From the experts:
This variation of Adele sounds somewhat more feminine than the original by virtue of the a- ending. It could join the army of popular A girls' names, especially with its tie to the recent celebrity-related success of Adele.
Adela, which came to England with the Normans in the eleventh century, has a royal pedigree via William the Conqueror's youngest daughter.
Adela Quested is a character in E. M. Foster's novel A Passage to India, and the name also appears in an Agatha Christie story. Journalist/novelist/screenwriter Adela Rogers St. .Johns was a colorful character in Golden Age Hollywood.
Adelia is another appealing variation, an alternative to the rising Amelia.