May: cumulonimbus

1   January: a Yellow Disc Rising from the Sea
2   February: the Arctic Circle
3   March: Seed Capsule
4   April: the First Day of Summer
5   May: cumulonimbus
6   June: Transit of Venus
7   July: murmuration
8   August: cloudpark
9   September: The Last Day of Summer
10   October: Cerberus
11   November: eleven
12   December: polarisation
13   Iliac Crest
14   Dead Space
15   Incubator