We group artworks with similar characteristics into periods or styles because it is easiest to do it that way - this way we can find common characteristics in these literary/artistic eras, and name them. For example, the Renaissance had a distinct style, which differs greatly from that of Romanticism.
2 that us the answer to your question
ocean - Large bodies of water
current flowing towards equator carry cold water from poles
current flowing away from equator carry warm water from poles
Ex - Humidity
