While Ley had been finishing his work and seeing it through the press, H. Hildebrand-Hildebrandsson and R. Abercromby had devised another modification which differed from Howard’s chiefly by the introduction of a new class, which they distinguished by the use of the prefix Alto. This scheme was formally adopted by the International Meteorological Conference held at Munich in 1891, and a committee was appointed to draw up an atlas showing the exact forms typical of each variety considered. Finally in August 1894 a small sub-committee consisting of Messrs H. Hildebrand-Hildebrandsson, A. Riggenbach-Burckhardt and Teisserenc de Bort was charged with the task of producing the atlas. Their task was completed in 1896, and meteorologists were at last supplied with a fairly detailed scheme, and one which was adequately illustrated, so that there could be no doubt of the authors’ meaning. It is as follows:—
The International Classification.
(a) Separate or globular masses (most frequently seen in dry weather).
(b) Forms which are widely extended, or completely cover the sky (in wet weather).
A. Upper clouds, average altitude 9000 metres.[2]
a. 1. Cirrus.
b. 2. Cirro-stratus.
B. Intermediate clouds, between 3000 m. and 7000 m.
a. 3. Cirro-cumulus.
4. Alto-cumulus.