equable
Unvarying, calm and steady; constant and uniform.
The cheerful influence of the morning seemed to have some effect, even upon his equable temper.
Free from extremes of heat or cold.
The indulgences of habit she would not resign, therefore held by the pleasures of the table; but the comforts of equable heat, which was particularly necessary, she denied herself much too long, and resisted warm clothing most determinately;...
Not easily disturbed; tranquil.
Where it comes from
Learned borrowing from Latin aequābilis, from aequō (“make level”), from aequus (“even, level”).
Adapted from the Wiktionary entry equable (CC BY-SA 4.0).