Question

What is the plural of "weather"?

Answer 1

It stays the same

Explanation:

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.

Answer 2

"weather" has no plural form.
(At least as far as I can imagine it).

Explanation:

Compare the following usage of boy / boys with a definite plural to parallel usage of weather.

That boy is happy.
`color(white)("XXX")`The weather is pleasant.

The boys in our class include Tom, Dick, and Harry.
`color(white)("XXX")`The weather in Canada includes snow, rain, and extreme heat.
`color(white)("XXXXX")`Note: we would not say, "The weathers in Canada include ...."