Changing patterns in NLC occurrence have motivated further investigations of these clouds.
Two pronounced changes are:
A dramatic increase in the frequency of NLCsThe 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