The instructions in this chapter relate to the state or appearance of the sky. Sky condition may be evaluated either automatically by instrument or manually. Clouds include obscuring phenomena aloft. Sky condition shall be evaluated at all stations with this capability. Automated stations shall have the capability to evaluate sky condition from the surface to at least 12,000 feet. Observers at manual stations shall evaluate all clouds and obscuring phenomena visible, i.e., the 12,000-foot restriction shall not apply.

12-2.   SKY CONDITION EVALUATION

A complete evaluation of sky condition includes the type of clouds or obscuring phenomena present, their stratification, amount, direction of movement, height of bases, and the effect on vertical visibility of surface-based obscuring phenomena.

12-3.   CLOUD FORMS AND OBSCURING PHENOMENA

If available, the WMO International Cloud Atlas, Volumes I and II, and the Abridged Atlas contain detailed instructions and photo-aids for identifying the various cloud forms. Additional aids may be used for identifying cloud forms (types) such as cloud code charts. Commercial products are also available that describe cloud forms and types. Descriptions of obscuring phenomena are included in Chapter 11, Weather Phenomena.