Fact 1: Cold fronts cause thunderstorms as they pass by.

Fact 2: Fog is common in the cold air ahead of the front.

Given the two facts above, answer the question "when is fog common?" with the following options: 
- summer end
 - before thunderstorms
 - The area was once underwater
 - dry and hot
 - It used to be covered by water
 - never
 - outer space
 - under water


 

  before thunderstorms       