صحو doesn't appear in Hans Wehr's Modern Standard Arabic dictionary, but Lane's Arabic-English Lexicon has 2 entries for it, distinguished by diacritics. Select one below for the full classical Arabic definition.

• صَحَا Form 1

  1 صَحَا ذ , said of a day, [aor. يَصْحُو,] inf. n. صَحْوٌ, It was, or became, cloudless: (TA:) and so : (Msb, K, TA, but not in the CK:) [it is said that] صَحْوٌ signifies the departing of the clouds: (S, Mgh, K:) [but] …

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• صَحْوٌ

  صَحْوٌ ذ [an inf. n. used as an epithet, and therefore applicable to a fem. as well as a masc. noun, and to a dual and a pl. as well as a sing.], applied to a day, Cloudless; (K, TA;) as also ; (S, TA;) and : (Mgh, Msb:)…

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