The weather is fine. Go downstairs for a walk

The sun is shining. Go out and get some fresh air.

Sunny day means sunny weather, blue sky, cloudless Wan Li and Wan Li. Fresh air means that the air outside is good and suitable for going out.

Weather refers to the ever-changing atmospheric state, which is not only the atmospheric state in a certain time and space, but also the continuous change of atmospheric state in a certain time interval. So it can be understood as a general term for weather phenomena and weather processes.