THE WEATHER

Hokitika Guardian, 7 June 1935, Page 2

THE WEATHER

PROCEDURE fN FORECASTING

Dlt KIRSON EXPLAINS

CHHTSTCHURCH, .r,im■ 0

Alodern methods of weather forecasting were outlined and some local applications of those methods made by Dr E. Kidson, Dominion Meteorologist, in an address to members of l.lie Philosophical Institute of Canterbury and the Canterbury College Scientific Society last evening. The president of the institute, Mr H. F. Baird, presided over a large attendance.

“The first aim of weather forecasters in official services,” said Dr Kidson. “is to know as much as possible ivbout the existing; weather over the area with which they are concerned and for as far as possible on all sides of it. The host way of summarising the information and making it readily ap-

preciable is to pilot it on a map. »«. some services, several diflerent maps are used, each being confined to the data for certain elements. This has certain disadvantages and by the use of appropriate symbols, and a good deal of ingenuity it is impossible t( got everything on to one map. “A close network of reporting stations is desired: and each reports the pressure and temperature of the air. the direction and force of the wind, and the- state of the weather at the time set for the observation. More complete reports tell how the pressure is changing, the type ol weather prevailing since the last observation, the amount and kind of cloud, the visahilitv and other details. W here the network of stations is close and the reports are complete, the forecaster ha* a mental picture of the sky over the whole area as well as knowing, the elements which are more directly measurable. In recent, times more and more data is being secured of conditions in the upper air. In New Zealand our knowledge in this direction is confined to t,'■ :it of the winds above W'ellingt-om and Christchurch.' It is hoped soon to have information supplied from Auckland as well.”

When this information had been placed on the map, said Dr Kidson. the distribution of air pressure was made more intelligible by the drawing of isobars or lines of equal pressure. The important fact was that the air flowed parallel to the isobars, with low, pressure on the right in the Southern Hemisphere to anyone looking in the direction of flow. Although proximity to the ground and hills and mountains might cause deviations find irregularities, in the free atmosphere parallelism with the isobars was very close.

“It is the centrifugal force due to the earth’s rotation, that forces the,;aiiV to move in this way instead of from high to low pressure,” continued the speaker. “The significance of this law is fundamental. It means that from successive isobar charts it is possible to deduce the speed and direction of motion t 0 the air over various parts of the area covered by the chart’. Tn other words, the history of the air masses can he traced and their day-to-day course followed. From, pressure charts alone, therefore, the characteristic of the air masses can, to a large extent, he deduced- —whether they are warm or hot, dry or moist, stable or unstable, and the kind of clouds they are likely to contain. A knowledge of the characteristics of the air masses 'helps us to anticipate their interaction on one another, the development of storms, and generally the resulting weather.”

Pressure could be measured accurately; it, did not vary rapidly and erratically from place to place, as was the ease with temperatures and relative humidity. Although the wind, temperature. and humidity at Christchurch ini"lit differ eonsideru 1 Iv from Akaroa the pressure would differ only slightly.

“Tn the modern methods introduced by the Norwegians the forecaster regards the atmosphere as being made up of air masses of different kinds,” continued Dr Kidson. “The most important quality of an air mass iswhemer it is warm or cold, for the place in which it finds itself or compared with adjacent air masses. Warmair masses will, of course, generally have had a northern origin, and cold ones a southern. It is the interaction between cold and warm air masses that leads to the development ol storm systems and the weather variations which most in torsi us..”

The speaker then discussed the stability or instability of rlr masses. The effect of -height on temperature (jepca.ded in extent on the amount of moisture, if a parcel of air was satur■’ted with moisture the temperature fell at a lower rate. When the temp* nature ; f the atmosphere fell off with height at a ; natter rate than a parcel of air r ised from the s irface, it was said to be unstable. Warmer and therefore lighter than the air surrounding it. the pared of air would go on rising. When the rate of fall of temperature with height was slow, any parcel raised from the surface became colder uu/1 heavier than its surroundings and tended to sink back. Such an atmosphere w,as stable.

The dividing line between Iwo air masses was called a ‘‘front.'' A warm front occurred when warm and light air was climbing up oven 1 a retreating wedge of cold and dense air. rain of a steady type usually occurring before the passage of such a front. With a cold front the cold, dense air invaded the warm air and wedged it up. the rain associated with it being showery. Sometimes a warm ami a cold front came together, this type being catted an occlusion.

Dr Kidson then explained series of weather charts from lantern slides, showing the development and passage of fronts of different types. Future movements and the resulting weather could lie anticipated from day to day by these, qharts.

“The weatner still remains extremely complicated, and such things as new waves and secondary fronts are always liahjie to. upset one's calculations,” concluded the speaker. “Even a tropical cyclone developing in the Pacific Island region can greatly influence our weather. In New Zealand the. mountains always make the weather complicated and difficult to describe. But I think it will be seen that we now know a great deal more about what happens than we used to know.”