peak of NLC season

[menno]

Normally you would think that the peak of the NLC-season is around the longest day -June 21st-.
I have the impression that the peak is somewhat shifted to the last days of June or perhaps the very first days of July. Perhaps someone has done some statistical investigations on this subject. Is my impression correct? And if so, what's causing this shift?

[Bill W]

Hi,
The brightest displays occur after the solstice because the atmospheric circulation continues to strengthen as the summer continues and stronger mesospheric cooling occurs.

The solstice represents the point where the sun is of course furthest north (in the northern hemisphere) and hence the zone of the planet that has the sun between the often quoted 6-16 degrees below the horizon, is at it's greatest possible extent furthest south.

This effectively means that the time available for "observing" NLC is at it's greatest. BUT due to continuing cooling, the mesopause is effectively freeze drying itself, the BRIGHTEST nlc are much more frequently seen after the solstice.

The time issue on the changes to mesospheric circulation are one reason why the reports of early NLC are received both with interest and caution.
Hope this helps.
Cheers,
Bill.