Noctilucent clouds: http://www.spaceweather.com/swpod200...firstlight.jpg

It would be interesting to examine the data of marked periods of these
things. I am guessing that they are the product of venting volcanoes
and might thus be shown to occur during periods where the North
Atlantic Oscillation is positively negative.

And we have a tool to examine sea level pressures for the North
Atlantic: http://www.wetterzentrale.de/topkarten/tkfaxbraar.htm

A powerful jet of superheated water could reach the upper atmosphere
becoming in turn vapour then ice whilst still a cohesive unit. At
those heights dispersal would only take place as gravity attracts the
cloud into the lower levels where it can be melted or deposited.

Another alternative would be severe cyclonic activity. Low pressure
raises huge amounts of water after all. In that case though the
pressure differences in the North Atlantic -as elsewhere, will be
markedly positive.

Again that last link would indicate an anomaly but this time, positive.