DROPS OF RAIN

Evening Post, Volume CVII, Issue 73, 30 March 1929, Page 13

DROPS OF RAIN

HOW PRODUCED

TYPES OF RAINFALL

EFFECT D¥ COOLED AIR

:' .... The most iniportant feature in weather forecasting'is precipitation, whether it be in the form of rain, hail, ot 'snow. '•' The winds and. temperature ■changes are also of importancoj but to precipitation they yield the first placo in importance. :. Unfortunately for the "meteorologist, whose, business it: is . :to issue forecasts.'of 4 th"e weather, precipitation is the most difficult and -most ..Complicated oX all meteorological ,phe-;-npinena:,;.says Dr. E. Kidson, Director

of' JMLeteoiolagicar Services, writing in the *"New Zealand Educational Gazettfe" : on"t-he -problem of- forecasting New Zealand weather.-' : : ';

■ "•First-of-all, he says,> a- thorough grasp should be 'acquired of tho fact that the oiily important means by which precipitation can be produced is by the cooling, of the air. Air always contains more or less water vapour, but at any particular temperature it can only con'.taina- cert.'iiri, amount, the amount incroasjng'^vith' the' temperature. . If, therefore, the'air be, cooled it.will finally reach a temperature at winch it cannot retain all-the ■moisture present in -the-form-of-vapour, ■■ Some of the vapour therefore condenses into droppots of 'water, which arc visible as a cloud or, if it is on the ground, a fog. If., the cooling is continued further; the -drops-increase in size-'until ■■ when their diameter is •about 1.. O.S mnu they will .fall. as.rain. . . Pxovious to that their rate pf- .'la-lf'-is, fop small' ;for, them; to produce--appreciable precipitation.. ■• ■ The. next .fact, t.o realise is that ■ tho: only--effective riveans of cooling tlio air sufficiently!, to produce rain is by raising it in the atmosphere. As the height-iu the. atmosphere increases, the pressure decreases owing to the' de.crease in. the.'amount of suponneumIbent "air. . If, therefore, :air be raised' in'-the- atmosphere, it. has to adjust itself t-o'coutinually decreasing pressure and expands in the process. As a result its', temperature falls; '!■ The fall is 18 degrccs'K "per kilometre rise if the air is dry, and 11 degrees F. if it is'saturated"wlth"moistlire. In the " second case, the .rate of cooling is retarded owing to thc-liberation of heat by tho condensing moisture. * Somo effect in producing precipitation will be exercised, by the fall'of pressure occurring as the air moves from a region of high to. a region of-low pressure, but.the pressure- changes involved,aro too small to be of much consequence. The air will sometimes cool itself, also, by radiation, tout this'process is slow, and unlikely to-be the direct .cause: of precipitation.

CYCLONIC RAINS.

Cyclonic rain, according to lijerknes, the emiuent Norwegian meteorologist, consists of either "warm front" rain or "cold front" raiii." The former is produced through the uplift of warm airby :cold. The warm air climbs up art inclined plane formed by the cold: air! It does' this ' partly through its lessor density and partly through ita greater velocity. The inclination of the boundary surface is very slight—of the order of 1 in 100 —so that the rain may cover a large area. The lino on the surface ■where the cold and warm air come, in contact is the "warm front." Owing to surface heating, broken, topography, and other effects, it is seldom ♦very marked;'in :New Zealand, and we often miss the warm front rain. Tlie warm air, however, usually appears in the tipper levels of the atmosphere shortly after 'the highest pressure duo to the anticyclone has been experienced. It is often marked by the appearance of high cirrus cloud. As the coming depression' advances, the clouds ' become lower and denser, until we have a sheet of covering the skies, to.be ■followed shortly by.nimbus and 'perhaps rain. ".Final!}',, at the surface, there is either a gradual or a more or less abrupt change to warmer temperatures. In■dßjcr'knes's typical depression there would then-be a- spell of fine weather as the.warm..air passes until the approach; of the "eold:front.V The warm air will be blowing from some'northerly direction. There were several clear examples of--warm-fronts and warm front rain in September,: 1928. .' . "

Just as the line of lowest pressure in •tKe- depression is' reached, there is it •sudden I''change- to winds from somej■southerly, direction. Coming;;.from ilie' south', the air of this wmil is cold,:aiid the boundary, between' the warm air and the cold, that. is., overtaking it is the ,-" cold.-i roiit." The cold.air is called "Polar-air,- 1' in. theßjerkncs theory,, and the. whole boundary between, tho warm a;rid; cold air the "polar front/.'. The use?;of. the w.or.d "polar" is in some respects unfortunate, because it is Habile, to-igiye the-impression-that' th.c air. haa: dq'me" direct from..the..Polar, regions. This, idea is tflready'very. prevalent, and has 'b'een"'!fifpp6rte(l"by; tire 'use of the term,. ."Antarctic ...depression"—another ,nris.--nomer. There results the belief that depressions come straight from the Antarctic"/.and that the latter is therefore "the,.soiirco'.6f. our: weather. This is-a misconception I'cm which explorers have battened , in. the past,, and/by which, some are endeavouring to profit at present;. . I'Polar-iair" or large portions of. it, .at.leasts-was .probably warm. " Equatorial -air " a week or so previ : ousljv- It may nob even have been cool? .ed-.by, a sojourn in some more southerly... latitude, but ;through lying over a :col'd continent or sea. In Europe, in fact, they now. recognise, "marine polar.-a-nd "continental polar" air. To' ..rotur;i,,.however,"jo the air. behind; the' cold..i'routi- thia. is advancing into' the warm. ak> than which it isvdensor, and wedging it up. In this case the warm air is forced up rather abruptly in front of and along the cold front, producing sometimes a wall" of 'cloud. The cold Troiit is "preceded by alto-cumulus clouds, •which si ream ahead of it and gradually vanish in the process. Tho alto-euinu-his gradually, gives way to nimbus and rain. The band of rain along the cold front is' typically narrower than that preceding the warm front, but we seldom faii to get rain from it, whereas from the warm front we frequently do. The ;ftold front is- followed. Shortly by clear intervals interspersed .with show-'-'ers,-which- gradually pease. ":-It would ■3>ei;ha]i/) ; -.>)0 >.b;ettpr to- call the Ay!arm and cold front rajns..'.'rail! associated.- yvit.H a depression," rather 'than "cyclonic -id-ins;" since it is not necessary to assume that tho depression is . of cyclonic form. !

