今天很热。

Just drop it into conversation whenever you feel the sun. You can say it to yourself as a sigh. You can say it to a stranger while waiting for the bus. It is a complete sentence on its own. If you want to sound more natural, add a 啊 (a) at the end. 今天很热啊！ makes you sound like you are really feeling the heat. It is like saying "Man, it's hot today!"

Use it when you walk out of an air-conditioned mall. Use it when you are sweating in the subway. Use it as a filler when you don't know what else to say. It works perfectly in text messages to explain why you are late. "I'm moving slowly because 今天很热." It is a universal truth that everyone can agree on. It builds instant rapport with the person next to you.

Don't use it if it is just "warm." For that, use 暖和 (nuǎnhuo). Avoid using it in a formal weather report; they use more technical terms. Also, don't say it if you are inside a cold room. People will think you have a fever! It refers to the ambient temperature outside. If you are personally feeling hot but the room is cold, say 我觉得热 (I feel hot).

In China, talking about the weather is a safe social lubricant. It is right up there with "Have you eaten?" Small talk is essential for building guanxi (relationships). During the "Three Fu" days of summer, this phrase is heard everywhere. People in cities like Chongqing or Wuhan (the "ovens" of China) say this with a sense of pride. It is a shared struggle that brings people together.

If it is insanely hot, swap 很 for 太 (tài). 今天太热了！ means "It's too hot today!" If you want to be dramatic, try 热死了 (rè sǐ le). This literally means "hot to death." It is very common among friends. You can also add 真 (zhēn) for emphasis. 今天真热 means "Today is truly hot." Each variation adds a little more spice to your complaint.