If you're talking about the mood or atmosphere:
* Cheerful
* Bright
* Happy
* Sunny
* Joyful
* Optimistic
* Hopeful
* Lively
* Vibrant
If you're talking about the weather:
* Sunny
* Clear
* Bright
* Warm
* Pleasant
If you're talking about a person's outlook:
* Positive
* Upbeat
* Hopeful
* Optimistic
The best opposite will depend on the specific situation and the meaning you want to convey.