The negative form of "extreme"
A common antonym of extreme, in contexts ranging from weather to politics, is moderate:
ODO:
- Average in amount, intensity, quality, or degree. ‘we walked at a moderate pace’
- (of a person, party, or policy) not radical or excessively right- or left-wing. ‘a moderate reform program’
American Heritage Dictionary:
- Being within reasonable limits; not excessive or extreme: a moderate price.
- Not violent or subject to extremes; mild or calm; temperate: a moderate climate.
- Of medium or average quantity or extent.
- Of limited or average quality; mediocre.
- Opposed to radical or extreme views or measures, especially in politics or religion.
Macmillan Dictionary:
neither very great nor very small in amount, size, strength, or degree
Cook the spinach over a moderate heat.
A moderate earthquake shook the San Francisco bay area this afternoon.
We need more housing for moderate-income families.
This is a new drug for treating mild to moderate depression.