A cumulonimbus.
Given the unique nature of a British summer (i.e. it’s like winter) it’s always a good time for cloud watching. One of our favourite books of recent years is Gavin Pretor Pinney’s The Cloudspotter’s Guide, which is a great book to have in the car and really makes you appreciate what fabulous things these huge, floating masses are.
If you want to hear more, there’s a wonderful slideshow narrated by the author here.
Or you can join the Cloud Appreciation Society here.