Standard flight plan has provision for using forecast winds, seasonal winds or Nil winds.
I would like to know:

1. What is the definition of Seasonal Winds for use in flight plan? The options are:

a) It is the monthly mean
b) It is mean for the winter(Dec-Mar), spring(Apr-May), summer(Jun-Sep), autumn(Oct-Nov).
c) Any other

2. Are they available for each degree latitiude/ longitude over the entire globe

3. What is the source of seasonal winds?

4. Is it free ? Else how much it costs?