Fact 1: Fog is formed by water vapor condensing in the air.

Fact 2: When air cools, the water vapor in the air condenses.".

Given the two facts above, answer the question "Fog is formed when" with the following options: 
- summer end
 - evaporation
 - Winter
 - movement
 - air cools
 - Wet weather
 - rainy
 - dry and hot


 

     air cools    