12-1.     Introduction

12-2.     Sky Condition Evaluation

12-3.     Cloud Forms and Obscuring Phenomena

12-4.     Ceiling

12-5.     Celestial Dome

12-6.     Cloud

12-7.     Cloud Movement

12-8.     Field Elevation

12-9.     Horizon

12-10.   Interconnected Cloud Layers

12-11.   Layer

12-12.   Layer Amount

12-13.   Layer Height

12-14.   Multiple Layers

12-15.   Obscuring Phenomena

12-16.   Sky Cover

12-17.   Sky Cover Classifications

12-18.   Summation Amount

12-19.   Summation Principle

12-20.   Surface

12-21.   Total Amount

12-22.   Variable Ceiling

12-23.   Variable Sky Condition

12-24.   Observing Sites

12-25.   Layer Amounts

12-26.   Summation Layer Amount

12-27.   Layer Heights

Figure 12-1.     Reportable Contractions for Sky Cover

Figure 12-2.     Priority for Reporting Layers

Figure 12-3.     Increments of Reportable Values for Layer or Ceiling
                        Heights

12-28.   Evaluation of Multiple Layers

12-29.   Amount of Obscuration

12-30.   Determining Amount of Sky Cover

12-31.   Evaluation of Sky Cover Amounts

12-32.   Sky Cover Classification

12-33.   Variable Sky Cover

12-34.   Non-Uniform Sky Cover

12-35.   Estimated Ceiling Heights

12-36.   Variable Ceiling Height

Figure 12-4.     Criteria for Variable Ceiling

12-37.   Significant Clouds

12-38.   Cloud Types (NA LAWRS)

12-39.   Operation of Equipment

Figure 12-5.     Examples: Summation of Sky Cover

Figure 12-6.     Convective Cloud-Base Height Diagram