"MEN," remarked the poet John Dryden, "are but children of a larger growth". And so, a fortiori, it might sometimes seem in the case of meteorologists. A hundred years or so ago they took to flying kites with gusto, and now their latest craze is model aeroplanes.
There is method, however, in this quasi juvenile frivolity. The objective was, and is, to explore the thermal structure of the upper atmosphere by finding novel ways to carry instruments aloft. Kites were convenient during the 1890s because the recording instruments of the time could be attached at regular intervals along the cable when the kite was reeled in, pressure and temperature data for the various levels could be read at leisure.
Radio, however, made the weather kite redundant: the hydrogen filled balloon with a radiosonde attachment proved to be a more effective means of obtaining readings from the upper air, and is the basic method still in daily use. But a balloon has the disadvantage that is survives only for an hour or so, and drifts unpredictably in the general direction of the wind. When it is desirable to investigate in detail the characteristics of a specific region of the upper atmosphere, the model aeroplane, is proving useful.
The Americans were the first to put the idea into practice when, about five years ago, they built a large unmanned aircraft called Perseus, designed to fly right into the eye of a Caribbean hurricane and send back information on its structure. Perseus, however, with a wing span of some 50 ft, was larger than many conventional single engined aircraft. The Australians are using a much smaller version for exploring thunderstorms.
The Australian model aeroplane is about 5 ft long with a wing span of 8 ft, and is called an aerosonde. For take off, it is strapped to the roof of a car from which it is released when a speed of about 40 mph has been attained. The aircraft is equipped with instruments to measure temperature, pressure, humidity and wind, and has a radio transmitter to get the information back to base. It navigates by means of the satellite based Global Positioning System, and is capable of flying a round trip of up to 5,000 miles on a single 10 litre tank of fuel.
The aerosondes are used to monitor conditions in the upper atmosphere in remote regions of the southern hemisphere where radiosonde ascents are few and far between, and they can undertake special missions, like flying into the very heart of a violent thunderstorm, without any risk to human life.