temperature which prevailed within a certain period of time.

When the bulb of the one thermometer lies on the right side,
that of the other is on the left, and on inclining the whole apparatus,
and striking it gently, the steel rod will fall by its weight on to
the column of mercury, and the glass rod to the very end of the
column of spirit. If we leave the apparatus thus arranged, the
steel rod will be pushed on by every ascent of the temperature,



DIURNAL VARIATIONS OF TEMPERATURE. 561

while the glass rod will be drawn back at every depression of the
temperature.

This instrument is especially calculated to give the maximum
and minimum of the diurnal temperature. On setting it in the
proper manner every evening, we may, the following evening, see
what has been the highest, and what the lowest temperature dur-
ing the last 24 hours.

Diurnal Variations of Temperature. In order to be able accu-
rately to follow all the variations of heat in the atmosphere during
the 24 hours, we must observe a thermometer at very short inter-
vals, as, for instance, from one hour to another. If such obser-
vations are to be pursued for any length of time, it is evident that
they cannot be conducted by one single individual, but that many
must combine for the same purpose ; in every case it is very la-
borious to institute a series of observations of this kind.

From such series of observations it has been shown that the
minimum of temperature occurs shortly before sunrise, and the
maximum a few hours after 12 at noon, somewhat later in sum-
mer, and somewhat earlier in winter.

This course may be easily explained. Before noon, whilst the
sun is constantly rising higher, the earth's surface receives more
heat than it radiates ; its temperature and that of the atmosphere
must, therefore, increase ; this continues somewhat beyond noon ;
but as the sun sinks lower, and its rays become less effective, the
heated earth radiates more heat than can be supplied by the solar
rays; this cooling naturally continues after sunset, until the
morning-dawn announces the return of the sun.

The diurnal variations in the thermometer do not always follow
this normal course, which may frequently be disturbed by foreign
influences, as, for instance, changes of weather, &c. ; in order,
therefore, to ascertain with exactitude the law of diurnal varia-
tions of heat, we must deduce the mean normal course from a
combination of as many numerous observations as can possibly
be instituted.

By taking the mean of every 24 hours' observations, we obtain

the mean temperature of the day.

As it is uncommonly wearisome and laborious to pursue for any
length of time these hourly observations of the thermometer, it is
of the greatest importance to meteorology to devise methods by



562 MEAN TEMPERATURE OF THE MONTHS.

which the mean diurnal temperature may be ascertained without
making these hourly observations. Twice in the day the ther-
mometer must indicate the mean diurnal temperature ; it, there-
fore, seems the simplest to calculate the hours in which such is
the case, and then limit our observations of the thermometer to
those periods of the day ; such a course may, however, easily
lead us into errors, since the thermometer varies most suddenly
exactly at this time, and we should thus commit a very considera-
ble mistake in our calculations, if our observations were made
either a little too early or too late. A far more correct result is
obtained by observing the thermometer at several similar hours,
for instance, at 4 and 10 A.M., and at 4 and 10 P.M. ; this method
is, as Brewster has shown, correct to T Vth of a degree ; we like-
wise obtain a very useful result by making our observations at 7
A.M. at noon, and at 10 P.M., and then taking the mean of these
three periods.

The mean of the highest and lowest degree of the thermometer
during the 24 hours varies so inconsiderably from the actual mean
temperature derived from hourly observations, that we may more
easily compute the mean diurnal temperature by aid of the ther-
mometrograph described at page 560.

Mean Temperature of the Months, and of the Year. When we
know the mean temperature of all the days of a month, we have
only to divide the sum of the mean diurnal temperatures by the
number of days, in order to obtain the mean temperature of the
month.

On taking the arithmetical mean from the mean temperature
found for the 12 months of the year, we obtain the mean tempera-
ture of the year.

In order to determine, with exactness, the mean temperature of
a place, we must take the mean of the mean temperatures obtained
from a large series of calculations. In general, the mean annual
temperatures do not vary much, so that we obtain the mean tem-
perature of a place with tolerable accuracy, even when we only
know it for a few years. For Paris, the mean temperatures of the
years intervening between 1803 and 1816, were as follows :

51 50,8 49,6

52 51,1 49,4



MEAN TEMPERATURE OF THE YEAR. 563

49,2 51 51

53,3 51 49,2

51,3 49,6

The highest of these mean annual temperatures varies only
about 4,1 from the lowest. On taking the mean of these 14
numbers, we obtain as a mean temperature for Paris 50,6, whilst
the amount derived from a series of 30 annual mean temperatures
is 51,4.

