In general, cloud height is measured from ground or AMSL (above mean sea level)?
If they measure from AMSL, how are low level clouds' height measured in higher terrain?
Cloud ceiling is measured from ground:
height of the base of the lowest clouds ... that cover more than half of the sky ... relative to the ground
Cloud base is measured from sea level:
... the lowest altitude of the visible portion of the cloud. It is traditionally expressed either in m or feet above mean sea level