A Popular Misconception Regarding Humidity and the Need for Closer Liaison Between Meterologists and Ecologists

Abstract

At school we are taught that the Gulf Stream greatly affects the. climate of the British Isles. We learn that a considerable belt around the coast of a continent differs from the interior in having a more humid and equable climate than the latter. This effect of bodies of water on the neighbouring land is apparently not confined to the great oceans. It is commonly believed, for instance, that the areas of unusually heavy rainfall on the northwestern side of Lake Victoria and at the north-east corner of Lake Nyasa are due to the presence of these lakes. Some years ago an eminent South African, Prof. Schwartz, wrote a book about the Kalahari. In it he proposed a scheme for damming the Zambesi and diverting much of its water southwards into the great hollows of the waterless country of the so-called desert of the Kalahari. He had two objects. The first was to irrigate great areas of new land, on which to settle land-hungry farmers. The second, the one which interes'ts us, was to save water which now runs down the Zambesi to waste in the Indian Ocean, and to spread it out over the land to evaporate directly and to be tranispired by the growing crops, with the object of raising the humidity of the air and increasing the rainfall of the droughtstricken north-western parts of the Union of South Africa. Prof. Schwartz's scheme so captured public imagination in South Africa, that the Government sent a special expedition of meteorologists, engineers and agriculturalists to Ngamiland to report on it. The belief in the effect of water in moistening the air of the country around is extended also to smaller lakes and even to small streams, ponds and marshes. In ecological literature one constantly finds reference to the humidity of the atmosphere in the hollows containing such bodies of water. Preferences for certain humidity belts have been used by a number of research workers to explain certain inove-nents of locusts. A few years ago I decided to take some measurements of temperature and humidity at Lake Rukwa in Tanganyika, in order to try to understand the behaviour of the Red Locust, which I was studying there. This lake is a large body of water, being about 85 miles long. The results surprised me greatly, as they seemed to be at variance with ideas which I understood to be generally accepted. I found that in the middle of the day in the dry season the air over the shallow flood water of the lake was certainly damper than inland, but the differences were very much less than I had expected. Even 200 or 300 yards gut over the water the air was very far from saturated, and relative humidities of only 40 or 50% could be read. Coming inland there was a decrease in humidity, but this ceased at a very short distance inland. Sometimes even places where there was still wet mud left by the retreat of the flood showed as low a humidity as the completely dry and burnt up areas hundreds of yards inland.