In order to find the true mean temperature of a month, we
must know the mean temperature of this month for a series of
years, and take the mean of these.

The greatest heat generally occurs in our latitudes some time
after the summer solstice, and the greatest cold some time after
the winter solstice.

July is, on an average, the hottest, and January the coldest
month. If the period of the highest and lowest temperature is
not exactly the same for all places of the same hemisphere, the
difference is only occasioned by local influences.

We may, on an average, consider the 26th of July as the
hottest, and the 14th of January as the coldest day of the year
for the temperate zone of the northern hemisphere.

It has been proved from numerous observations on temperature,
that the mean annual temperature generally occurs on the 24th of
April, and the 21st of October in the northern temperate zone ;
the annual course of the heat in these parts is therefore as follows.
The temperature rises from the middle of January at first slowly,
more rapidly in April and May, and again more slowly until the
middle of July, from which period it diminishes but slowly in
August, more rapidly in September and October, finally reaching
its minimum again in the middle of January. This admits of an
easy explanation. When the sun, after the winter solstice, again
ascends, this ascent goes on so slowly, and the days increase so
little, that as yet no more powerful effect from the sun's rays is
possible. On this account, the minimum of the yearly temperature
occurs after the winter solstice ; a rise of temperature first takes
place when the sun has returned somewhat farther north. About
the time of the equinoxes, the sun's progress in the heavens to-



564 MEAN TEMPERATURE OF THE YEAR.

wards the north is quickest: the increase of temperature for this
reason is at this time the most perceptible.

When the sun has attained its highest position, the earth has
not yet become so warmed that the heat which the ground loses
by radiation is equal to the quantity of heat, which it receives
from the sun's rays; the balance would only be restored after the
sun had remained a longer time at its northern solstice. But now
the sun goes back after its summer solstice, very slowly at first.
The effect of the sun's rays is for some time quite as powerful as
at the moment of the solstice ; the temperature, therefore, will
still rise after the longest day, and indeed even to the middle of
July, and then again fall. These considerations lead to the divi-
sion of the year into four seasons.

The astronomical division, when the seasons are limited by
the equinoxes and solstices, is the most suitable to meteorology.
It would be better were we to divide the year in such a manner,
that the hottest month (July) should fall in the middle of summer,
and the coldest month (January) in the middle of winter. Ac-
cording to this, winter would include the months of December,
January, and February; spring, March, April, and May; summer,
June, July, and August; and autumn, September, October, and
November. According to this signification, we must understand
the seasons given in the following table, which contains the mean
annual temperature, mean temperature of individual years, and
the hottest and coldest months for a large number of places scat-
tered over different parts of the earth's surface.

The numbers of this table are only mean numbers, from which
the true temperature inclines sometimes towards one side, some-
times towards the other, and thus, too, the mean temperatures of
the hottest and coldest months by no means indicate the limits
between which the thermometer may fluctuate at one and the same
spot. It thus happens, that even in districts enjoying a warm
climate and a mild winter, an extraordinary degree of cold is
often felt; thus, for instance, in the year 1507 the harbor of Mar-
seilles was frozen over its whole extent, for which a cold of at
least 0,4 was requisite; in the year 1658, Charles X., with
his whole army and their heavy artillery, crossed the little Belt.
In 1709 the Gulf of Venice, and the harbors of Marseilles, Genoa,
and Cette were frozen over; and in 1789 the thermometer fell at



MEAN TEMPERATURE OF FORTY-THREE PLACES.



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566 MEAN TEMPERATURE AT DIFFERENT PLACES.

Marseilles to 16. The following table gives the highest and
lowest degrees of temperature observed at different places.

Minimum. Maximum. Difference.

Surinam .... 70,3 90,1 19,8

Pondicherry . . . .71 112,3 41,3

Esna (Egypt) . . . 117,3

Cairo .... 48,3 104,2 55,9

Rome .... 22,1 100,4 78,3

Paris . . . 9,3 101,1 110,4

Prague . . . 17,5 95,9 113,4

Moscow . . . 38 89,6 127,6

Fort Reliance (North America) 70

Considerable deviations from the normal annual course of heat
do not occur locally, but are scattered over wide districts; thus,
for instance, the winter of 1821 and 1822 was very mild in Europe,
but in the December of the latter year a severe cold prevailed
over the whole of Western Europe. A similar very considerable
deviation never has, however, been spread over an entire hemi-
sphere. The northern hemisphere is generally divided in a
northern to a southern direction into two halves, upon which op-
posite deviations from the normal temperature may be observed;
these deviations are greatest in the middle of the two halves,
while a more average temperature is perceived where they ap-
proach each other.

