The Four p.m. REPORT

Radio Record, Volume IV, Issue 19, 21 November 1930, Page 1

The Four p.m. REPORT

The Weather "Man

Unravels those puzzling "B's" and "OD’s" and > Tells you what the Forecast Means PLAIN ENGLISH

E are still frequently asked the meaning of such terms as "cyclone" and "anticyclone." Even the expressions "districts with a westerly aspect" and "milder temperatures" appear to cause some difficulty, although it seems to us that there should be no ambiguity about them. It must be confessed that our forecasts are expressed in somewhat stilted and technical language. No doubt some of the stiltedness is due to the fact that after preparing fore-

casts day-in, day-out, on Sundays, week-days, and holidays, for a number of years in succession, one loses the first freshttess of one’s interest in the weather as a subject of conversation and is liable to sink into routine modes of expression. But far more "important than this is the fact that the number of words we can use is strictly limited. Our reports have to be telegraphed to different parts of the country, and that takes time and costs money. Consequently, in our efforts to try to get the maximum amount of information into the smallest compass, we have to use technical and semi-technical terms and a condensed style. In this respect we are at a disadvantage as compared with the local weather prophets who flourish in some of our cities. These gentlemen have the benefit of our reports, they are able to issue their forecasts some hours later than we, and, with the support of a local paper, are able to spread themselves of a large area of print. ,

i will try, then to describe some of the more important processes and principles of weather forecasting, and in doing so elucidate a number of technical terms. Weather forecasts are based on a knowledge of conditions in the surrounding fas on all sides, and the greater the area over rhich we know the weather conditions and the greater the detail of our information, the better we are pleased. The reports come to us by telegraph, cable and wireless message. In New Zealand we receive, in addition to local mesSages, reports from about a dozen places in Australia, from Lord Howe Island in the North Tasman Sea, from Norfolk and Chatham Islands, and from a varying number of ships at sea. As the reports come in they are written on to maps which are specially prepared for the purpose. The direction and speed of the wind is shown by arrows which, as they are drawn .on the map, fly with the wind. The point of the arrow lies in the position on the map cor-

responding to the place from which the report comes. The barometer reading, or air pressure, at sea level, the temperature, and the weather are also shown on the map. The next thing is to draw’ the isobars, or lines of equal pressure. The isobar for 30.0 inches, for instance: passes through all points at which the pressure is 30.0

inches. On one side of it barometers would read higher than 30.0 inches, and on the other lower. Isobars are drawn for each tenth of an inch. When they are drawn, it will be found that they do not run. haphazardly over the map, but are fairly regular and tend to curve round certain areas on the chart where the pressure is higher or lower than its surroundings. These areas are usually marked "High" or "Low" on the map, and, indeed, some weather services speak of them as "Highs" and "Lows." In describing and defining isobars, they are often compared with contour lines, or lines of equal height on an ordinary map. According to this analogy an area of high pressure would correspond with a mountain, a mountain, it is true, of huge extent and of gentle slopes. The low pressure area would correspond with a valley or depression in the surrounding country. An anticyclone is, then, merely one of these

pressure mountains, while for the low-pressure areas meteorologists use the same term "depression" as used when referring to low-lying land areas. Now, a very pronounced feature of the average condition of the atmosphere in the Southern Hemisphere is a continuous ridge of high-pres-sure surrounding the globe in sub-tropical latitudes, with a corresponding valley or trough of. low pressure in the sixties of latitude: or just north of the Antarctic Circle. There is thus a downward slope of pressure over New Zealand from north to south. This slope is not a uniform slope, however, but is seamed with valleys or broken by basins of low pressure. The valleys open to the south into the great trough in subAntarctic regions and are widest at their southern ends. The isobars consequently . take the shape on the map of inverted V’s. Depressions of this kind are, therefore, sometimes called V-depressions. (Continued on page 2.)

