The Americans
consider June 21st to be the start of summer,

That's interesting, as most parts of the USA seem to have a later
summer compared to us. I've noticed (from weather reports) that most
parts of the USA are as likely as NW Europe to have cool, wet weather
in say May, while September and October are notably warmer and sunnier
than NW Europe. I remember seeing some monthly average maxima for New
York, and amazingly, New York is warmer in October (mean max 21C) than
May (mean max 20C).

Nick