Pilots need to know how high the lowest cloud base sits above the ground. That height is the ceiling. When the sky is covered by more than half with clouds, the ceiling equals the height of the lowest layer that meets that coverage.

Ceilings are reported in hundreds of feet. A ceiling of 500 feet is low and can restrict visual flight. Clear skies or scattered clouds mean no ceiling is reported. Fog sitting on the surface produces a ceiling of zero.

Automated sensors and human observers both measure ceilings using laser ceilometers or visual estimates. Aviation forecasts always include ceiling information because it directly affects whether aircraft can land or depart under visual rules.

• Height of lowest significant cloud layer
• Critical for aviation operations
• Measured by ceilometers or observers
• Zero when fog covers the surface