ORIGIN OF DEW

Otago Daily Times, Issue 27040, 26 March 1949, Page 2

ORIGIN OF DEW

VAPOUR GIVEN OFF BY EARTH AND PLANTS

Dew is not derived entirely from moisture in the air, says a bulletin issued by the British Ministry of Agriculture. Much dew has its origin in vapour given off by the earth, or from grass or other plants on which dew appears. In dew the total nitrogen content is just under three parts in a million, while in rain it is less than one part in a million. When atmospheric moisture condenses into rain drops it is extremely pure water. But as it passes through the air it rapidly absorbs oxygen, carbon dioxide and ammonia. Hence, when it reaches the earth it is no longer pure water, but water plus a “ little something.” Over towns rain can contain as much as 35 parts in a million of sulphuricacid, and over four parts in a million of ammonia. In inland country districts the sulphuric acid content of rainwater is only about five parts or less in a million and the ammonia well under one part. Rain at the beginning of a shower has a higher -plant food value than the later rain, as has also rain after a long drought when compared with that which falls towards the close of a rainy season.