Understanding Clouds is the theme of World Meteorological Day 2017, which falls annually on 23 March.
The official site of the World Meteorological Organisation’s (WMO) International Cloud Atlas describes the classification system for clouds and meteorological phenomena used by all WMO members.
The Atlas provides a common language to communicate cloud observations, and ensures consistency in reporting by observers around the world. It serves as a training tool for meteorologists, as well as for those working in aeronautical and maritime environments, and it has become popular with weather enthusiasts and cloud spotters. – World Meteorological Organisation