Significance is a type of Meaning. Reference is a type of Meaning. Reference is related to Drop. Drop is related to Ground. 
Clear is the opposite of Fog. Fog is the opposite of Sky. you are likely to find Balloon in/at/on Sky. 
Significance --IsA--> Meaning <--IsA-- Reference <--RelatedTo--> Drop <--RelatedTo--> Ground 
Clear <--Antonym--> Fog <--Antonym--> Sky <--AtLocation-- Balloon 