The first is the state of the weather yesterday. 
The first is how the weather is.
The first is how the weather was last week.
The Clean Air Act in now way recognizes visibility as something that should be protected.
Some people walking through a field.
The kids are running.
A woman is holding two puppies in her hands.
I've never been there.
THeir study covered about half of the Class I areas in the US.