Mildness, n.
The quality or state of being mild; as, mildness of temper; the mildness of the winter.
Shades of meaning
Fernald has no entry of its own for mildness; he lists it under mercy.
Where he writes about it:
Clemency is a colder word than mercy, and without its religious associations, signifying mildness and moderation in the use of power where severity would have legal or military, rather than moral sanction; it often denotes a habitual mildness of disposition on the part of the powerful, and is matter rather of good nature or policy than of principle. — from mercy