This is what IIOCTOR KIDSON told 2YA Listeners

(Continued from page 1.) The basins are what we call "cyclones." They are shown on the weather chart’ by a series of isobars surrounding a centre of low pressure. It is worth while distinguishing between the valley and the basin, the Vdepression and the cyclone, that is, becatise they have very different types of weather asSociated with them. But our pressure hills, basins, ‘and valleys differ ‘from the’ corresponding land features. in one important characteristic, that of movement. It is true that there is a scriptural "reference to "valleys being exalted, and hills being laid ‘low’; but in spite of recent experiences in connection with the Murchisom earthquake, the process is usually a slow one. I did once read a-story of a vessel which, in order to avoid the hazards of the sea, took:-to the land, but had the misfortune to strike a landstorm, in which the mountains heaved and sank, and rocks came aboard in quantities. The authenticity of that story has not, however, yet been proved. But there is no doubt that: our pressure systems,’ both. anticyclones and depressions, can and do move with considerable rapidity. The average speed, infact, is something like 600-miles a day. At this time of year it is more than that, which accounts for the rapidity of the weather changes.. Three of‘these V-depressions. have passed us this week. . There is, however, fortunately: for the forecaster, one important restriction on the movement of pressure systems, and that is that ‘in these latitudes they are practically always moving from west toward east. It may be from some direction rather north or south of west, or’even,; in the northern portion of our region, from due north, but never from the east. Since the pressure systems carry their weather with them, a knowledge of the rate of movement from the west naturally allows us to anticipate the weather. changes they will bring. The value of this prineiple to the forecaster is, however, limited, because the rate of movement, especially of the depression, is irregular and the ‘shape and the accompanying weather is constantly changing. Two Important Principles. BEPORE going any further, I must explain two other important principles which aid the forecaster. These refer to the relationship between wind and pressure, or, what is the same thing, wind and the isobars. The first, which is often referred to as the "law of storms," is that, if you stand with your back to the wind, you will have low pressure on your right and higher on your left. That is in the Southern Hemisphere; the rule is reversed for the Northern Hemisphere. Using, again, the analogy between pressure and altitude, we may say that instead of flowing down a ‘slope, the air flows across it, with the downward side to the right. As a matter of fact, the flow is not quite directly across .the slope, but slightly ‘downhill. In other words, the wind blows nearly parallel to the isobars, crossing them from high toward low pressure at a slight angle. The other principle is that the steeper the slope, the more rapid will be the -flow,or the steeper the pressure gra. dient, the stronger the wind. Where -the pressure gradient is steep, the iso bars will be close together. Therefore the closer the isobars the stronger the wind, These two principles account fo:

most of the characteristics of antieyelones and depressions. An Antieyclone. AS a result of the first of these rules, the wind tends to blow round an anticyclone in a direction opposite to that: of the movement of the hands of a clock, while round a depression the, movement is clockwise. The mountains of New Zealand, of course, deflect the wind very considerably, and this accounts for local irregularities of both wind and weather. Turning again to our pressure moun: | tain, the anti-cyclone, we find that it has a broad and gently-sloping summit, so that the winds are usually light and variable and the weather, though sometimes cloudy, is fine about its centre. We do not like a very high-pressure mountain or, as we call it, an intense anti-cyclone, however, in New. Zealand, because it usually has. a steep slope. on some side leading to a deep depresison of some kind at the bottom. Very often this depression is of the basin form, or a cyclone, and we get some of our worst weather and our heaviest rainfalls from it. This applies particularly to the eastern: districts of New Zealand. At this time of year the anti-cyclone centres, with the accompanying fine weather, are too far north for us to get much benefit from them, We pass rapidly from one depression to another. Passing on to the V-depression or pressure-valley, as has been explained, it is usually a valley in a general slope to the southward. ‘The general slope gives the winds a westerly tendency, according to the law of storms. (It is, in fact, the origin of the winds of the "roaring forties.") The prevailing winds, therefore, are north-west, west and south-west. Owing to this fact and to the regular movement of the depressions from the westward, they are sometimes called "depressions of the westerly type." They are our commonest type of storm. The valley of our V-depression runs, usually, in a more or less north-west to south-east direction rather than due north and south. The course of events during the passage of one of these denressions is somewhat as follows: At some stage we will be in an area of light winds and fine weather in an anti-cyclone or on a ridge connected with one. Then, as this moves away eastward, we get on to the slopes of the valley of the. V-depression, The winds will begin to blow from the north, Coming from the north,- they

