General bad weather:
* Il tempo è brutto. (This is the most common and direct translation.)
* Fa brutto tempo. (This is also very common and emphasizes the feeling of the weather.)
More specific bad weather:
* Il tempo è cattivo. (This implies a more severe or dangerous type of bad weather.)
* Fa un tempo pessimo. (This emphasizes the extreme badness of the weather.)
* C'è brutto tempo. (This translates to "there is bad weather" and focuses on the presence of the bad weather.)
Other options:
* Il tempo è schifoso. (This is a more informal and slangy way to say the weather is bad.)
* Il tempo è terribile. (This emphasizes the terrible nature of the weather.)
You can also use more specific adjectives depending on the type of bad weather, for example:
* Fa freddo. (It's cold)
* Fa caldo. (It's hot)
* Piove. (It's raining)
* C'è vento. (It's windy)
* C'è la nebbia. (It's foggy)
Ultimately, the best way to say "the weather is bad" in Italian depends on the context and the level of formality you want to achieve.