The Nature Column.

Digger (Invercargill RSA), Issue 19, 23 July 1920, Page 7

The Nature Column.

Rain, the source of water supply, has come into- prominence of late owing to tbe activity in connectkm with hydro-elee--tric undertakings. The lack of sufficient rainfall data must handicap our hydro engineers, in that they have either to under-develop the powers or run a risk of failure during exceptional periods of drought. No lake control system can fully safeguard a power supply unless the amount of water entering the lake for a series of years can be foretold with some accuracy. If the average annual draw-off amounts to more than the average annual inflow there must inevitably come a time of stringency. This presupposes that all the water entering the lake is impounded. But where great seasoned changes talce place in the rainfall it may not be economical to construct a dam to hold all the inflow. This makes it all the more necessary to ascertain the inflow during the longest likely dry period. It is held by some authorities that the guaging of a stream over a short period, without taking the rainfall at the same time, and comparing it with long period rainfall statistics is unsafe. Unfortunately it is not possible to make this comparison properly in the Dominion, for the waterpowers are mostly in inaccessible places, with sparse population, and the Government with a not unusnal lack of foresight has not encouraged scientific investigations. We should not wait for the Government so far as our own system is concerned. For less than £200 five or six automatic rain ga-uges- could be installed round Lake Monowai and would afford inost valuable information. The guages would need to be read once a year and the work could probably be done for nothing if the Board invited the assistance of the right people. These guages would only afford close approxi^tion, as rainfall varies enormously betAeen stations only a short- distance apart, and greatly from month to month. In the Sudbury watershed, Boston, which is comparable to the Monowai in size and is liilly, the maximum run-off in July, the di-y month, was 20.9in. and the minimum 3.6, and many other months showed similar. differences. The average rainfall in England is 25 inches, yet ip one place it averages nearly 139 inches. The world 's record rainfall is held by Cherra Poonjee, in Assam, with an average of 439 inches. If the figures given by Mr Fowler in a lecture previously publislied in this paper are correct, some places -round Monowai must approach this record. Fortunately for us, the rainfall of New Zealand, owing to its configuration, is very steady, but even so there is a great discrepancy between the amount of rain on the West Coast and on the Canterbury plains. The cause of this is as follows : — Rain is generally preceded by cloud focination, being the condensation of the invisible water vapour in the air. The only process in nature by which this condensation can occur to produce rainfall, is the cooling involved by transfer of air from a lower to a higher level. We are all aware that air compressed in a bike pump gets hot, conversely when it is expanded it becomes cooler. Air at different temperatures has the power of holding a certain amount of water vapour. The hotter it is the more vapour it can hold. As we ascend through each 180 feet the temperature of the air, owing to its expansion, will f all about 1 degree. If the ascent of the air be sufficient it will be cooled to the dewpoint. The dewpoint is that degree of temperature (varying with the" amount of moisture) at which the water vapour can no longer exist as such. The dewpoint having been reached small drops of rain are formed, and it is necessary for this formation that minute particles of dust shall exist in the air. As the process goes on more water is thrown out and the droplets grow until they ultimately become large enough to fall to the ground as rain. While the droplets are small they float like so much thistledown. Now we can see why the rainfall in the Fiord country and on the West Coast is heavy. The wind sweeping over the ocean becomes saturated with moisture, on striking the Southern Alps or other mountains it is forced abruptly upwards, a lowering of temperature takes place and heavy precipitation follows. A large amount of what is known as latent heat is lccked up in water vapour and the condensation of this vapour releases the heat. Systematic weather records would do much to boom Southland, and stations should be established all over the province as they are in England. Most of the observations in the Old Country are made by unpaid enthusiasts who vie with one another in producing the best sunshine and rainfall records. We have a longer day in summer than Auckland and we should endeavour to prove by records that this end of the Dominion is not the cold bleak spot pictured by our northern oeighbours.