will tend to be warm and, since the previous weather will have been cool or cold, we forecast "mild or milder The strength of the northerlies will depend on the steep-

ness of the valley slope. More or less rain falls,as we descend the slope, especially in’ ‘western districts. Bx the more’ nearly due north and sout the valley runs and the deeper it is; the niore general and heavy will be the rain.. If the valley is wide open to the south, so that the V of the isobars is very blunt, the westerly component in the winds will be unusually marked. Rain will, on these occasions, be confined mainly to those districts which meet. the westerly winds directly, -or "districts with a westerly -aspect." The steeper the slope presented to the wind ‘the heavier will be the rain. Rain of this kind, therefore, is heaviest on the: western side of the ranges of the South Island. It is erratic at Wellington and in the Taranaki Bight, fairly heavy on the high countr¥round Mount Egmont and extending eawijward to Tongariro and Ruapehu and ‘northward to Raglan, and, if it extends so far northward, also on the higher parts of the Auckland Peninsula. Frequently the rain does not extend north of Taranaki. In the eastern districts, "especially in Hawke’s Bay and Canterbury, warm and dry conditions are experienced in this weather of the pronounced ‘westerly type. Squally Weather. TEIN we reach the bottom of the V- _, depression we come suddenly on the upward slope, the rising country, of ‘the other side. The wind must now ‘plow from some southerly direction, usually south-west. The change from north-west to south-west is generally very sudden and often accompanied by, a squall. There is an immediate dro in temperature and almost invariably some rain. The steeper the slope of a valley side: the rougher is the going, and so in our depression the steeper the rise in pressure the stronger the winds. After the southerly winds have been blowing for some time the sky begins to break and gradually clear the wind dies ‘away. Another anticyclone or ridge of high pressure approaches, Sometimes, however, our depression is not a simple one, there are several.dips in the.same main valley. The.first depression may be followed by one or more "secondaries," as they are. called. These complicate the work of the forecaster very considerably. . The basin-shaped depression, or cyclone, is less regular in its movements than the westerly depression, and its centre may he found in any part of our region. .. As would be exp , it has a more complete wind syste™; winds circulating completely ro it. Consequently, on the eastern and svuthern sides we have winds from between north-east to east and southeast. In facet, any persistent strong east wind in New Zealand is nearly always due to a cyclone. The weather is usually very dull in the eastern half of the cyclone, and fogs are very com-~-mon. Rain usually falls, and is frequently very heavy. -Since the winds are mainly from the east, the heaviest rains are usually in eastern districts. ‘The West Coast of the South Island is protected by the main ridge, and the best weather is found there. If the. centre is far north, Otago and Cant#rbury have comparatively little rain. The winds and weather. in the western half of the cyclone are generally similar to those in the western half of a Y-depression. | The southerlies of the south-west quarter are, however, frequently south or south-east rather than (Concluded on page 29.)

