AIM Text
- Ceiling, by definition in the CFRs and as used in aviation weather reports and forecasts, is the height above ground (or water) level of the lowest layer of clouds or obscuring phenomenon that is reported as “broken,” “overcast,” or “obscuration,” e.g., an aerodrome forecast (TAF) which reads “BKN030” refers to height above ground level. An area forecast which reads “BKN030” indicates that the height is above mean sea level.
- Pilots usually report height values above MSL, since they determine heights by the altimeter. This is taken in account when disseminating and otherwise applying information received from pilots. (“Ceiling” heights are always above ground level.) In reports disseminated as PIREPs, height references are given the same as received from pilots, that is, above MSL.
- In area forecasts or inflight advisories, ceilings are denoted by the contraction “CIG” when used with sky cover symbols as in “LWRG TO CIG OVC005,” or the contraction “AGL” after, the forecast cloud height value. When the cloud base is given in height above MSL, it is so indicated by the contraction “MSL” or “ASL” following the height value. The heights of clouds tops, freezing level, icing, and turbulence are always given in heights above ASL or MSL.
Source: FAA Aeronautical Information Manual · current edition · paragraph 7-1-14.
The short answer
AIM 7-1-14 is FAA advisory guidance on how cloud heights are reported. Ceiling, defined in the CFRs, is the height above ground (or water) level of the lowest broken, overcast, or obscuration layer. It explains that TAF heights are AGL, area forecast heights are MSL, PIREP heights are MSL as received from pilots, and tops, freezing level, icing, and turbulence are given above MSL/ASL.
Research Notes
Common Questions
Is a ceiling height given AGL or MSL?
What reference do PIREP cloud heights use?
The AIM notes pilots usually report height values above MSL because they determine heights by the altimeter, and in disseminated PIREPs height references are given the same as received from pilots — that is, above MSL.
How are cloud tops, icing, and turbulence heights referenced?
According to the AIM, the heights of cloud tops, freezing level, icing, and turbulence are always given in heights above ASL or MSL. In forecasts, ceilings are denoted by the contraction “CIG” and AGL or MSL/ASL contractions clarify the reference.