Example sentences
jīn今tiān天tiān天qì气zěn怎me么yàng样？
How's the weather today?
tiān天qì气biàn变lěng冷le了。
The weather turned cold.
About this word
天 is "sky, day" and 气 is "air, breath, vapor". 天气 is the air of the sky, meaning the state of the weather. 气 turns up all over Chinese as a word for a mood or a force: 生气 (angry, literally "born air"), 客气 (polite), 力气 (strength). Weather talk takes adjectives directly: 天气很好, 天气冷了. Do not insert 是, since 天气是好的 sounds wrong. The standard small-talk opener is 今天天气很好.
The story behind the character
天
大 is a person with arms spread. Add one stroke on top and you get 天. The space above a person's head. So it didn't start as "sky," it started as "the top of your head." Only later did it climb up to mean heaven.
气
Three strokes of rising vapor. Xu Shen wrote 气, 云气也: "cloud vapor." Side note: the traditional 氣 is actually a different character (it meant "to gift grain"); they got merged when characters were simplified.