WINDS AND THE WEATHER

Korero (AEWS), Volume 2, Issue 14, 17 July 1944, Page 26

WINDS AND THE WEATHER

Twentieth Century Advances in Forecasting A KORERO Report

Once upon a time, before the invasion, there was a man well known and highly respected for the accuracy with which he could predict the next day’s weather. He knew all about rain, wind, frost, and sunshine, and, it was common knowledge in the district, a lot more besides. His advice was greatly, and constantly, in demand. On fine days he would sit on the rustic seat in the shade of the walnut tree in his garden ; when it was raining or the day was too chill for comfort he could be found in front of the stove in his kitchen. Always he was pleased to share his knowledge ; and his callers varied from the neighbouring Chinese marketgardener, who was wondering, please, whether it was too early to plant out honourable hothouse seedlings, to the local schoolmistress, who was planning a picnic in the country for some of the older pupils with an interest, a marked interest, in botany. This weather expert, and he wasn’t so old considering he knew so much, was held in high esteem ; it was even mentioned that he was to be nominated as candidate for mayor. As a preliminary to this he was elected unopposed to the drains committee of the local council. Then his secret was discovered, the secret of his knowledge. The people’s shame that such a hoax had not been seen through for so many years was so general that everybody in the town went round with ear-tips blushing red, too thoughtful to speak. It was found that this imposter, a man of no conscience, had risen early every morning, an hour before anybody else, and collected his morning newspaper from the lawn. Asked for his advice later in the day, he shamelessly offered as his own work the exact opposite of the information given in the daily weather forecast. This simple story may or may not be true ; at least it shows people’s interest

in the weather and the prediction of its changes. From early in this century weather forecasting has been scientifically studied and advanced : progress in aviation has brought further technological development ; and in the fight to victory of World War II meteorological prediction has become of great value and importance. One of the interesting stories after the war will be of co-opera-tion between the meteorologist and the strategist —of operations planned to success with the help of the weather. Axis and allied forces have learned to use weather purposefully as an ally. Mark Twain’s complaint that everybody talks about the weather but no-one does anything about it could not be made to-day. Weather Prediction Helps Business Weather forecasting is important not only to the needs of war ; in our daily lives it is more definite and timely than ever before. Among its benefits are : less hazardous aeroplane travel; less respiratory disease ; reduced costs of perishable foods, especially fruits ; a decrease in forest fires ; less damage from storms ; fewer shipping losses ; and a a great financial saving to business generally. Floods can be predicted, their extent estimated, and the necessary precautions taken. Weather men can tell engineers responsible for dams and roads how bad will be the worst possible storm ever to hit that region, and therefore what is probably necessary to make the roads or the dams safe. ■ Why weather is so important to business can be illustrated by two examples. Several racing clubs in New Zealand make a practice of “ laying the odds ” with insurance companies against a fall of rain on a race-day that will result in the cancelling of the meeting. A long-scale prediction and a knowledge from past records of the average rainfall at that time of the year is valuable in the laying of the odds.

A large baking company in New York is saving approximately a year by following weather predictions closely. When the weather is to be fine, about one-half of the firm’s products are distributed in the city and one-half through small retail shops scattered through the suburbs. But when the weather is stormy, 75 per cent, is kept in the city or distributed at points close to suburban railwaystations. The company has discovered that when the weather gets bad enough, about one-half of its women customers in the suburbs will telephone their husbands in the city asking them to buy bread and other bakery products either before they get on the train or just after they leave it. Savings on unsold goods amount to the total mentioned. The most important, and certainly the most sensational, advance in weather forecasting was claimed early this year be Dr. C. G. Abbot, United States weather physicist. After experiments lasting more than twenty-five years, Dr. Abbot believes that by a proper combination of observations on solar heat, air-mass movements, and local disturbances it is possible to predict weather at least two weeks ahead with almost perfect accuracy. Last year his system worked so well that he was able to forecast almost to a drop how much rain would fall in Washington and on what days. Dr. Abbot is confident that he has developed a workable system, and his predictions so far certainly prove his claim. This explanation of the types and causes of New Zealand weather was written by Miss D. F. McLeod and is reprinted from the Education Gazette. Cyclones and Anticyclones An anticyclone, or high, is a highpressure area in the atmosphere from which pressure decreases outwards in all directions. There is a tendency for air to move outwards from high-pressure centres towards low-pressure centres ; but this is greatly modified by the rotation of the earth. Consequently, in the atmosphere the winds blow parallel to the isobars instead

