45 THE OBSERVATION OF ATMOSPHERIC PRESSURE

174. ANEROID BAROMETER AND BAROGRAPH.

It was said in the earlier chapters on Basic Meteorology that the only meteorological instruments the average small boat navigator is likely to possess are an aneroid barometer and thermometers for measuring the air temperature, sea temperature and relative humidity. Of these, the barometer is unquestionably the most important, because pressure is the mainstay of the weather map and falling or rising pressure has a lot to say about the prospects of wind.

Although an aneroid barometer is not as accurate as a mercurial barometer as far as the absolute pressure value is concerned, it will, nevertheless, accurately indicate pressure tendency, which is of more importance to a navigator and, in any case, a mercurial barometer is too big and unwieldy an instrument to be accommodated in most small craft. The advantages of the aneroid are:-

• • • It is simple to read.
    • It is easily transported and may be placed or hung in any convenient position.
    • By adjusting the shorter pointer, by means of the metal thumb screw at the centre of the glass-face, it is readily apparent whether the barometer is rising or falling

We have previously stressed the importance of checking an aneroid barometer against a mercurial barometer at a nearby harbour office or Weather Centre, and have described how an aneroid barometer can be adjusted to the correct reading so obtained. On all occasions of checking the aneroid barometer, details should be entered on a check card attached to the instrument. This card, which can be drawn-up by the navigator and renewed as necessary, should be similar in layout to that shown in fig. 45-1. As pressure changes are relative, the units used are not important, but for accurate comparison when checking, it is advisable to buy an instrument calibrated in millibars, rather than inches.

When the aneroid barometer is kept in the cockpit of a small craft then, like the thermometer, it needs the protection of a strong box, so that hasty feet en route to shorten sail cannot wreck it. Of course, some nook or recess in the cabin will keep it safe, but with for’ard hatches closed and a following wind blowing down the companionway it will read a higher pressure than it should. The effect is not great, but it might just mask the first signs of a fall.

Before reading an aneroid barometer, gentle tapping of the glass-face with the pads of the fingers will ensure that any stickiness in the movement is shaken out. Barometers will oscillate with the pressure changes induced when a strong wind blows around the craft are usually small, but the fact should be taken into account.

Some owners may decide to invest in a barograph (fig. 45-2). 

This instrument is an aneroid barometer provided with a lever and pen which records variations of pressure on a chart attached to a revolving drum. The drum is driven clockwise, and makes one revolution in seven days. The paper charts (called barograms), which fit on the drum, are graduated to show the day and time of day, as well as the height of the barometer in millibars and inches. A barograph is useful, not only because it gives a continuous record of the barometric pressure for reference, but also because it can register minor fluctuations of atmospheric pressure, seldom detected in the action of either an ordinary aneroid barometer or a mercurial barometer.

Being somewhat delicate, a barograph must be kept in a position as free from shocks as possible. Barographs in small craft are best fitted as near amidships as possible, with the pen arm athwart the craft, so as to minimise variations induced by punching into, or being buffeted by, the sea. Such instruments also require dessicators to absorb the high humidity found in small craft.

The pressure of the pen on the paper is adjustable and should be reduced to a minimum, consistent with a continuous trace on the barogram. An ink bottle is supplied with the instrument and the pen should be filled when necessary (usually once a fortnight). The ink congeals after a time and the pen should then be washed in methylated spirit or water. The position of the pen can be adjusted to agree with a corrected mercurial barometer reading (as with the aneroid barometer), either by adjusting the height of the fulcrum of the principal lever, by means of the milled head screw on the central bridge at the top, or (in other types of instrument) by raising or lowering the point in the base plate to which the lowest of the set of aneroid boxes is fixed. Should it be necessary to make an adjustment of more than 5mb., the pen should be disconnected from its spindle by easing up the screw which secures the gate suspension to the spindle, setting the pen arm to the approximate reading, tightening up the screw and completing the operation by the fine adjustment described above.

When renewing or reversing the chart each week, it is customary to remove the drum by removing the clock key and unscrewing the milled nut which holds the drum in place, but, with care, the operation can be carried out with the drum in place, provided the stylus has been withdrawn from the chart by means of the near side of the bottom of the case. A time mark should be made before removing it. When fitting a new barogram, care must be taken the flange on the bottom of the drum. When clamping the two ends of the chart with the two spring clips, top and bottom of the drum, be sure that the last portion (end of the week) of the barogram comes on top of the first portion, so is not changed at the end of seven days, the pen will not catch on tear it, or damage itself. The drum should be placed approximately in position, i.e. nearly at the right time; a final adjustment can be made by gently rotating it the pen is nearly in position.

175. OBSERVING ATMOSPHERIC PRESSURE

While the average atmospheric pressure is 1013mb., the deepest Lows have central pressures around 940mb., and the most intense Highs push up abve 1040mb. The absolute limits may be 10mb. outside these values, but a range of 100 millibars is convenient to remember. So, the total variation is of the order of 10% of the total pressure, which is relatively small. Actual pressure, unless it is well under 1,000mb., or above 1020mb., has no real prognostic value. It is tendency which counts, and this should be the change noted over the previous three hours if it is to accord with normal practice.

The snag with a single barometer is that its message is about just one spot, and what is happening to the pressure in the thousands of square miles which are covered by pressure systems, such as Lows and Highs, cannot be gauged from a single barometric reading. Yet, a falling barometer is allied closely to the incidence of bad weather. The reason, stated simply, is that low pressure regions, where the air has a generally-ascending tendency, breed bad weather, while high pressure ones lead to subsiding air and clearing skies.

The average atmospheric pressure for the area in which a navigator is sailing can be obtained from the appropriate Admiralty Sailing Directions (Pilot Book), and from various meteorological charts published by the Met. Office. If the barometer reads 4mb. or more below the average for the time of year and is steady or falling, the chance of fair weather continuing for the next 24 hours is small. A low barometer indicates unsettled weather and, if it is falling, increasing wind force. If the barometer is 4mb. or more above the normal and is steady or slowly rising, fair weather will probably continue for the next 12 hours and if the pressure is 8mb. above normal and slowly rising, fair weather will continue for at least 24 hours. Generally speaking, a rising “glass” (as the barometer is familiarly called) indicates improving weather, and a falling one a deterioration but, if the speed of rise or fall is rapid, the change is likely to be short-lived, because you are probably near the path of a Low, behind it with a rapid rise and in front of it with a rapid fall. On the other hand, a slow steady fall indicates the approach of a Low and a warm front (see previous Chapters on ‘Basic Meteorology‘). The rate at which a barometer falls is not a reliable guide to wind strength, as the pressure will fall more rapidly if you are sailing towards a Low than if you are stopped or sailing away from it, and the Low is overtaking you; and also because you may be sailing along the line of an isobar with steep pressure-gradients on either side of you. Only when the tendency is 10 or more millibars per three hours can a gale be confidently anticipated. It is usually a falling tendency which presages a gale, but sharply-rising pressure can also precede a gale. The strongest winds may be engendered, not in the centre of a deep depression, but between that depression and its flanking High when the former is deepening and the latter intensifying at one and the same time.

Since pressure is only one of the elements affected by an approaching Low, the prudent seaman will not confine his attention solely to the barometer.

In the event, the only real way to forecast wind is with carefully drawn weather maps, for only then can the pressure variations over wide areas be assessed. However, some help for the lone forecaster is given in the preceding paragraphs, and in the next section.