38 THE ATMOSPHERE, PRESSURE AND WIND

139. THE ATMOSPHERE

The atmosphere, or air, covers the whole surface of the Earth in the form of a film over 100 miles in depth although over half the total weight of this air is within 3½ miles of the Earth’s surface. It consists mainly of the gases Nitrogen (75%) and Oxygen (23%), together with a few other rare gases. These gases have no effect on the weather, but some other bodies present in the air, Such as water vapour, salt, and certain products of combustion, have a considerable effect.

The amount of water vapour in the atmosphere is always a variable quantity as it is increased by evaporation and decreased by rain and dew, and this accounts to some extent for the wide variations of weather experienced at any particular place. The temperature of the atmosphere falls off with height above the Earth’s surface at an average rate of about 1.7°C. (3°F) per 300 metres (1,000ft.), so that at a height of 3050 meters one can expect the temperature to be about 16.7°C (30° F) below that at sea level.

At a height which varies from about 9km. (5 miles) over the Poles to about 18km. (10 miles) over the Equator, temperature stops falling with height and thereafter there is little material change of temperature with height. The level at which the temperature stops falling is known as the tropopause, and it separates the atmosphere into two distinct layers, known as the troposphere below and the stratosphere above (see fig. 37-1).

Because the tropopause is considerably higher over the equator than it is over the poles, the lowest temperature in the atmosphere occurs in the stratosphere of equatorial regions, despite the high surface temperatures.

The troposphere contains nearly all the “weather” as we understand it, namely clouds, precipitation, and winds.

140. ATMOSPHERIC PRESSURE

The atmosphere, or air, does not escape into space because the gravitational force of the Earth is sufficient to retain it. Air, although light, has a sensible weight, and the resulting pressure on a given area can be measured accurately with a mercurial barometer. A column of air equal in height to that of the atmosphere (about 240km. or 130 miles), resting on one square inch of the Earth’s surface at sea level weighs just under 151b. It is, however, customary to use an international or metric unit of pressure known as a millibar (mb.), which is roughly equivalent to 1 gram per square centimetre. The average atmospheric pressure at sea level is 1013 Mb. Because the pressure of the atmosphere is due to the weight of air immediately overhead, and as one ascends some of the air is left below, it is evident that the pressure must decrease with height above the ground, and it does so at the rate of about 34 MB for every 300 metres (1,000ft.) of ascent near the Earth’s surface.

A decrease of pressure causes air, like any other gas, to expand or occupy a greater volume so that there is, for example, less air in a cubic foot at 300 metres above sea level than there is at sea level in other words the density of the air varies with the pressure.

Measurements of atmospheric pressure made regularly at a given place for a long period of time show that it is seldom constant for Longitude From the above description of pressure it will be seen that a change of atmospheric pressure at one place can only be produced by a change in the weight of the atmosphere above that place, and this, in turn, can be produced only by the addition of, or removal of, air from a column of fixed cross—section above that place. This can occur (a) by expansion or contraction resulting from the heating or cooling of the atmosphere, or (b) by means of air passing into the column at a different rate from that at which it is passing out. Such changes of pressure with time and place are closely associated with wind and weather and their changes, and for this reason, atmospheric pressure is one of the most important factors in weather forecasting, and the barometer is the most important instrument.

The atmosphere, or air, does not escape into space because the gravitational force of the Earth is sufficient to retain it. Air, although light, has a sensible weight, and the resulting pressure on a given area can be measured accurately with a mercurial barometer. A column of air equal in height to that of the atmosphere (about 240km. or 130 miles), resting on one square inch of the Earth’s surface at sea level weighs just under 151b. It is, however, customary to use an international or metric unit of pressure known as a millibar (Mb.), which is roughly equivalent to 1 gram per square centimetre. The average atmospheric pressure at sea level is 1013 MB. Because the pressure of the atmosphere is due to the weight of air immediately overhead, and as one ascends some of the air is left below, it is evident that the pressure must decrease with height above the ground, and it does so at the rate of about 34 MB for every 300 metres (1,000ft.) of ascent near the Earth’s surface.

A decrease of pressure causes air, like any other gas, to expand or occupy a greater volume so that there is, for example, less air in a cubic foot at 300 metres above sea level than there is at sea level in other words the density of the air varies with the pressure.

Measurements of atmospheric pressure made regularly at a given place for a long period of time show that it is seldom constant for Longitude From the above description of pressure it will be seen that a change of atmospheric pressure at one place can only be produced by a change in the weight of the atmosphere above that place, and this, in turn, can be produced only by the addition of, or removal of, air from a column of fixed cross—section above that place. This can occur (a) by expansion or contraction resulting from the heating or cooling of the atmosphere, or (b) by means of air passing into the column at a different rate from that at which it is passing out. Such changes of pressure with time and place are closely associated with wind and weather and their changes, and for this reason, atmospheric pressure is one of the most important factors in weather forecasting, and the barometer is the most important instrument.

