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Clouds
- Introduction and principles of cloud classification
- Definitions of clouds
  - Useful concepts
    - Height, altitude, vertical extent
    - Levels
  - Definitions of clouds
- Descriptions of clouds
- Orographic influences on clouds
- Upper atmospheric clouds

Height, altitude and vertical extent

(Section 2.2.1.1)

Height: Vertical distance from the point of observation on the Earth’s surface to the point being measured.
Altitude: Vertical distance from mean sea level to the point being measured.
Height/Altitude of cloud base: For surface observations, height of the cloud base above ground level; for aircraft observations, altitude of the cloud base above mean sea level.
Vertical extent: Vertical distance from a cloud’s base to its top.

The text enclosed in grey-shaded boxes, like this example, comprises Annex I to the Technical Regulations (WMO-No. 49) and has the legal status of standard practices and procedures.