Wind, fog, rain, and lightning primarily (7)
I believe the answer is:
mistral
'wind' is the definition.
(I know that mistral is a type of boreas)
'fog rain and lightning primarily' is the wordplay.
'fog' becomes 'mist' (mist is a kind of fog).
'primarily' indicates taking the first letters.
The initial letters of 'rain and lightning' is 'ral'.
'mist'+'ral'='MISTRAL'
(Other definitions for mistral that I've seen before include "Strong cold dry wind" , "Strong north wind of southern France" , "Famous wind in the Rhone Valley blowing towards the Med" , "That's a blow" , "Nice chiller" .)