141. THE ANEROID BAROMETER

The mercury barometer is about a metre in length and, therefore, too cumbersome an instrument to install in a small boat, but the aneroid barometer (fig. 38-1) is much more compact and although slightly less accurate than the mercury type, quite adequate for marine use if properly maintained and adjusted. Its essential part is a small capsule or concertina-like box, partially exhausted of air, which is squeezed or stretched as the atmospheric pressure rises or falls. The box is connected by a system of levers to a pointer and dial, which indicates the pressure.

As his aneroid barometer is the most important meteorological instrument a mariner possesses, and one on whose accuracy he may have to depend, he should check it fairly frequently against the corrected readings of a mercury barometer. This can be done free of charge through any Port Meteorological Officer and in many other places, a list of addresses for all these being given in Reed’s Nautical Almanac. This check should certainly be made before sailing if the barometer has not been checked for some time. The moving pointer of an aneroid barometer can be adjusted by means of a screw at the back of the instrument. To make this adjustment, the aneroid should be in the place where it is to be used, and the correct atmospheric pressure (corrected to sea level) obtained from either a nearby ship or Met. Office. Set the aneroid to this reading by turning the adjusting screw at the back; this adjustment should be limited to a maximum of about 5 millibars – for larger adjustments, the glass should be removed and the pointer (which is a push-on fit) removed and re-set near the correct reading, final adjustment then being made with the screw at the back. As long as it remains at the same height, the aneroid should now continue to read the correct sea level atmospheric pressure.

When buying an aneroid barometer care should be taken to obtain one with the dial graduated in millibars. Some domestic aneroids are marked only in inches.

Should you have to use an old aneroid marked only in inches, tables for converting atmospheric pressure in inches to millibars will be found in most nautical tables and nautical almanacs.

The ‘weather-glass’ legends of ‘Very Dry‘ to ‘Stormy’ are based on long observation of the type of weather associated with a barometer reading, but only tendency is truly valuable. So long as the capsule is not punctured or the linkage broken or sticking, then the absolute correctness of an aneroid barometer is of little consequence because it will still show tendency, if the readings are recorded every hour or so.

Barometric Tendency is the change in the barometric pressure with time, usually restricted to the change in the three hours preceding an observation. The symbols shown in fig. 38-2 are used on weather charts to indicate the barometric tendency.

142. PRESSURE DISTRIBUTION AND PRESSURE SYSTEMS

If atmospheric pressures are taken simultaneously at a number of different places, corrected to what they would be at sea level for uniformity and marked on a chart, a fair estimate of those at intermediate places can be made and lines can be drawn connecting places where the pressure is the same. Such lines of equal pressure are called isobars. A chart for a particular time which includes data such as weather, wind, visibility, cloud and isobars for a number of observing stations is termed a weather map, or more frequently a synoptic chart, because it gives a synopsis or general view of the weather conditions over a large area at a given instant of time.

Fig. 38-3 shows the common pattern of isobars corrected to mean sea level. It is often found that some of the isobars drawn to fit the values form closed curves around centres of either high or low pressure. A centre of high pressure is known as an Anticyclone or High, whilst a centre of low pressure is termed a Depression or Low.

An outward extension in the isobars from a centre is called a ridge or wedge if from a High, and called a trough if from a Low. An area of almost constant pressure (and, therefore, few isobars) between two Highs and two Lows is known as a col.

143. RELATIONSHIP BETWEEN PRESSURE AND WIND

The vertical movement of air, although not felt on the Earth’s surface, is evident in clouds, and causes the bumping often experienced in aircraft. Warm air rises and cooler air flows in to take its place (fig. 38-4). This cooler air has large horizontal components which flow over the surface of the Earth and which we call wind. Thus, it will be seen that the primary cause of wind is a difference in temperature, although this, in turn, can cause a difference in atmospheric pressure. It is the relationship between wind and atmospheric pressure in the horizontal plane which is the major factor in forecasting wind movement and strength. Air tends to flow from an area of relatively high pressure to an area of low pressure. Theoretically, it should take the shortest route and flow directly towards the centre of low pressure at right angles to the isobars (fig. 38-5(a)), but in fact, it blows almost directly along the isobars with the low pressure to the left in the northern hemisphere and to the right in the southern hemisphere. Fig. 38-5(b) shows a weather map covering part of the northern hemisphere with isobars drawn and with winds plotted by using wind arrows in a conventional way.

Each wind observation is represented by a shaft drawn from the observing point in the direction from which the wind is coming, and ‘feathers’ indicate speed, one whole feather representing ten knots and half a feather five knots. From this figure, it is apparent how the wind blows nearly parallel to the isobars with the lowest pressure to the left of the direction of motion, and also that the wind speed is greatest where the isobars are most closely spaced. These two factors will now be explained.

