The San Francisco call., 1896-05-31

often misinterpreted, was explained as long
ago as 1853 by Franklin in the following
characteristic and simple words: "The
spout appears to drop or descend from the
cloud though the materials OF which it is
composed are all the while ascending, for
the moisture is condensed faster in right
line downwards than the vapors them
selves can climb in spiral line upwards.
Such is the modern theory of the torna-
do, developed by Ferrel, exhibited in our
illustrations, and found to be in complete
accordance with recorded facts.
Theory shows that tornadoes will prevail
where the geographical conditions are
favorable to the facile movement of cold
air from the north alongside or above
warm air from the south. Such an area
exists par excellence over the fiat river
basins of the Mississippi, Missouri and
Ohio, and as fact we find the States en-
braced by their area most visited by tor
nadoes. independently of geographical
locality such a crossing of currents, es-
pecially cold over warm, occurs in the
S. Sc octant of cyclone and it is pre-
cisely in this part of cyclone or what is
frequently termed "low" that tornadoes
are generated. The same octant when the
vertical temperature dltferences are less
marked is frequently the seat OF a small
subsidiary Glow in which thunderstorms
occur. A thunderstorm, waterspout and
tornado are in fact first cousins.
Another factor favorable to tornado for
nation is a sluggish movement OF the air,
allowing a local accum ulatlon of heat and
moisture, causing what Is termed oppres-
sive weather, and since such heating up
near the surface, with cold air overhead,
occurs most readily in the early summer
it is not surprising to find that the re-
searches of Lieutenant Finley place the
month of May as that of greatest tornado
frequency. When the gun is loaded it
only requires the exercise of small mus
cular effort to release an immense poten-
tial of energy.
Similarly in the case of the tornado, the
heat and other conditions may have been
accumulating for some days, but the mo-
ment most favorable 10 its sudden trans
formation is when the daily tenn perature
has reached its highest point, that is in the
afternoon, and though tornadoes have been
recorded at all hours of the day they are
most common in the afternoon. Again,
since they occur on the southeast side of a
cyclone they travel in the same direction
as the air moves in this part, VIZ: from
southwest to northeast, and as the condl-
tions favor their development chiefly on
this side of the area if there is more than
one tornado they will usually travel along
parallel paths.
In addition to these general motions a
tornado whirl, owing to the variations in
the local conditions which regulate its
development, frequently oscillates up and
down. now descending to the earth and
again whirling harmlessly overhead and
swaying from side to side, occasionally
doubling over its path. but only within
narrow limits, and always maintaining q
| generally easterly course.
| From the theory 1t appears that while
the velocity with which the tornado is
propagated is no greater than that of the
middle stratum of the cyclone within
which it is generated, viz, forty-four miles
an hour, according to Finley the velocity
| of the air round the central vortex may
rise to 500 miles or more per hour.
AS a matter of fact. Finley mas measured
effects which can only have been produced
by such velocities, and we may take it that
the horizontal force of the wind. near the
center, is frequently as much as $50 pound;
to the square foot, sufficient to wreck any
/ of the arrival of the travelling vacuum
over it, the air within exerts this force in
bursting the roof and walls outward. In