What unit is used in PIREPs to report cloud base heights?

Cloud base heights in PIREPs, or Pilot Reports, are reported in feet MSL (Mean Sea Level). This measurement denotes the height of the cloud base relative to mean sea level, providing a standardized reference point for pilots and air traffic controllers to understand the altitude of clouds concerning the earth's surface. This information is crucial for assessing visibility and weather conditions pertinent to flight operations.

Using feet AGL (Above Ground Level) is not suitable in this context because cloud heights need to be referenced consistently to mean sea level for uniformity across diverse terrains. Altitude in meters may be a valid measure in some situations, but aviation in the United States primarily uses feet as the standard unit, making feet MSL more applicable. Flight levels are also not used for reporting cloud bases, as they denote altitudes expressed in hundreds of feet, typically for aircraft operating at higher altitudes in the flight levels system.

Thus, feet MSL serves as the most accurate and widely accepted unit for reporting cloud base heights in PIREPs.