On account of the centrifugal (or Coriolis) force of the Earth’s rotation, air which is drawn in towards a centre of low pressure is deflected to the right in the northern hemisphere, and a circulation is set up in an anticlockwise direction about the centre of low pressure. Similarly, a clockwise circulation is set up around a centre of high pressure in the northern hemisphere. In the southern hemisphere, these directions are reversed. The effect of this deflection is to cause the wind to blow parallel to the isobars because the deflecting force varies with the speed of the wind and is exactly equal, and opposite, in direction to the pressure force (as shown in fig. 38-5(a)).

Secondly, the closer the isobars are together the greater the pressure gradient (or the rate of change of pressure at right—angles to the isobars), and, therefore, the pressure force and the speed of both the wind and the deflecting force. The theoretical wind direction and force resulting from the calculation of these factors is called the geostrophic wind, and weather maps used by meteorologists have a geostrophic wind scale from which they can measure the speed of the wind for a given distance apart of isobars in any given latitude.

In practice, however, the surface wind (below about 460 metres (1,500ft.) above sea level) is only about two-thirds of the geostrophic wind speed over the sea and about half of it over land, while the direction of the surface wind is inclined to the isobars at an angle of 10° to 15° over the sea and 20° to 30° over land, in both cases towards the low-pressure area. This is due to the frictional effect of the Earth’s surface, and the land being generally rougher, it drags the lower layers of air back more than does the sea.

The relationship between the direction of wind and isobars was first put into the form of a law by the Dutchman, Professor Buys Ballot of Utrecht, in 1857. Buys Ballot’s Law states:

Stand facing the true wind and the centre of Low pressure will be between 90°and 130° on your right hand in the northern hemisphere, and on your Left hand in the southern hemisphere.

It follows from this law that in the northern hemisphere, winds form a clockwise circulation around an anticyclone but anticlockwise around a depression. Buys Ballot’s Law only applies to the middle latitudes of either hemisphere and cannot be used with local winds such as land and sea breezes, and mountain and valley winds (see ‘Local Weather‘ – 6 of this Manual).

144. THE MEASUREMENT OF WINDS 

Wind direction is always quoted as the direction from which the wind blows, since an observer is basically concerned with the type of weather the wind is bringing with it, measured in relation to True North and usually quoted as a compass point thus:- NW, SSE, W, etc.

Two terms are used to describe changes in wind direction. When the direction of the wind changes in a clockwise direction the wind is said to veer. When the direction of the wind changes in an anticlockwise direction the wind is said to back (see fig. 38-6).

Wind strength, as described above, depends on the pressure gradient which is related to the perpendicular distance between the isobars. Isobars spaced closely together indicate a steep pressure gradient which gives rise to strong winds, while widely-spaced isobars indicate a slack or weak pressure gradient and relatively light winds. The international unit of wind speed is the meter-per-second, but in many countries, including the United Kingdom, the nautical mile per hour, or knot, is widely used. The strength of the wind by and for seafarers, however, is usually given in the Beaufort Scale (see fig.38-7).

The Beaufort Wind Scale, originally devised by Admiral Sir Francis Beaufort in 1808, allots numbers to various wind strengths from 0 (calm) to 12 (Hurricane), and describes the state of the sea (and the range of the wind-speed in knots) roughly corresponding to each number in the scale. Mariners throughout the world use the Beaufort Scale of Wind Force to estimate wind strength, as there is no more accurate means of determining it (anemometers at sea do not register the true wind speed owing to the effect of the vessel’s own motion). Accurate estimates of the Beaufort wind force are only acquired by experience; there is a tendency for inexperienced observers to over-estimate the force of light winds and to under-estimate the strong ones, and to make insufficient allowance for the wind created by their own vessel’s movement.

The Pressure Gradient is the change in pressure per unit distance, the distance being measured at right-angles to the isobars. For example, in fig. 38-8, point B is 300 miles from point A, but their distance apart (AC to BD) when measured at 90° to the isobars is only 100 miles and, since the pressure difference between the two places is 3mb., the pressure gradient is 3mb., per 100 miles.

An indication of wind speed at sea in the middle latitudes from the isobaric spacing on a weather map can be derived from the rough rule that a pressure gradient of 1mb. in 15 nautical miles gives a surface wind speed of about 24 knots. Translating this into the terms of the Beaufort Scale and to the most usual 4mb isobars used on weather maps would give rise to the very rough guide given in scale below: –

Distance apart of 4 mb.         Wind Force

    Isobars in miles               (Beaufort Scale)

  45                                      8

  60                                      6

  90                                      4

180                                      2

360                                      1