Changing patterns in NLC occurrence have motivated further investigations of these clouds.

Two pronounced changes are: 

A dramatic increase in the frequency of NLCs
The recent propagation of NLCs to middle latitudes

These changes are demonstrated below

The figure below is from Gadsen [1997],  and shows the number of NLC sightings versus time.  Note that the number of NLCs has doubled during the past 35 years.

The map below shows areas where NLCs typically occur (poleward of 50 degrees latitude),   land areas where ground observers typically see NLCs,  and towns in the USA where NLCs have been sighted in recent years,  for the first time ever. Note that NLCs are now occurring nearly 700 miles farther south than ever before.  


US towns where NLCs were recently sighted:
  • Twin Falls,  Idaho
  • Logan,  Utah
  • Boulder,  Colorado
  • Glen Ullin,  North Dakota
  • McGuire,  New Jersey