Humid current around river (8)
I believe the answer is:
tropical
But I can't explain it at all! Could you help me understand?
(Other definitions for tropical that I've seen before include "Hot and humid, as north and south of the equator" , "Belonging to hot regions" , "Hot and humid type of weather" , "Fervidly hot" , "April-Oct, unusually humid and very hot" .)