1. 这是冷的： when i have salad for breakfast, i would say "这是冷的", emphasize the thing i touch, or i eat, or i feel is cold, 2. 这很冷： same usage as "这是冷的"， but it means colder 3. 这非常冷 : same usage as "这是冷的" , but it means coldest, sounds that you are upset, not satisfied with something. 4. 天气是冷的: talk about the weather. the weather is cold. a little odd. 5. 天气很冷: same usage as "天气是冷的"， but it means colder. sounds natual, you think the weather is cold. 6. 天气非常冷：same usage as "天气是冷的" , but it means coldest，emphasize the weather is extremely cold. sounds you need sweater.