Clouds from Selsey, England imaged by Pete Lawrence (Digital
Sky). ©Peter Lawrence.
The last few nights
have seen bright NLCs over
The clouds are our highest. They are over 50 miles high and a few
miles below the mesopause which
at -90°C is the coldest part of our atmosphere.
The skeins of tiny ice
crystals shine by reflected sunlight. At northern latitudes the sun
is still above the horizon at their altitude. Summer is also the
time when the mesopause is at its coldest. Changes in
the upper atmosphere could be making the clouds appear more often.
Here it might appear that the moon is illuminating them. Not so,
the clouds are so tenuous that the light of the moon is quite insufficient
to make them visible.