From Academic Kids
The Cloud base is the lowest portion of a cloud.
A ceilometer is used to measure the height of the cloud base.
Knowing the height of the cloud base is important in aviation as aircraft require a minimum safe altitude on landing approach to see the runway lights. If the cloud base descends below the minimum safe altitude then an airport may need to be closed for safety reasons.
- National Science Digital Library - Cloud Base (http://www.nsdl.arm.gov/Library/glossary.shtml#cloud_base)
- Measuring Cloud Heights (http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?Measuring_Cloud_Heights) : A lesson plan from the National Science Digital Library.