Learn Italian: Il tempo oggi è pessimo. - The weather today is terrible.

pessimo is the irregular absolute superlative of cattivo (bad). It means "extremely bad" or "terrible". While you can technically say molto cattivo or cattivissimo, Italians use pessimo very frequently to say something is just awful.

If pessimo is a superlative, does its ending change like a normal adjective?

Yes! Even though it is an irregular superlative, it still acts like a normal adjective. It must match the noun it describes in gender and number. Here it is pessimo to match the masculine singular noun tempo. If it described a feminine noun like scelta (choice), it would be pessima.

I thought tempo meant "time"?

It means both "time" and "weather"! You can usually tell which one is meant from the context. If you are talking about rain, sun, or the current conditions outside, tempo means "weather".

What if I just wanted to say the weather today is "worse" than yesterday?

To say "worse", you use the irregular comparative form peggiore. For example, you would say Il tempo oggi è peggiore di ieri. You use pessimo only when the weather is absolutely terrible, not when comparing it to something else.