-_ light, air, 2-3, slight: breeze, 27: 3, [HS above is a portion of the extended weather report which is broadeast: by 2YA during the 7 :0’elock session. In the ‘first column is given the direction of :the-wind, fol‘lowed ‘by’ its force reckoned on the Beaufort Scale. This translated into velocity in miles per hour at 16 feet above ground is 0, calm, 0-1 m.p.h oly gentle: breeze 8-11; 4, breeze, 12-16; 5 fresh breeze, 17-21; 6, strong breeze, 22-27: %, high wind, 28-33; 8, gale, 34-40; ‘9, ‘strong gale, 41-48; "10, ‘whole gale,- 49. 56; 1, storm, 57-65 ¢ 12, hurricane. ~The next column is the barometric pressure in inches of mercury while the temperature is degrees farenheit, or the ordinary temperature as we know it. The fourth column deals with the weather, and for this the following legend is used :-~ ‘Weather.-B., blue sky; B .C., blue sky and ‘detached ‘clouds; C., cloudy ; O., overcast; G., gloomy, dull; U,, ugly threatening: R., continuous or steady rain; S., snow; hal P., ‘passing "showers; a squally; L., lightning ; T., thabaee? F., fog; M., "mist ; Z.; haze. A’ fifth column can be’ added, the matter for which is broadcast, "This refers to'sea conditions, such as calm, light swell, or rough, as the case may be. This, however, is quite evident, and needs no explanation. There is one. point which sometimes confuses listeners, that is when the letter "O" (denoting cloudy weather) is.followed by particular of the sea. Some ‘Tisfeners mistake the letter, "QO" for. the: ‘word: "sea."" There -is’ no connection, and it is only by chance that the. words come together. 1 ae at een. aun a, ae ee, a ee oe ee on ee a ee

Weather Reports

(Continued from page 2.) south-west and stormy, dull and wet weather is more persistent than with & depression of the westerly type. In conclusion I may add that the’ Meteorological Office has printed forms on which the reports may be written as they are received. These forms include a map of the New Zealand and Tasman Sea area and by means of them those interested can draw their own weather charts. |The forms are printed in books of 100 and may be obtained from the Meteorological Office, Wellington, at cost price, which amounts, with postage, to 4/-, Appendix, AN order to render the above remarks « more intelligible ‘and to give a more permanent record, two specimen weather charts have been added in figures 1 and 2, Figure 1 actually shows the chart for 9 a.m. on Saturday, the day on which the address was given. In a figure on so small.a scale it is not possible. to include all details, so that the isobars and a few of the wind arrows only are shown. The number of feathers on the arrows indicate the strength of the wind on the "Beaufort Scale." An arrow with eight feathers represents a gale, one with four feathers a moderate breeze, and soon. A black dot at the point

of an arrow means that rain was falling at the time of observation. The chart shows anti-cyclones just coming on to Western Australia and over the North Tasman Sea respectively. A ‘rather sharp V-depression has just crossed New Zealand, while one of & more pronounced westerly type is located south of Victoria. The first mentioned depression brought only light and scattered showers which fell for the most part in

districts with a westerly aspect. Figure 2 shows the chart for August 4 of this year. ‘A very good example of a cyclone is located west of the North Island. The isobars curve round a centre of low pressure, around which the winds blows in a clockwise direction. The isobars are close together, or, in other words, pressure gradients are steep, and the winds are therefore strong. The easterly tendency of the winds in New Zealand and the wet weather will be noticed. Over South Australia is centred an ‘anti-cyclone which is an intense one, the pressure being over 80.40 inches at its centre. This illustrates the fact that an intense anti-cyclone frequently has a cyclone to one side of it. The weather in Australia, except along the east coast, is fine with light winds.

) ‘Station. Wind. a PRP Tem. W.: ery — : -Patea .. NW5 = 29.81 58 P , "Wanganui W 5 29.82 61 BC’ % Foxton .. W8 -- 57 = Q C Campbell NH i 29.65 60 B Westport . SW 4 29.75 52 BCP Diinedin ... NE 1 29.67 49 C Blu& os.) Ni 29.54 49 OD '.Alexandta: Not: received. > Puyse- ; ‘gur.Pti. .. N.4 29.49 47 OP Cape Maria V,D. ec WE 80.04 70’ CZ , -. SW 3 29.90 64 BC Taupo, .. NW.2 -, 54.0 "Hast Cape. W 3 29.82 60 BC