182 ON TWO STORMS EXPERIENCED THROUGHOUT THE UNITED STATES, . 
of the earth is moving on harmoniously in one direction. How is rain produced in 
such an atmosphere? The first requisite appears to be a change of direction of the lower 
stratum of air. This appears, in winter, to be frequently the effect of a preceding storm. 
The prevalent westerly current being temporarily checked in its progress by a violent 
storm, soon acquires force sufficient to break down all opposition. It supplants the rare- 
fied air of the storm, and not only restores the barometer to its mean height, but the 
momentum of the excited mass carries it considerably above the mean. ‘This excess of 
pressure causes a reverse current a little to the westward of a violent storm; and hence we 
sometimes have a long series of violent storms succeeding each other, at nearly equal 
intervals; and hence also a violent storm succeeded by an unusually high barometer 
affords ground for expecting a second storm within one or two days. But this explana- 
tion will not apply to all cases, for then, if the barometer should ever settle down to its 
mean height all over the globe, we never could have another storm. ‘The case here sup- 
posed is not likely ever to happen, but even if it should, we cannot admit the conse- 
quences attributed to it. Admit such a case to occur, and the sun’s heat would be com- 
petent to generate a new storm. Different portions of the earth’s surface absorb the sun’s 
rays in unequal degrees, and afford unequal quantities of moisture for evaporation. The 
result is, that we find bodies of air in close proximity of unequal density, arising from 
unequal temperature or humidity. Hither case would be sufficient to cause a deflection 
of the lower stratum of air from its normal direction. Suppose, then, we have the mass 
of the atmosphere pursuing its wonted course from west to east, while a stratum of a 
mile or so in height, next to the earth’s surface, blows in some different direction. If 
this direction be from the south to the north, then this current must be cooled in its pro- 
gress by change of latitude. This effect may be aided by the inequalities of the earth’s 
surface, and by friction upon the upper stratum of colder air. At the surface of the 
earth, where the temperature is probably five or ten degrees above the dew point, no 
remarkable effect may follow. But at a certain elevation, the air is always saturated 
with vapour. A very slight reduction of temperature causes cloud, and its density and 
extent will be proportioned to the energy of the causes in operation. If the wind should 
blow from the north, it might happen that no cloud would be formed; but if the direction 
should be easterly, being partly opposed to the normal current, some portion of this mass 
would almost necessarily be elevated from the earth’s surface, and being cooled, its vapour 
be condensed. The first stage of the process, then, is an abnormal current at the earth’s 
surface; the second is the production of cloud. At this stage the sky is covered with a 
veil which checks radiation; the thermometer rises above the mean from this cause, and 
also from the heat liberated in the condensation. This only adds to the energy of the 
first abnormal current. More cloud is thus formed, and presently the particles of water 
having acquired sufficient size, fall rapidly to the earth. The wind being southerly, the 
thermometer rises. A portion of the atmosphere being thus unusually heated and loaded 
with vapour, while the upper limit of the atmosphere remains nearly invariable, the 
barometer necessarily falls. Thus, these causes might continue to operate a long time, 
acquiring energy by their own action. A limit, however, is soon attained. The rare- 
faction thus produced creates a tendency in the surrounding colder and heavier air, to 
rush in and occupy its place. Moreover, if the wind be at all easterly, as is usually the 
