How do weather symbols represent sunny days?

11:00, 16:00, and 17:00 represent sunny days, while 12-15:00 represent sandstorms.

Weather phenomenon symbols specified in the surface meteorological observation specifications of the China Meteorological Administration:

Weather graphic symbols issued by the China Meteorological Administration: Extended information

Weather symbols are representative Special symbols for various weather phenomena, cloud shapes, sky conditions, etc. Includes weather phenomenon symbols for observation records and weather graphic symbols for media dissemination.

When a meteorological observer observes a certain weather phenomenon, the corresponding symbol should be recorded in the "Weather Phenomenon" column of the observation book on that day, except for a few phenomena, and its start and end times should also be recorded. Based on these symbols, combined with the intensity, coexistence and certain characteristics of weather phenomena, a set of symbols that can be filled in on weather maps is called map-filling symbols.

In order to facilitate public understanding and recognition, a set of images representing weather phenomena is developed for use on television or other public media.

Reference: Baidu Encyclopedia-Weather Symbols