Ateih i, 1899,] THE TROPICAL 
AGRICULTURIST. 
oa ship-board for the transport of frozen provisions 
from New Zealand and Australia. 
Bearing in mind, then, this faot— -that air in expand- 
ing and driving aside the air into which it expands 
is always coolel— let ns see how this applies to the 
case before us, the production of cloud and rain. 
The volume of a given weight of air — in other 
words, the spaos it occupies— depends on the pressure 
to which it is snhj ot : the less this pressure the 
greater its volum . If we suppose the atmosphere 
divided into a numbsr of layers superimposed on 
each other, the bottom layer is clearly subject to 
the pressu e of all those that rest on it. This is 
equal to about 14| pounds on every square inch of 
bu face. Another layer, say 1,000 feet above the 
ground, will clearly re under a less pressure, since 
1,000 feet of air are below it; and this 1,000 feet of air 
weighs slightly less tr a'i half a pound for every 
square inch of horizontal surfaoe. At 2,000 feet the 
pressure will be less by nearly one pound per square 
fnoh, and bo on. If, then, any mass of air begins 
to asoend through the atmosphere, it will be con- 
tinually subject to less and less pressure as it 
ascends j and therefore, as we have already seen, 
it expinds, and becomes co >ler by expansion. 
Cooling fr.m this oause is termed dynamic cooling. 
Its rate may be aocurately computed from the work 
it has to do in expanding. 
It am unts to 1° for every 183 feet of ascent if 
the air be dry or free from vapour, and if, as is always 
the case, it contains some vapour, the height will 
not be very much greater so long as there is no con- 
densation. Bub so soon as this point is passed, and 
the vapour begins to condense as cloud, the latent 
heat set free retards the cooling, and the height 
through which this cloud-laden air must ascend to 
cool 1° is considerably greater, and varies with the 
temperature and pressure. When the barometer 
stands at 30 inches, and at the t smperature of freez- 
ing, the air must Jse 277 f-et to lose 1°, and if the 
temperature is 60° nearly 400 feet. 
Conversely, dry air d scending through the atmo- 
sphe e and becoming denser as it descends, since it is 
continually becoming subject to an increased pressure, 
is heated 1° for every 183 feet of descent ; and fog 
and cloud-laden air at 30 inches of pressure and the 
freezing point will be warmed 1° in 277 feet only, or 
if at 60° nearly 400 feet of descent, owing to the 
re -evaporation ot the fog or cloud and the absorption 
of latent heat. 
Now let us see how these facts explain the formation 
of cloud; and first 1 will take the case of the common 
cumulus or heap cloud, which is the commonest cloud 
of the day-time in fine weather. 
"When after sunrise the air begins to be warmed, 
the lowest stratum of the atmosphere, which rests 
immediate'y on the ground, is warmed more rapidly 
than the higher strata. This is because the greiter 
part of the sun's heat passes freely through a clear 
atmosphere without warming it, and is absorbed by 
the ground, which pives it out again to the air imme- 
diately in contact with it. Sr> soon as the vertical 
decrease of temperature exceeds 1° in 183 feet, the 
warm air below begins to ascend, and the cooler air 
above to descend, and this interehange gradually ex- 
tends higher and higher, the ascending air being 
gradually cooled by expansion, and ceasiDg to rise 
when it has fallen to the same temperature as the 
air around it. This ascending air is more highly 
charged with vapour than that which descends to 
replace it, since, as was mentioned before, most 
land surfaces furnish a large amount of moisture, 
which evaporates when they are heated by the sun. 
This process goes on until some portion of the 
ascending air has become cooled to the point of con- 
densation. No sooner does it attain this, then a 
small tuft of cumulus cloud appears on the top of the 
ascending current, and the movement which was 
invisible before now becomes visible. In a oalm at- 
mosphere each tuft of cloud has a fiat base, which 
marks the height at which condensation begins, but 
it is really only the top of an ascending column of air. 
