"The other day when I got to the airport, there was definitely an overcast cloud deck, but the automated weather for the airport was reporting a clear sky. How can that be?" - Robert J.
“This typically occurs when the bases of those clouds are very high. Surface observing equipment such as the Automated Surface Observing System or ASOS, employs a sensor called a ceilometer to measure the height of the cloud bases. This ceilometer is a vertically pointing laser that measures the height and infers the amount of cloud elements that pass over the sensor. It operates continually and once every minute it determines the sky condition based on a weighted average over the most recent 30 minutes. The sensor does not measure or know what is happening along the horizon, nor does it report on clouds that may exist above 12,000 feet.
At airports with a human observer, the sky coverage and ceiling height can be augmented to report ceilings above 12,000 feet. So when you see an automated surface observation that reports the sky as clear (CLR), just remember that this means the sky is clear below 12,000 feet above the ground level.”