T SHOWERS AICD DRIZZLES.

- , .Wh.at are called instability ...showers art; produced in either two ways. -' Instability may be brought about by heating over a warm sea surface. Air is -linst-flbio wh-pii-the decrease of temperature v.'itli .'illitiidc.or tlio "lapse rate," as it. is called, approaches that which would-occur in air that was raised from the- surface. ' In this condition, if a mass of air at the surface is. heated to a temperature ''above that of its surToundings it will be forced up owing to its decreased density. Its 'temperature ■will fall as its height increases, but it wi.ll remain .warmer than its surrounding.?; aiid" therefori: cuiiUnuc 16 rise, Until':cither it is gradually, mixed wilh the surrounding airor it mevt's-n stable layer. Ju this way heated air may rise sufliciently for-thc'consequent cooling to cause rain to fall. In unstable air passing over a warm sea, therefore, there are likely to be scattered showers. The

heating at the sea surface will itself tend to produce the-necessary instability. Tho. chances of unstable conditions being produced will bo greater tlm greater the contrast in temperature between (he air and the sea surface. Thercf'Tc, instability is most likely to be produced in Polur air. A similar'effect may be produced by tho unequal heating oC cold, unstable air over a warm laud surface. The intermittent showers occurring i" the rear of a depression consist largely of instability showers. It will be noticed that the rain clouds are markedly cumnliform, at times developing into fowering cumulo-nimbus and even thunderstorms. Wo had an instance of this in ■Wellington on Sunday, ltMh August, 1928, when, on what was, generally speaking, a beautifully fine day with a light and cool southerly wind, thunderstorms were produced in the Upper Hutt Valley and other parts of tho eastern .districts of the North Island.

v -When air is stagnant over the landin, summer, unstable conditions may bo. produced after several days by the accumulated effect day after day of the heating at the ground' surface by the sun, amd local showers or thunderstorms develop. Rain of this type is particularly common in mountainous regions. ' i

■ A very-light rairi may be. produced in 'the lowest ■ layers by direct cooling ■■ft*' the: air by:a cold sea or land surface, but the quantity of rain thus falling can never amount to more than a point or two.

MOUNTAINS AND WINDS.

.Orogra,phical rain is produced when a mountain range lies across the path of a wind and the air is forced up over the range and consequently cooled. Now, air does not tend naturally to (.•limb over a mountain; energy is consumed in lifting the air up against gravity,-and the more stable the atmosphere the. greater .the forces required to do the lifting. In general, therefore, air tends to travel round mountains. This 1 accounts partly for t'ho strength of the winds in Cook Strait, and, to a .'lesser, extent, in Foveaux Strait. There is..a ;■. concentration of. tho air flow through. these channels. , Some of the air corning through Cook Strait in a westerly wiild, for instance, has probably came round from as far south as Westport. But when a wide current meets a long obstacle such as the Southern .. . Alps, some .of the air is sure to be 1 forced over; and the wider the wind current,- the longer the mountain range, and the more directly it lies across the path of the wind, the greater will be the quantity flowing over, and the greater the rainfall it is likely to produce. The nearer the air approaches to unstable conditions, the easier it is to force over mountains. When;" therefore, tho rain is a. combination of instability rain and orographical rain the falls are likely .to be, very heavy. ,It is only necessary to look' at the rainfall map of New Zealand to realise how important orographical' rair is in tho Dominion.

.. It is interesting" '■ to consider what happens to. the air. when it reaches the other side of the mountain after drop-, ping much of.its moisture. It will bo difficult for it to descend unless it is potentially, as dense as the air . beloAV it oil the loo,side —that is, unless when it is brought to the lower levels it is as dense as or. denser than the air at those levels. • The contrary is very like-ly-to ,be. the case. Therefore, we may have, as frequently happens in Canterbury, the.'current from the windward side continuing on the lee sido as an upper' current only and overriding tho air at' lower levels, which may bo relatively stagnant, or be drifting slowly round the mountain barrier. lii Canterbury, then, there are frequently strong north-westerly winds at about 3 or <t kilometres with, light north-casters beneath them.

The turbulence of the wind .as it comes over the mountains, however, tends to make it drag out and replace the. lower layers. Sometimes, too, tlio temperature conditions will favour the descent of the air'on passiug the mountain'crest. We then get' the well-known Fohn wind, a hot and dry wind which is better'developed at very few places in the world than on tho Canterbury ■Plains. -In addition to its other qualities, it is a very turbulent wind in which there is much vertical motion..

There are other affects ' produced by mountains. ..The.march of cold and warm fronts may be profoundly affected by them and. tho weather modified accordingly. For instance, a wedgeshaped, mass of air near a warm front may, be held back, the warm air continuing to climb over the cold wedge. In. this way warm front rain may be prolonged on the weather side of a mountain range. '■ ■ •-' ■'