Chapter Twelve CHAPTER TWELVE Isobaric Patterns Out of seven characteristic isobaric patterns there are only two types of weather systems which are fundamental-the depression and the anticyclone-the remainder being either outward extensions from one of these or a neutral area between them These seven distinctive isobaric forms are: Depression Anticyclone Secondary depression Trough Wedge or Ridge Depressions, fronts and anticyclones are Col discussed in greater detail in later chapters Straight Isobars DEPRESSION (or LOW) An area of low barometric pressure surrounded by an area in which the pressure is relatively high The isobars are roughly circular or oval in shape and, in accordance with Buys Ballot's Law (see Chapter 9) the wind flows in an anticlockwise direction round the area of low pressure in the northern hemisphere (see Figure 12.1) and clockwise in the southern hemisphere It should be noted that in both hemispheres the surface wind flows slightly in towards the central area (see Convergence in Appendix 1) where the worst weather is usually encountered Depressions are of greatly varying intensities and are usually associated with bad weather-Leo much cloud and precipitation with strong or gale force winds, especially near the centre of the system, the severity of the weather being governed mainly by the steepness of the pressure gradient and the moisture content of the surface air, but there are other important factors involved Despite the availability of official weather forecasts it is important for a mariner to be able to recognise the precursory signs of bad weather and to know the weather sequence, shift of wind, etc which may be expected with the approach, passage and retreat of a well defined depression Indeed, the safety of his ship may well depend on such knowledge and his ability to act on it 104 Chapter Twelve The term depression is commonly applied to cyclones in latitudes which lie outside the tropics but may also be used to describe a weak tropical cyclone As a depression develops the pressure gradient becomes steeper and the winds stronger, isobars on each successive weather chart are drawn closer together and the depression is said to deepen A weakening or dying depression is said to be filling up Depressions tend to move towards areas of low or falling pressure and to steer round high pressure regions ANTICYCLONE A region of high pressure surrounded by an area of relatively low pressure The isobars are roughly circular or oval in shape In the northern hemisphere the wind circulates in a clockwise direction round the centre of high pressure: the converse is true for the southern hemisphere (See Figure 12.2) General characteristics The pressure gradient is slight, winds are light near the centre Weather is usually quiet dry and settled Land and sea breezes (see Chapter 9) are marked especially during the warmest months of the year In summer the weather is generally dry, sunny and warm but within the outer portions of the system it is often cloudy with some rain In winter the weather may be one of two types: (a) Cloudless sky with sharp frosts at night or radiation fog (b) The sky completely covered by stratus cloud Dull, cold and foggy or misty weather may persist for some days (See Anticyclonic gloom in Appendix 1) High pressure systems are usually slow moving by comparison with other systems and often remain stationary for long periods An anticyclone is said to intensify as the pressure within the system rises, whereas when pressure falls thereby weakening the system, it is said to decline A SECONDARY DEPRESSION A secondary depression is one which forms within the isobaric pattern of another (primary) depression When the primary depression is old and filling up, the secondary may develop and deepen till it completely absorbs all traces of the primary The secondary in Figure 12.3 has a steeper gradient and lower pressure at its centre than the primary or parent depression Note that secondary depressions often develop into much more vigorous systems than their primaries 105 Chapter Twelve TROUGH OF LOW PRESSURE This is distinguished on the weather chart by a system of isobars which appear sharply curved (concave towards low pressure) along a line called the trough line within a depression (See Figure 12.6) The wind circulation, anticlockwise in the northern hemisphere, is indicated by the arrows In the southern hemisphere the wind flows clockwise round the low pressure centre Note that the isobars are closer together near the centre The wind circulation, clockwise in the northern hemisphere, is indicated by the arrows Note that the isobars are more widely spaced near the centre of high pressure and that the surface wind tends to flow outwards from the centre 106 Chapter Twelve A trough may be termed deep or shallow according to whether the curvature of the isobars is acute or gentle, respectively The weather associated with a trough is generally cloudy with precipitation (See Line squall in Appendix 1) Fronts Fronts, which are dealt with in later chapters, are all troughs; but a trough is not always a front When the isobars of a depression or a tropical cyclone are circular the term trough refers to a line drawn through the centre of the system at right angles to the line of progression of the centre RIDGE (OR WEDGE) OF HIGH PRESSURE A wedge shaped extension of an anticyclone between two areas of low pressure (See Figures 12.4 and 12.6) The isobars assume their greatest curvature along the axis of the ridge It is generally associated with the fair weather of the anticyclone, often having light winds along the central portion A ridge in which the isobars are sharply curved generally moves faster than a "flat" ridge 107 Chapter Twelve 108 Chapter Twelve COL An area of indeterminate pressure located between two highs and two lows which are arranged alternately (See Figures 12.5 and 12.6) It is generally associated with light variable winds, often thundery in summer and dull or foggy or misty in winter In Figure 12.5 the wind circulation round the four systems is shown by arrows It is easy to see why the winds are variable within the col area STRAIGHT ISOBARS An atmospheric pressure distribution in which the isobars run in more or less parallel straight lines across a large area (See Figure 12.7) It is usually the outlying portion of a large and distant depression or anticyclone 109 Chapter Twelve QUESTIONS Name the seven characteristic isobaric forms Define the terms depression and anticyclone (a) Sketch the isobars, fronts and wind circulation in a typical southern hemisphere depression (b) What are the two main factors which govern the severity of the weather in any middle-latitude depression? In which direction depressions tend to move? Describe the general characteristics of an anticyclone: (a) In summer (b) In winter Describe the weather associated with a co!: (a) In summer (b) In winter What mainly governs the weather conditions to be expected in a system of straight isobars? 110