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Phenomena Of Revolving Storms
Observations
Storms Hurricanes And Typhoons
Localities For Additional Observations
Lower Latitudes
Appendix
Phenomena Of Atmospheric Waves
Seasons For Extra Observations
Avoiding The Centres Of Rotating Storms
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Times Of Observation
Avoiding The Centres Of Rotating Storms
Seasons For Extra Observations
Phenomena Of Atmospheric Waves
Appendix
Lower Latitudes
Localities For Additional Observations
Storms Hurricanes And Typhoons
Observations
Phenomena Of Revolving Storms

Storms Hurricanes And Typhoons

The solution of the question--How far and in what manner are storms
connected with atmospheric waves?--must be extremely interesting to
every one engaged in either the naval or merchant service. As we have in
the former chapters directed attention to their connexion, our great
object here will be to endeavour to mark out such a line of observation
as appears most capable of throwing light, not only on the most
important desiderata as connected with storms, but also their connexion
or non-connexion with atmospheric waves. We shall accordingly
arrange this portion of the instructions under the following
heads:--_Desiderata_; _Localities_; _Margins_; _Preceding and Succeeding
Accumulations of Pressure._

_Desiderata._--The most important desiderata appertaining to the subject
of storms, are certainly their origin and termination. Of these initial
and terminal points in the course of great storms we absolutely know
nothing, unless _the white appearance of a round form_ observed by Mr.
Seymour on board the Judith and Esther, in lat. 17° 19' north and long.
52° 10' west (see Col. Reid's 'Law of Storms,' 1st edit. p. 65), may be
regarded as the commencement of the Antigua hurricane of August 2, 1837.
This vessel was the most eastern of those from which observations had
been obtained; and it is the absence of contemporaneous observations to
the eastward of the 50th meridian that leaves the question as to the
origin of the West Indian revolving storms unsolved. Not one of Mr.
Redfield's storm routes extends eastward of the 50th meridian; this at
once marks out, so far as storms are concerned, the entire space
included between the 20th and 50th meridians, the equator and the 60th
parallel, as a most suitable area for observations, under particular
circumstances hereafter to be noticed, with especial reference either to
the commencement or termination of storms, or the prolongation of Mr.
Redfield's storm paths.

_Localities._--The three principal localities of storms are as
follows:--I. The western portion of the basin of the North Atlantic; II.
The China Sea and Bay of Bengal; and III. The Indian Ocean, more
particularly in the neighbourhood of Mauritius. The first two have
already been marked out as areas for the three-hourly observations; to
the latter, the remark as to extra observations under the head of
Desiderata will apply.

_Margins._--Mr. Redfield has shown that on some occasions storms have
been preceded by an unusual pressure of the atmosphere; the barometer
has stood remarkably _high_, and it has hence been inferred that there
has existed _around_ the gale an accumulation of air forming a margin;
barometers placed under this margin indicating a much greater pressure
than the mean of the respective localities. With regard to the West
Indian and American hurricanes--any considerable increase of pressure,
especially within the space marked out to the eastward of the 50th
meridian, will demand immediate attention. Upon the barometer ranging
_very high_ within this space, three-hourly observations should be
immediately resorted to; and if possible, _hourly_ readings taken, and
this is the more important the nearer the vessel may be to the 50th
meridian. Each observation of the barometer should be accompanied by an
observation of the wind--its direction should be most carefully noted,
and the force estimated according to the scale in page 21, or by the
anemometer. It would be as well _at the time_ to project the barometric
readings in a curve even of a rough character, that the extent of fall
after the mercury had passed its maximum might be readily discernible by
the eye. A paper ruled in squares, the vertical lines representing the
commencement of hours, and the horizontal tenths of an inch, would be
quite sufficient for this purpose. The _force_ of the wind should be
noted at, or as near to the time of the passage of the maximum as
possible. During the fall of the mercury particular attention should be
paid to the manner in which the wind changes, should any change be
observed; and should the wind continue blowing steadily in _one_
direction, but gradually _increasing_ in force, then such increments of
force should be most carefully noted. During the fall of the barometer,
should the changes of the wind and its increasing force indicate the
neighbourhood of a revolving storm, (independent of the obvious reasons
for avoiding the focus of the storm,) it would contribute as much to
increase our knowledge of these dangerous vortices to keep as near as
possible to their margins as to approach their centres. The recess from
the centre towards the margin of the storm, will probably be rendered
apparent by the _rising_ of the mercury; and so far as the observations
may be considered valuable for elucidating the connexion of atmospheric
waves with rotatory storms (other motives being balanced), it might be
desirable to keep the ship near the margin--provided she is not carried
beyond the influence of the winds which characterize the latter half of
the storm--until the barometer has nearly attained its usual elevation.
By this means some notion might be formed of the general direction of
the line of barometric pressure preceding or succeeding a storm.

Should a gale be observed commencing without its having been preceded by
an unusual elevation of the mercurial column, and consequently no
additional observation have been made; when the force of the wind is
noted in the usual observations at or above 5, then the three-hourly
series should be resorted to, and the same care taken in noting the
direction, changes, and force of the wind as pointed out in the
preceding paragraph.

The foregoing remarks relate especially to the central and western
portions of the North Atlantic; they will however equally apply to the
remaining localities of storms. Under any circumstances, and in any
locality, a _high_ barometer not less than a low one should demand
particular attention, and if possible, _hourly_ readings taken some time
before and after the passage of the maximum: this will be referred to
more particularly under the next head.

_Preceding and Succeeding Accumulations of Pressure._--Mr. Redfield has
shown in his Memoir of the Cuba Hurricane of October, 1844, that two
associated storms were immediately preceded by a barometric wave, or
accumulation of pressure, the barometer rising above the usual or annual
mean. We have just referred to the importance of _hourly_ observations
on occasions of the readings being _high_ as capable of illustrating the
marginal phænomena of storms, and in connexion with these accumulations
of pressure in advance of storms we would reiterate the suggestion.
These strips of accumulated pressure are doubtless crests of atmospheric
waves rolling forwards. In some cases a ship in its progress may cut
them transversely in a direction at right angles to their _length_, in
others very obliquely; but in all cases, whatever section may be given
by the curve representing the observations, too much attention cannot be
bestowed on the barometer, the wet and dry bulb thermometer, the
direction and force of the wind, the state of the sky, and the
appearance of the ocean during the ship's passage _through_ such an
accumulation of pressure. When the barometer attains its mean altitude,
and is rapidly rising above it in any locality, then _hourly_
observations of the instruments and phænomena above noticed should be
commenced and continued until after the mercury had attained its highest
point and had sunk again to its mean state. In such observations
particular attention should be paid to the direction and force of the
wind preceding the barometric maximum--and the same phænomena succeeding
it, and particular notice should be taken of the time when, and amount
of any change either in the direction or force of the wind. It is by
such observations as these, carried on with great care and made at every
accessible portion of the oceanic surface, that we may be able to
ascertain the continuity of these atmospheric waves, to determine
somewhat respecting their length, to show the character of their
connexion with the rotatory storm, and to deduce the direction and rate
of their progress.

Next: Seasons For Extra Observations
Previous: Localities For Additional Observations

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