temperate
Mild and not extreme in weather, climate, or behavior.
Temperate describes weather, climate, or behavior that's moderate and mild, avoiding extremes. A temperate climate has warm summers and cool winters, but nothing too harsh: not scorching deserts or frozen tundras. Most of the United States has a temperate climate, which is one reason so many different crops can grow there.
When describing a person, temperate means showing self-control and moderation. A temperate response to bad news stays calm instead of flying into a rage. Someone with a temperate personality doesn't swing wildly between moods. They're steady and balanced.
Think of Goldilocks seeking porridge that was neither too hot nor too cold: she wanted something temperate. The opposite would be intemperate, which describes behavior or conditions that go to extremes. An intemperate person might shout angrily over small problems, while a temperate person would stay composed.
When scientists describe the temperate zones of Earth, they mean the regions between the tropics and the polar areas, where conditions stay relatively moderate year-round. These zones have made it easier for human civilizations to thrive because the climate doesn't swing between dangerous extremes.