晴天。
Qíng tiān
Clear sky.
Single noun, simplest possible
今天天气很好，阳光灿烂。
Jīn tiān tiān qì hěn hǎo, yáng guāng càn làn
Today weather very good, sunshine brilliant.
Natural full sentence
今天阳光明媚，出去走走吧！
Jīn tiān yáng guāng míng mèi, chū qù zǒu zǒu ba!
Today sunshine bright and beautiful, let's go for a walk!
阳光明媚 = classic idiom for brilliant sunshine; 走走 = casual stroll
More Chinese (Mandarin) Weather Phrases