打雷了。

You can use this phrase all by itself. It’s a complete sentence. If you hear a loud 'boom' while sitting on the couch, just look up and say 打雷了. You can also add it to other sentences. For example, you might say 打雷了，快回家吧 (It's thundering, let's go home quickly). It’s very versatile and works exactly like the English equivalent.

Use it whenever the weather turns stormy. It’s great for breaking a silence when a storm starts. You can use it while texting a friend to explain why you’re staying indoors. It’s also perfect for small talk with a neighbor or a colleague near a window. If you're at a restaurant and hear a rumble, saying this helps everyone decide if they should move inside.

Don't use this if it's just raining without the sound. For just rain, you say 下雨了. Also, avoid using it metaphorically for 'angry' unless you are much more advanced. In a very formal business presentation about finances, saying 打雷了 out of nowhere might make people think you're literally hearing a storm outside. Stick to the actual weather for now!

In Chinese culture, thunder is often associated with the 'Thunder God' (雷公). People used to believe thunder was a sign of heaven's power. Today, it’s mostly just weather talk. However, there is a famous proverb: 雷声大，雨点小 (Loud thunder, small raindrops). This describes someone who talks a lot but doesn't take much action. It's like saying "all bark and no bite."

If the thunder is really loud, you can say 打大雷了. If it’s just starting, you might hear 要打雷了 (It’s about to thunder). Sometimes people combine it with rain: 打雷下雨了. If you want to be more descriptive, you can say 雷声很大 (The thunder sound is very loud). But for most daily life, the simple three-word version is your best friend.