Whether: used when talking about a choice you have to make or about something that is not certain
Example: She was uncertain whether to stay or leave.
Weather: the temperature and other conditions such as sun, rain, and wind
Example: What's the weather like in Canada in winter? It's snowy.
Wither: If plants wither, they become drier and smaller and start to die.
Example: If you do not water the plants for a week, they will wither.
Wether: (1) Wool from previously shorn sheep
(2) A castrated ram