Phenomena Of Revolving Storms

It is the object of the following pages to exhibit, so far as

observation may enable us, and in as brief a manner as possible, the

connexion, if any, that exists between those terrific meteorological

phænomena known as "revolving storms," and those more extensive and

occult but not less important phænomena, "atmospheric waves."

To the great body of our seamen, whether in her Majesty's or the

mercantile service, the subject can present none other than the most

interesting features. The laws that govern the transmission of large

bodies of air from one part of the oceanic surface to another, either in

a state of rapid rotation or presenting a more or less rectilineal

direction, must at all times form an important matter of inquiry, and

bear very materially on the successful prosecution of the occupation of

the voyager.

In order to place the subjects above alluded to in such a point of view

that the connexion between them may be readily seen, it will be

important to notice the principal phænomena presented by each. Without

going over the ground so well occupied by those able writers on the

subject of storms--Redfield, Reid, Piddington, and Thom--it will be

quite sufficient for our present purpose simply to notice the essential

phænomena of revolving storms as manifested by the barometer and vane.

The usual indications of a storm in connexion with these instruments are

the _falling_ of the barometer and the _freshening_ of the wind, and it

is generally considered that a _rapid_ fall of the mercury in the

hurricane regions invariably precedes the setting in of a storm.

There are three classes of phænomena that present themselves to an

observer, according as he is situated _on_ the line or axis of

translation, or _in_ either the right or left hand semicircle of the

storm. These will be rendered very apparent by a little attention to the

annexed engraving.

In this figure the arrow-head is supposed to be directed true north, and

the hurricane--as is the case in the American storms north of the 30th

parallel--to be moving towards the N.E. on the line N.E.--S.W. If the

ship take the hurricane with the wind S.E.,--the letters within the two

larger circles indicating _the direction of the wind in the storm_

according to the rotation as shown by the circle of arrow-heads, and

which it is to be particularly noted is in the northern hemisphere

_contrary to the direction in which the hands of a watch move_: in the

southern hemisphere the rotation is reversed--the only phænomena

presented by the storm are as follows:--The wind continues to blow from

the S.E., increasing considerably in force with the barometer falling to

a very great extent until the centre of the storm reaches the ship, when

the fury of the winds is hushed, and a lull or calm takes place,

generally for about half an hour, after which the wind springs up mostly

with increased violence, but from the opposite quarter N.W., the

barometer begins to rise, and as the storm passes off, the force of the

wind abates.

The point to which we wish particularly to direct attention in connexion

with this exposition of the phænomena attending the transmission of a

storm is this:--If the observer so place himself at the commencement

that the wind passes _from his left hand towards his right_, his face

will be directed towards the centre of the storm; and the wind

undergoing no change in direction, but only in force, will acquaint him

with this important fact that the _centre_ is not only gradually but

surely approaching him: in other words, in the case before us, when he

finds the wind from the S.E., and he places himself with his face to the

S.W. he is looking towards the centre, and the wind rushes past him

_from his left to his right hand_. Now the connexion of the barometer

with this phase of the storm is _falling with the wind from left to

right, the observer facing the centre while the first half is

transiting_.[1] During the latter half these conditions are reversed,

the observer still keeping his position, his face directed to the S.W.,

the barometer _rises_ with a N.W. wind, which rushes past him _from his

right to his left hand_ with a decreasing force. We have therefore _a

rising barometer with the wind from right to left during the latter half

of the storm, the observer having his back to the centre_.

The above _general_ enunciations of the barometric and anemonal

phænomena of a rotating storm hold good with regard to the _northern_

hemisphere, whatever may be the direction in which the hurricanes

advance. This may be placed in a clearer light, as well as the remaining

classes of phænomena shown, by consulting the following tables,

constructed for the basin of the Northern Atlantic, and comparing them

with fig. 1. In this basin, with lower latitudes than 25°, the usual

paths of the hurricanes are towards the north-west, in higher latitudes

than 30° towards the north-east. The tables exhibit the veering of the

wind with the movements of the barometer, according as the ship is

situated in the right or left hand semicircle of the hurricane. It must

here be understood that the right and left hand semicircles are

determined by the observer so placing himself that his face is directed

towards the quarter to which the hurricane is advancing.