Concerning the pressure of the atmosphere in Polar latitudes, I would remark, particularly in the winter and spring months, it is liable to sudden and very considerable variations, and a careful study and observation of these is necessary to enable the watchful mariners to anticipate the approach of storms.
The following are the relations which, in Polar latitudes, I have been enabled to trace between the barometer and the weather:—
1. A hard westerly gale, with snow, occasions the greatest depression of the mercury; and a light easterly wind, with dry weather, the greatest elevation.
2. The rising of the mercury foretells the subsidence of wind or rain, a change of wind or fine weather; and its falling, rain, snow, or a change or increase of wind.
3. The mercury rising unusually high, and then becoming stationary, indicates, in the months of April and May, a continuance of fine weather; but in June or July, foggy weather.
4. If, in the month of April, the mercury fall with some rapidity an inch or more, a storm will most certainly succeed, however contrary appearances may be, which will probably be the more severe in proportion as it approximates the east, and will frequently continue, with unabated violence, for fifty or sixty hours.
5. The rising of the mercury usually precedes the cessation of a storm, but does not invariably determine the period of its continuance, as storms frequently blow for a day or two after the first rise of the mercury.
6. Sudden and repeated fluctuations are indicative of unsettled weather; but the rapid fall of the mercury is no indication of a short gale, though, in other regions, the reverse is said to be the case; for, before storms that continue two or three days, the barometer frequently falls an inch within twenty-four hours; and indeed, in a gale as long and as heavy as I almost ever witnessed, the fall of the mercury was above an inch in twelve hours.
7. Before very heavy storms, when the barometer falls uncommonly low, the mercury seems to get below its natural level, and often rises two or three tenths of an inch as soon as the predicted storm commences; hence this first rise of the mercury is no indication whatever of an abatement of the wind.
8. On account of the different states of the barometer in west and east winds, the usual level of the mercury, with a moderate wind at west, not being much higher than with a gale at east, a change of wind from one of these quarters to the opposite may be accompanied with the greatest alteration in the strength of the wind, without producing any effect on the barometer.