of across them, though, near the ground, surface friction causes the winds to blow across the isobars in a slightly slanting direction (at an angle of about io" to 30°). Winds thus blow round the anticyclone in a counter-clockwise direction in the Southern Hemisphere (see Fig. i). The anticyclones in our latitudes are often large enough to cover Australia. They generally bring fine weather, with a calm or light breeze such as land and sea breezes or mountain and valley winds near the centre. Although anticyclones in this area move eastwards, they may remain stationary for a week or so, with consequent long spells of fine weather. A cyclone, or low, is a low-pressure area. In the Southern Hemisphere the winds blow round the centre in a clockwise direction (see Fig. 2). Cyclones mean changeable weather and rain. Meteorologists generally restrict the use of the term “ cyclone ” to centres of severe storms and use the term “ depression ” to describe the average low. New Zealand weather is dominated by a belt of high pressure which consists of eastward-moving anticyclones. The most recent research indicates that these anticyclones are not, strictly speaking, part of what geography books term “ the tropical, high-pressure, belt.” They originate from strong outbreaks of cold air which flow northward into the southern Indian Ocean from the Southern Ocean. These outbreaks usually occur in the rear of deep cyclones of the Southern Ocean and have a tendency to occur in a rhythm of about a six-day period. As a result anticyclones usually pass a given locality separated by a six-day period, and thus there is a scientific backing for the recurrence of wet or fine weekends. Figure 3 shows the “ favourite ” tracks of these anticyclones across New Zealand at different seasons of the year. To the south of this high-pressure belt passes a succession of antarctic depressions, the centres of which are well to the south of New Zealand — e.g., about 70 degrees south latitude. Winds As may be seen from Fig. 4, a counterclockwise circulation of air round the

anticyclones of the high pressure belt produces the following winds : — (1) Westerly winds— “ prevailing westerlies ” —blow along the southern fringe. These winds bring rain to the west coast of New Zealand, but are generally warm and dry on the eastern side of the mountains, owing to the watervapour content being decreased and the air being warmed by the descent to the plains. (2) South-east winds—i.e., the southeast trades —blow along the northern edge. These winds affect New Zealand very little. (3) Northerly winds blow behind each anticyclone— i.e., on the western side of the high pressure. These winds are a warm air mass ; whether they have a high humidity depends on whether the air has been blowing for a considerable period across warm seas. When a warm air mass with a high water-vapour content is cooled, condensation readily occurs, resulting in mist, fog, or drizzle, or low cloud on the windward side of mountains. On the leeward side there are generally blue skies and warm sunshine. A week of northerly winds means that there is an extensive anticyclone to the east of New Zealand. The wind tends to become progressively warmer and damper. (4) Southerly winds blow ahead of each anticyclone i.e., on the eastern side. When a mass of cold air from the Antarctic Ocean is moving northward to temperate latitudes the lower layers are

warmed by contact with the ground. Thus the lower layers become considerably warmer than the upper layers of the atmosphere and convection currents are set up. The rising air expands and cools ; towering cumulus clouds form ; often there are heavy showers and sometimes hail. Local Winds. — (a) Easterly winds bring a fair amount of rain to east coast districts. These winds are usually connected with cyclones moving across the north of New Zealand. For example, if a cyclonic storm passed to the north of New Zealand, the easterly wind along the southern fringe might blow across the east coast (see Fig. 5). In summer, tropical cyclones sometimes move down off the Queensland coast and pass close to, or across, New Zealand, causing specially strong easterly or north-easterly winds in eastern districts of the North Island. Where the winds cross hills, as in Hawke’s Bay, floods sometimes occur as a result of the high rainfall. (d) The Canterbury nor’-wester is a wind of the Fohn type. (c) Mountain and valley winds are common in hilly districts e.g., the Barber —a cold wind which blows down the Grey River valley in Westland. (d) Land and sea breezes occur in coastal districts in anticyclonic weather, when there is no strong general wind to mask their effects. The Effect of Depressions You will see in Fig. 6 that a front, known as the rneridianal front, extends

northward from these Antarctic depressions. Along this front other depressions form. What is a front ? The term “ front ” is a metaphor. A front is a battle-line, where two opposing forces meet. The opponents in this case are the warm and cold air, which struggle for balance. According to the Polar Front theory., which is accepted as a working hypothesis by practically all meteorologists, depressions originate along the front which separates a warm air mass from a cold air mass. In other words depressions originate along the front which separates warm winds, such as our northerlies, from cold winds, such as our southerlies. In Fig. 7 the warm northerly wind behind one anticyclone can be seen blowing alongside the cold southerly wind ahead of the next following anticyclone, like two trains moving in opposite directions. As we can see from newspaper maps, military fronts sway backwards and forwards and develop “ bulges ” where there is a weak place. Just as a military front develops a “ bulge ” so a weather front may develop a “ wave ” (see Fig. 8) when a tongue of warm air penetrates into the cold air. This tongue of warm air is the warm sector of a depression. Figure 9 shows the grouping of the isobars round the centre and the clockwise wind circulation. The figures are the last two figures of the pressure, expressed in millibars. Pressure varies from 1,015 mb. to below 975 mb. The depression consists of three parts • (1) the warm front ; (2) the warm sector ; (3) the cold front and southerly wind. Depressions move polewards along the front. Figure 10 shows a warm-sector type of depression passing over New Zealand. (1) With the approach of the warm front a thin sheet of high white cloud

