This paper describes some interesting properties of waves in, and oscillati
ons of, the interstellar medium (ISM) in the direction normal to the plane
of the Galaxy. Our purpose is to examine possible reasons for four observed
phenomena: the falling sky in the northern hemisphere; the apparent presen
ce of clouds in absorption spectra when a sight line is occupied primarily
only by warm intercloud gas; the peculiar structuring of spiral arms involv
ing clumps, spurs, and feathering; and the existence of an abundance of hig
h-stage ions far off the plane of the Galaxy. We explored the reaction of t
he interstellar medium-in the vertical direction only-to large imposed dist
urbances (initial displacements, expansive velocities, and compressions) an
d to the introduction of small-amplitude waves via oscillation of the midpl
ane. Our findings included (1) the anticipated growth in amplitude of high-
frequency waves with height; (2) the four lowest normal modes for the oscil
lation of the atmosphere as a whole, as functions of the height of the oute
r boundary; (3) the time for material to "bounce" from one unusually dense
state to the next as a function of height; and (4) the tendency for the dis
k to develop a hot outer halo, either after the passage of a single shock f
rom a large event or in response to a continuous stream of small-amplitude
waves. We discovered that three of the four observed phenomena targeted are
likely to be closely connected. Following a large expansion, material near
the plane falls back first, with material at higher z then falling in upon
it. This provides precisely the sort of velocity segregation observed in t
he northern sky, at about 50 Myr after the event. In addition, this bounce
time (and/or the period of the subsequent smaller oscillations, which is ab
out twice the bounce time) may be linked to structure in the spiral arms, w
ith vertical oscillations having been provoked by initial compressions in t
he arms. Oscillations of the fundamental symmetric (breathing) mode of the
ISM also produce substantial disturbances in the outer atmosphere. This can
result in the production of an extensive layer of hot gas overlying the co
oler disk material, i.e., a hot Galactic halo with a significant population
of high-stage ions. Hence, three of the four phenomena may be natural resu
lts of the simple existence of strong local compressions at the spiral arms
and the associated vertical motions in a thick Galactic disk. Finally, the
somewhat mysterious appearance of clouds in some absorption spectra can be
produced by small-amplitude waves in the ISM. Under the right conditions,
clouds will seem to appear through "velocity crowding," when in fact there
are no density concentrations in space.