Hi Jacqueline - most adjectives go after the noun, so it's easiest to learn the exceptions and assume the rest go after. With weather, the obvious exception would be "un beau jour" (a fine/nice day).
However, you do need to be a little careful with weather expressions and descriptions as you often can't translate them word for word from the English. "A cold day" = "un jour froid" and "A cold and rainy day" = "un jour froid et pluvieux" but "a snowy day", would be "un jour de neige".
There will be many cases like that where a 'de' + noun is used instead of an adjective. "un jour de brouillard" (a foggy day).
If we are talking about the weather now, then we also have the "il y a" and "il fait" expressions:
Talking about the weather in French - il y a + [nom]
Talking about the weather in French - il fait + [adjectif]
Notice that the "il fait" expressions use adjectives but the "il y a" expressions use preposition + noun.
Hope that helps.