(cirro-stratus) develops. Gradually the cloud thickens and lowers and becomes a depressing sheet of grey. More or less steady rain develops over an area one hundred and fifty miles to three hundred, miles wide. Along that portion of the front known as “ warm,” the warm air is climbing up as a gently-sloping surface of cold air. The gradualness of the rise accounts for the overcast sky and widespread rain. In Fig. io note the wide belt of rain along the warm front, ■ in contrast to the narrow belt along the cold front. (2) As the warm front moves on, the northerly winds characteristic of the warm sector prevail. If the wind is fairly humid there is likely to be mist and drizzle or rain on the windward side of hills and clear skies and warm sunshine on the leeward side. This lasts for perhaps half a day ; longer if the place is on the outskirts of the warm sector. (3) Then come the cold front and southerly wind. Along the part of the front known as “ cold,” the cold air is pushing under the warm air and wedging it up abruptly. Masses of towering, cumulus cloud form, sometimes darkening to cumulo-nimbus. There is heavy rain over an area about twenty miles tosixty miles wide. If the slope of a cold front is very steep, the warm air may be forced up with some violence, causing squally conditions and even hail and thunder (characteristic of violent upward currents) along a belt about twenty miles to sixty miles wide for hundreds of miles along the front. Less-vigorous cold fronts may be accompanied by only a few showers. Temperatures become cold. (See Fig. 11.) (4) As the cold, dense air flows in from the south, the barometer rises. Scattered showers of the convectional type may be experienced as the lower

layers of the air become warmer, but gradually the proportion of blue sky increases and the wind moderates, and if we are lucky an anticyclone becomes established. The above section describes the typical results when the centre of a conventional, well-behaved depression passes across New Zealand. However, as we know, .such regularity .in New Zealand weather is the exception rather than the rule. Here are some other types of weather common in New Zealand :— (a) An Occlusion. — The cold air along the cold front pushes under the warm air of the warm sector, wedging it up. As the cold front thus pushes forward it “ catches up ” the warm front, wedging the intervening warm air off the ground. Thus a portion •of the front becomes “ occluded.” The occluded portion extends until all the air in the warm sector has been wedged off the .ground. The depression continues as a weak, dying whirl of cold air and finally disappears. (See Fig. 12.) (&) Secondary Cold Fronts. — Sometimes •another cold front develops behind the first, • and perhaps others behind that. The wind becomes variable just before the arrival of the secondary front; lowering cloud and : showers accompany the change to a squally southerly. This is believed to be because the southerly arrives not continuously, but in surges, and these extra surges are the secondary cold fronts. (See Fig. 13.) (c) A series of Antarctic, or Westerly Depressions, the centres of which pass to the south of New Zealand. These naturally have most -effect in spring, when the track of the anti- ■ cyclones is farthest north. (See Fig. 3.) When the northern part of an Antarctic •depression crosses New Zealand — i.e., a -A-shaped depression —we have a cold front only. A northerly or north-

westerly wind backs to the south-west or south, and there may be a period of showers or rain as the change occurs. Often successive depressions of a series pass with great rapidity. The southerly veers gradually back to north-west on the approach of the next depression ; then comes another southerly change. It is during the passage of one of these depressions that our weather maps show one of the most characteristic “ isobar patterns,” a A-shaped depression between two anticyclones. (See Fig. 6.) Types of Rain in New Zealand (1) Cyclonic vain, due to the passing of warm and cold fronts. (2) Ovogvaphical rain, due to topographical features. This includes not only mountain rain, but any rain caused by physical features. It is believed that a fair proportion of Wellington’s cloudy days (often with light drizzle or rain) are caused by air flowing into the narrow channel of Cook Strait. Just as the tide rises in a narrow estuary, so air rises in the narrow channel of the strait. The rising air expands and cools, causing mist and drizzle. (3) Convectional rain, occurring : — (a) After a southerly, when the cold air in contact with the warm ground is heated in its lower layers until it is much warmer than at higher levels ; and (&) On very hot days in inland districts, when the ground, and consequently the air in contact with it, becomes very warm. In concluding these notes I would like to thank the Meteorological Department for their helpfulness in furnishing information about the most recent research.