expression, i-adjective
cloudless; without a single cloud
Describes a completely clear sky with no clouds at all. Often used in weather descriptions or poetic expressions.
今日は雲ひとつない青空だ。
Today the sky is blue and cloudless.
雲ひとつない快晴の下でピクニックを楽しんだ。
We enjoyed a picnic under a perfectly clear, cloudless sky.
快晴 is a weather term meaning 'fine weather' or 'clear skies', often used in forecasts. 雲ひとつない is more emphatic and colloquial, stressing the total absence of clouds.
晴れ simply means 'sunny' or 'clear weather', while 雲ひとつない specifically highlights that there is not a single cloud.
A fixed phrase composed of 雲 (cloud), ひとつ (one), and ない (not exist), literally 'not even one cloud'. The expression is straightforward and transparent.