Thus, in February, 1828, it was very cold in Kasan and
Irkutzk, unusually mild in North America, whilst Europe remained
unaffected between these two opposite deviations. In December,
1829, this maximum of cold inclined towards Berlin, while it
also continued to be very marked at Kasan; in North America,
however, the weather was unusually mild ; but in December, 1831,
the excessive cold was limited to America. Generally speaking,
these deviations from the average range of heat are observed to
be similar in Europe and Asia, and opposite in America.

Frequently, although not so remarkable, the boundary line of
opposite deviations runs from east to west.

A deviation from the mean temperature often continues for a
long time in the same direction. Thus, from June, 1815, to the
December of 1816, there prevailed in Europe an unusually low
degree of temperature, which occasioned the failures in the crops



ISOTHERMAL LINES. 567

in 1816 ; 1822 was a remarkable year for the vines, the unusual
heat continuing then from November, 1821, to November, 1822.

From this it follows, that the opinion so prevalent of a cold
winter succeeding a hot summer, and a warm winter a cold sum-
mer, is altogether erroneous, since the contrary often occurs, as
may be seen from the examples above given ; thus, too, the hot
summer of 1834 succeeded a very mild winter.

These deviations from the mean range of heat are more marked
in winter than in summer.

From all this it appears highly probable that the same quantity
of heat is always distributed over the earth's surface, although
unequally. A cold winter is the consequence of a long preva-
lence of north-east winds, and a cold summer is induced by the
continuance of south-west winds; these alternating exclusively
prevalent currents of air being, as Dove has shown, the controlling
agents in the relations of weather. If a hot summer is to succeed
a cold winter, the north-east wind must prevail throughout the
whole year ; while, on the other hand, the wind must blow chiefly
from the south-west for the same space of time to bring a cold
summer after a mild winter.

Isothermal Lines. A table of the kind given at page 565, con-
tains many of the elements, from which we may calculate the
distribution of heat over the earth's surface. At all events, we
may see from such a table that all places lying under the same
degree. of latitude have not the same mean temperature. Thus,
for instance, the mean annual heat at the North Cape is 32,1 ;
whilst Nain, on the coast of Labrador, has a mean annual tem-
perature of 25, although Labrador is 14 south of the North
Cape. Humboldt was the first to give us a clear view of the
distribution of heat over the earth, making use, for this purpose,
of his isothermal lines, by which he connected together all such
places in the same hemisphere having equal mean annual tempe-
ratures.

If we suppose, for instance, a traveler starting from Paris to
make a journey round the earth in such a manner as to visit all
places of the northern hemispheres which have the same mean
annual heat as Paris, that is, 51,4, the course he will thus pursue
will be a line of equal mean annual heat, consequently an isother-
mal line; this line, instead of corresponding with the degree of



568 ISOTHERMAL LINES.

latitude of Paris, will be irregular and curved, passing through
places having a very different latitude from Paris.

Fig. 518 represents the earth's surface in Mercator's propor-
tions, with the isothermal lines at every 5 degrees. At the ter-
restrial equator, the mean temperature of the sea-coast is 81,5,
although somewhat less upon the western coast of America and
Africa ; in the interior of these two continents, especially in that
of Africa, the mean temperature is higher than on the sea-shore ;
the mean temperature of the equator in the latter continent is
above 84.

An examination of the chart in Fig. 518, will spare us a fur-
ther description of the course of the isothermal lines. We observe
how considerable their curves become in the northern hemisphere
the further we remove from the equator; the isothermal line of
32, for instance, ascends from the southern end of the coast of
Labrador across Iceland towards the North Cape, in order to de-
cline again considerably in the interior of Asia.

Where the isothermal lines incline the farthest towards the
south, they describe a concave ; and where they ascend the highest
towards the north, a convex vertex. The southern turning points
of the isothermal lines lie in the east of North America and in the
interior of Asia, while the northern turning points lie on the
western coasts of Europe and America.

