If a climate is intemperate, its temperatures might be extreme. If a person is intemperate, his moods might be extreme. Being intemperate is all about avoiding moderation.
Intemperate is a combination of the prefix in- meaning "not" and the Latin temperantia meaning "moderation." When you are intemperate, you are not doing things in moderation; you lack self-control. It's often a word used when describing the tendency someone has to indulge excessively in liquor. An alcoholic is intemperate — overindulging and depending on alcohol.