In complex displays, two or more forms may be seen simultaneously. It is not unusual for two intersecting groups of long bands to occur, and these give rise to bright knots where waves cross. Complex structures can be categorized as follows:
O: A form that does not fit into types I–IV above
S: A noctilucent cloud with bright knots in the structure
P: Billows crossing a band
V: A net-like structure
Artificial noctilucent clouds can be created by rocket launches into space, where exhaust products of water vapour and solid particles are introduced into the mesosphere. These noctilucent clouds can be observed at latitudes below 45°.