What is the plural of "weather"?
It stays the same
It is a non-count word such rain,milk...
So, to pluralize non-count words it is necessary to add a word that signifies a unit or measurement of quantity.
"weather" has no plural form.
(At least as far as I can imagine it).
Compare the following usage of boy / boys with a definite plural to parallel usage of weather.
That boy is happy.
The boys in our class include Tom, Dick, and Harry.