Those who have said it is measured by l@ser are barking up the wrong tree.
Oh dear, people need to get out more
Measuring Cloud Height | Laser Ceilometer Model 8340 | Weather Instruments
The 8340 Laser Ceilometer measures cloud height and thickness, in addition to vertical visibility, detecting up to four cloud layers simultaneously to a distance of 40,000 vertical feet. Its precision makes it ideal for applications requiring the highest in performance and reliability, such as aviation and meteorological studies.
A laser pulse is emitted into the atmosphere and backscatter analyzed. Using the speed of light, the height of each cloud base and top is determined. Due to poorly defined borders or a sparse composition, some clouds are much more difficult to measure than others. Depending on the current and historical sky conditions, an adaptive algorithm determines the number of returns needed to maintain accuracy.
Accurate measurement of cloud height and thickness in all weather conditions, including heavy precipitation and low clouds, can cause serious errors in other ceilometers. Proprietary algorithms and digital techniques from 20 years of cloud height detection research and manufacturing are applied, allowing the 8340 ceilometer to provide accurate information even in difficult circumstances.
What about the AWIB's?
There's no ATC where there are AWIB's. (generally)
An AWIB unit can measure different levels of cloud base if you've listened to them carefully...
Or:
The manual way.
1. Find the difference between Surface Temperature (°F) and Dew Point (°F)
2. Divide the difference by 4.4
3. Multiply the quotient by 1000
4. Add the product to the Field Elevation (ft)