National Geographic : 1904 Jul
Fig. 5 . -Cumulus Clouds Cumulus clouds, as in the above, illustrating the formation of a central core of ascending warm air, generally precede local rains or thunderstorms by a few hours. Photos by Henry Fig. 6. -Near View of Large Cumulus Clouds The small detached clouds on the lower left-hand margin of the cloud are almost invariably seen in advance of thunderstorms. Cumulus clouds, like those shown, rarely give rain at the point of observation, since their prevailing drift in these latitudes is eastward.