Extremely high Summer clouds.

[Trevatanus]

Noting some really high white clouds with this gorgeous summer weather.
Did not realise just how high they were until I saw an airline which was clearly very high, pass below them.
What are they called, and are we experiencing some unusual phenomena to see them (apart from it being hot and sunny in the UK obviously!)

[DocJock]

High cirrus.

[Eric Mc]

I've seen that a few times. It's quite rare and strange to see a high altitude aircraft leaving a contrail which is BELOW the level of a higher cloud layer.

Another phenomenon you sometimes see is an aircraft ploughing directly through a high altitude but thin layer of cloud. When that happens, the wing vortices leave a twin furrow in the cloud layer which gradually spreads apart.

[Beati Dogu]

There's this one of a Boeing 787 creating quite the vapour trail:

https://www.youtube.com/watch?v=8v6sxIs0csk