"Today is sunny too."
I think you mixed up the two concepts. …が…に…いる is the construct for the existance of animated subjects.
However …ている has two different meanings and is different from above.
Action verb + ている means the continuous action of the verb..e.g. 見ている "is looking," 食べている "is eating."
State verb + ている means the continuous state of the verb. e.g. 始まっている=始まった "something started/is starting," 着いている=着いた "someone arrived," 終わっている=終わった "something finished."
Languages do not correspond one to one. For example, we say "I am hungry" but we say "J'ai faim" in French or "Ich habe Hunger" in German. "Faim" and "Hunger" are nouns instead of adjective! Similarly we have to say "私はおなかがすいた" or ”腹が減った" or "空腹" to represent "hungry." All these sentences are the common ways of saying hungry in the respective countries.
This is also true for sunny. There is no common adjective for sunny in Japanese. In Japanese the way to translate is to use ～ている or ～た together with a state verb. Examples are 晴れている, おなかがすいた or 腹が減った. If you look at my other comments, ている with a state verb represents a continuous state, not a continuous action.