The relations of temperature of the southern hemisphere are not
nearly so perfectly known to us as those of the northern hemi-
sphere; we may, however, consider it as established, that the
southern is colder than the northern hemisphere, although the
difference may, perhaps, be less considerable than we are gene-
rally disposed to assume it. The circumstance that has probably
contributed to the opinion that the southern is so much colder
than the northern hemisphere, is, that the relations of temperature
of the southern part of America have been compared with those
of like northern latitudes in Europe, where the isothermal lines
ascend so very considerably to the north; the matter is very
different when we compare districts of South America with those
lying equally far from the equator on the east side of North
America.

That the southern hemisphere is somewhat colder than the
northern, arises probably from the fact, that in the former, water,
and in the latter, land, predominates. The continent is much more



ISOTHERMAL LINES.



.069




48'



570 ISOTHERMAL AND ISOCHIMENAL LINES.

heated by the absorption of the sun's rays than the sea, which
reflects a great portion of them.

Isothermal and Isochimenal Lines. We have thus stated that
all places lying on the same parallel circle have not the same
climate; here, however, the question arises, whether all places on
the same isothermal lines, consequently such as have the same
mean annual heat, have likewise otherwise equal climatic rela-
tions. We need only look at the table, page 565, in order to
convince ourselves that such is not the case. Thus, for instance,
Edinburgh and Tubingen have the same mean annual temperature
of 47,6; at the former place, however, the mean temperature of
winter is 38,6, at the latter 32,2 F.; Tubingen, consequently,
has a far colder winter than Edinburgh. But then, again, the
mean summer temperature of Tubingen is 62,7 F., while it is
only 58,4 F. for Edinburgh. With a like mean annual tempe-
rature, Edinburgh has, therefore, a milder winter and a colder
summer than Tubingen.

In order to know the relations of heat of a country, it is not
sufficient to be acquainted with its mean annual temperature; we
must also know how heat is distributed during the different
seasons of the year. This distribution may be shown upon an
isothermal chart, by setting down, according to Humboldfs idea,
the mean summer and winter temperature against the different
places upon one and the same isothermal line, which could not be
done on our isothermal chart, owing to its small size ; we shall
thus see, that in the immediate vicinity of the convex summit of
the isothermal lines, the differences between the mean summer and
winter temperature are the least; the same reasons, consequently,
which cause the isothermal lines upon the western coast of Europe
and America to rise so far to the northward, make the difference
between the summer and winter temperature less considerable.
A very good idea of the distribution of heat in winter and summer
may be obtained by means of a chart, in which all places having
the same mean winter temperature are connected together by
curved lines, as are also all the places that have the same mean
summer temperature. The lines of like mean winter temperature
are termed isochimenal, and those of like mean summer tempe-
rature, isothermal. Fig. 519 represents a small chart of Europe
with the isothermal and isochimenal lines drawn at every 5
degrees.



ISOTHERMAL AND ISOCHIMENAL LINES.



571



Fig. 519.



flOC



20 C. +4 F.




The curves, whose corresponding temperatures are on the right
the chart, are the isockimenal, and the other the isothermal
. We may easily see from this chart, that the western coasts
the southern part of Norway, Denmark, a portion of Bohemia
Hungary, Transylvania, Bessarabia, and the southern extre-
mity of the peninsula of the Crimea, have the same mean winter
temperature of 0- (32 F.). Bohemia, however, has the same
immer heat as the districts lying at the mouth of the Garonne,
in the Crimea the summer is far hotter. Dublin has the
ame mean winter temperature, viz., 5 (41 F.) as Nantes, Upper
Italy, and Constantinople, with the same summer heat as Dron-
theim and Finland.

The isothermal line of 20 (68 F.) passes from the mouth of the
Garonne, nearly over Strasburg and Wurzburg to Bohemia, the
Ukraine, the country of the Don Cossacks, somewhat to the north
of the Caspian Sea ; how different, however, is the mean winter
temperature at different places upon this line ! On the western
coasts of France it is 5= '41- F.), in Bohemia (32 F.), in the
Ukraine 5 C (23 F.), and somewhat to the north of the Caspian
Sea even 10 (14 F.).



572 THE CLIMATE ON LAND AND AT SEA.

The Climate on Land and at Sea. The consideration of the last
map, and the table at page 565, leads us to the important differ-
ence between the climate at sea and on the land, or, as we may
also express it, between the continental and littoral climate. The
differences between the summer and winter temperature increase
with the distance from the sea ; on the sea-side the summers are



Online LibraryJohann Heinrich Jacob MüllerPrinciples of physics and meteorology → online text (page 45 of 55)