Thirdly, by the force of the wind. If a wind freshens, the moisture and direction of the wind remaining the same, the glass will fall.
These three causes do not often act in accord; one is generally affecting the glass in a way opposite to the other two. It is for this reason that an observation of the barometer alone will often mislead us. It must be read in conjunction with the thermometer and also with the hygrometer, in order to determine the true cause of the rise or fall of the mercury.
Admiral Fitzroy’s two well-known rules are—
The Barometer Rises for northerly wind (including from N.W. by the N. to E.), for dry or less wet weather, for less wind, or for more than one of these changes; except on a few occasions, when rain (or snow) comes from the N. with strong wind.
The Barometer Falls for south wind (including from S.E. by the S. to W.), for wet weather, for stronger wind, or for more than one of these changes; except on a few occasions, when moderate wind with rain (or snow) comes from the northward.
The following rules are selected from the official computation, which is very comprehensive and should be studied by every yachtsman. Admiral Fitzroy’s book should be on every yacht’s library shelf.
If the barometer has been at its ordinary height—about thirty inches at sea-level—and is steady or rising, while the thermometer falls, and dampness lessens, N.W., N., or N.E., or less wind may be expected.
If the barometer is falling, the thermometer rising, and the dampness increasing, wind and rain, or snow may be expected from S.E., S., or S.W.
The most dangerous shifts of wind and the heaviest gales from N. happen after the mercury first rises from a very low point.
A rapid rise of the barometer indicates unsettled weather; a slow rise, or steadiness with dryness shows fair weather.