No sooner is this cloud formod then the ascent bo- 
comes more, rapid, bocause the cooling which ohecked 
its further ascent now takes place at a much slower 
rate, and therefore the cloud grows rapidly. 
On a summer afternoon when the air is warm and 
very damp, such cumulus cloud ascends sometimes 
to very great heights, and develops into a thunder- 
cloud, condensing into rain. Rain differs from fog and 
cloud only in the size of the water drops. In fog and 
cloud these are so minute that they remain suspended 
in the air. But as the cloud becomes denser, a 
number of th°m ooalesce to form a rain-drop, which 
is large enough to overcome the friction of the air. It 
then begins to fall, and having to traverse an enormous 
thickness of cloud below, it grows larger and lirger 
by taking up m"ra and more of the cloud corpusoules, 
so that when finally it falls below the e'oud it may 
have a con-iderable size. 
Such, then, is the mode in which rain is formed in 
an ordinary summer shower j and the more prolonged 
rainfall of stormy wet weather is the result of a 
similar process, viz. the » scent and dynamic cooling 
of the moist atmosphere. But in this case the move- 
ment is on a far larger scale, being shared by the 
whole masa of the atmosphere, it may be, over hun- 
dreds or thousands of square miles ; and to understand 
this movement we shall have to travel somewhat 
further afield, and to inquire into the general circula- 
tion of the great atmospheric currents set in move- 
ment by the sun's action in the tropics, and modified 
by the earth's diurnal rotation and the distribution 
of the continents and oceans on its surface. 
Before, however, entering on this subject, which 
will require some preliminary explanation, and in 
which we shall have to take account both of ascend- 
ing and descending currents on a large scale, I will 
draw your attention to another and simpler case, in 
which both these classes of movements are promi- 
nently illustrated, and in which they exhibit their 
characteristic features in a very striking manner, 
In the valleys of the Alps, more especially those to 
the north of the central chain, in Switzerland and the 
Tyrol, there blows from time to time a strong warm dry 
wind, known as the Fohn. It blows down the valleys 
from the central chain, melting the snows on its 
northern face, and although there is more or less clear 
sky overhead, all the southern slopes of the moun- 
tains are thickly clouded, and heavy rain falls on 
the lower spurs and the adjacent plain, replaced by 
snow at the higher levels up to the passes and the 
crest of the range. Cloudy weather also prevails to 
the north in Germany, and the weather is stormy 
over some part of Western Europe. 
It is only since the general introduction of tele- 
graphic weather reports and the construction of daily 
weather charts have enabled us to take a general 
survey of the simultaneous movements of the atmo- 
sphere over the greater portion of Europe, that this 
Fohn wind has been satisfactorily explained.* It is 
found that when a Fohn wind blows on the north of 
the Alps, the barometer is low somewhere to the 
north or north-west, in Germany, Northern France, 
or the British Isles, and high to the south-east, in 
the direction of Greece and the Eastern Mediterra- 
nean. Under these circumstances, since the winds 
always blow from a place of high barometer to one 
of low barometer, a strong southerly wind blows across 
the Alps. On their southern face it is forced to 
ascend, and therefore, as just explained, it is cooled 
and gives rain in Lombardy and Venetia, and snow 
at higher elevations. But having reached the crest 
of the mountains, it descends to the northern valleys, 
and being by this time deprived of a large part of its 
vapour, it becomes warmed in its descent, owing to com- 
pression, absorbs and re-evaporates the cloud carried 
with it, and is then further warmed at the rate of 1° 
for every 183 feet of descent. Thus it reaohes the 
lower levels as a warm dry wind, its warmtn being 
the effect of dynamic heating. 
Other mountain chains afford examples of the same 
phenomenon. A very striking instance, which much 
impressed me at the time, is one that I witnessed 
* The explanation was originally given by Prof, 
J. Haun of Vienna. 
