This cloud formation is the reult of instability in the midlevel regions. This cloud forms when colder air slides across an area of altocumulus cloud. It is a very good indicator for thunderstorms later in the day. With further surface heating during the day the mid level cas grow into cumulonimbus. The first pic below is from a decaying altocumulus late in the day showing virga trails.
The second pic is ML instability also late in the day and ahead of Thunderstorms that moved over to Ireland from Wales later in the evening.
Pic by nilhg
Pic by Snowbie