One of the duties of an officer of the deck is to keep the captain advised of the weather changes, and in particular of any appreciable rise or fall of the barometer. It is just as true now as it ever was that a rapidly falling or rising barometer indicates a change of weather conditions. However, an analysis of the causes of rate of pressure change may cause quite different interpretation of its significance.
The different factors to be considered in such an analysis are: (1) the diurnal variation of the barometer, (2) the general barometric pressure distribution, and (3) the course and speed of the ship, if under way. Before going any farther it might be well to define the diurnal pressure change. As with the tides, having a high and low twice a day, so too with the barometer, normally it has two maximum readings and two minimum readings, but, unlike the tides, with the high and low tide occurring at different hours on consecutive days, the barometer attains its maxima and minima at nearly the same time each day. The time of the maxima is usually about 10:00 a.m. and 10:00 p.m. and that of the minima 4:00 a.m. and 4:00 p.m. The amount of change for different stations has been computed by taking the mean hourly readings for a long period. The general pressure distribution of course refers to the location of the high and low pressure areas as shown on a weather map.
To best show how the various factors enter into an analysis of the pressure change, an example will be given for the various conditions.
Condition 1.—Ship at anchor, weather conditions continuing unchanged.
A typical set of the barometer readings would show the lowest pressure at about 4:00 a.h., then the 10:00 a.m. reading would be about .05 inch higher than the 4:00 a.m., the barometer having risen steadily; by 4:00 p.m. the reading should have dropped .04 inch; when again it should start to rise giving another maxima at about 10:00 p.m. about .02 inch higher than the 4-:00 p.m. reading to be followed by another fall of about .03 inch by about 4:00 a.m.
Note: The above numerical values are taken from the records for San Diego, California. They are slightly different in other localities.
Condition 2.—Ship steaming directly towards the center of a low pressure area center stationary.
Assume there is a center 600 miles due west of the ship, and in this the barometer reads .60 inch lower than the ships barometer, also the ship is steaming west at 10 knots.
Expected pressure changes:
At 10 knots, the ship should steam the 600 miles in 60 hours, and the barometric pressure should drop .60 inch divided by 60, or .01 inch per hour or .06 inch in 6 hours.
Condition 3.—Same as condition 2, except the ship is steaming away from the center of low pressure.
Comparing conditions 2 and 3 it is quite obvious that the rate of rise when steaming away from a low pressure center is not the same as the rate of fall when steaming towards one.
Condition 4.—Same as condition 1 (ship at anchor) except that the low pressure center is moving east (directly towards you) at 20 knots.
At 20 knots it will take 30 hours for the center to reach you, or the rate of fall will j>e .60 inch divided by 30 or .02 inch per hour, or .12 in. in 6 hours.
Condition 5.—Same as condition 4, plus ship moving directly towards center at 10 knots.
Condition 6.—Same as condition 5 except ship is moving directly away from the center at 10 knots.
Another set of conditions may be set up for the center moving away, or moving at an angle to you, but it is felt that the above are enough to illustrate the main points sufficiently and will serve to show
that when a pressure change is noted, that unless the three factors stated above, diurnal pressure change, general pressure distribution, and the course and speed of the ship, are considered the full significance of the report will not be properly appreciated and cannot be intelligently interpreted. Also any steady pressure drop between approximately 4:00 A.M. and 10:00 a.m. and 4:00 p.m. and 10:00 p.m. means a whole lot more than if the drop occurs during the two other 6-hour periods of the day—10:00 a.m.-4:00 p.m., and 10:00 p.m.-4:00 a.m.