In your first sentence, either rainy or raining could fit, depending on what you actually want to say; "... because it is raining" indicates that water is physically falling from the sky right now, while "because it is rainy" indicates that it is the sort of day where rain is extremely likely to happen, but doesn't necessarily mean that rain is ...
What are the best foods to have on a rainy day? What are the best foods to have for a rainy day? Which preposition would be appropriate here?
grammar - ON a rainy day or FOR a rainy day? - English Language ...
The reason is that in the first sentence, "today is rainy", today is the object being described directly, so you don't need the pronoun 'it'. In the second however, there is a comma so after the comma, the 'it' pronoun is needed to make the sentence correct (hence the 'it's').
is it correct to say "today is rainy" or it is "today, it's rainy"?
Today is good. "Rainy" and "good" are both adjectives, so: Today is a rainy day. Today is rainy. But "frost" is a noun, so to make parallel sentences, you would have to use the adjective, "frosty": Tomorrow will be a frosty day. Tomorrow will be frosty. Alternatively, if you want to use the noun "frost", you could say, "Tomorrow there will be ...
word order - "Today is rainy" Vs. "It's rainy today." - English ...
word usage - "It is raining" or "it is rainy"? - English Language ...
It is perfectly idiomatic to say “it is rainy” to mean “it is raining” and vice versa, m.m., the same for snowy, icy, etc. It is not necessary for snow or ice to accumulate to use these descriptions for the weather.