WHAT MAKES IT RAIN?

Dunstan Times, Issue 3334, 16 August 1926, Page 2

WHAT MAKES IT RAIN?

(London Daily Chronicle Correspondent.)

In spite of the great advances that have been made in the study of weather science during the present century, meteorologists axe not yet in a position to give a complete and satisfactory answer to what may at first sight seem a simple and even fundamental question—“ Why does it rain?” Leading physicists of many nations have applied themselves to the solution of this problem, and as a result of their labors a number of matters that were fonnerly obscure arc now explained, but we are still far from having arrived at a full understanding of the mechanism of our atmosphere. The reason for this is that, living as wc do at the bottom of the great ocean of air that surrounds the earth, we can clearly sec and examine only two dimensions of a three-dimensional problem. Our position is, in fact, analogous to that of deep sea fishes anxious to understand the structure of the ocean of water and the causes of its motions and changes. Exploration by means of aircraft and unmanned balloons has taught us something of what goes on up to a height of twenty miles or so, but it is certain that' the earth’s atmospheric cloak is at least 100 miles thick, so that four-fifths of it or more is practically an unknown region. Observations of meteorites and their trails have lately led British scientists to the astonishing conclusion that beyond the permanent zone of intense cold that has long been known to exist within the first few miles above the ground there is an equally permanent area of tropical heat. This is a discovery which, may prove of far-reach-ing importance, and it is, in any event, a very notable contribution to our knowledge of the upper atmosphere. If it is not yet possible to trace the ultimate causes of rain, the immediate causes at least are now fairly well understood, and they may be sunnnui - ised 1 in a very few words.. Practically all rain, hail and snow are p-oduced by the rising and consequent chilling of air currents, and there are three distinct ways in which this may hap*, pen.

1. Cyclonic rain.—To various nhenomena comprised by the terms “cyclone” and “depression,” of which wo read so continually in the weather reports, are the chief source of rain over most of England, and their functioning mnv lie briefly outlined as follows Two drifts of ajr at different temperatures are clashing, and the colder of the two (usually from E. or S.E.X being • he denser, and therefore the heavier, forces the warmer and lighter drift <usually from W, or S.W.) to rise bodily over it. In so doing, the warmer nr current becomes cooled by expansi ni as it mounts into the less dense layers above, and its moisture .consequcn :ly condenses, producing, first, cloud consisting of tiny water particles, and. finally, as these particles coalesce, rain. For the amount of water vapor that can rett'iain in the uiieondcnsod and invisible state in a mass of air depends solely on the temperature.

2. Conveetional Rain.—The second way is exemplified by what happens n u. summer thunderstorm. In this case the layers of air near the ground are rapidly heated by the sun’s rays, and so expand and rise far into the ••■■■ld upper reaches o': the atmosphere. “Convection” is the physical name for this process. The ascending •.in' is charged with moisture that it has gathered from the ground, vegetatam. nr other sources, and as it goes on mounting and cooling the moisture condenses into cloud, and finallv into ram when it reaches a height where it can no longer hold water vapor in the in vis. ible state.

The uprush of air in this case is often so rapid that it prevents the water drops and hailstones from falling until they have, grown sufficiently large and heavy to overcome the resistance. This accounts for the great intensity of rain and hail during thunderstorms, and for the extreme instances called “cloud-bursts.”

Hail is. of course, formed when the temperature at which the condensation takes place is below freezing-point, and as low temperature in the upper air is essential for thunderstorms, hail is a very general feature of such cases.

■}. Arographic Bain.—This type of rain occurs when air is cooled through being pushed upwards by mountain ranges or hills, the condensation resulting from the process already outlined. The uplands of the West of Scotland, Wales, and England are so notably wet because the prevailing warm S.W. winds, charged with moisture accumulated during their long journey over the Atlantic, are forced to ascend on reaching the mountains, and so. becoming cooled, condense the water vapor then and there. The vfleets of this process are noticeable on n small scale even in London, where Hampstead gets, on the average, 10 to 15 per cent, more rain than the lower districts, owing to its position on a hill 150 feet high.