Kautilya's Arthashastra 2.24.10

2.24.10

vātam ātapayogaṃ ca vibhajan yatra varṣati /
trīn karīṣāṃś ca janayaṃs tatra sasyāgamo dhruvaḥ //

Three are the clouds that continuously rain for seven days; eighty are they that pour minute drops; and sixty are they that appear with the sunshine--this is termed rainfall. Where rain, free from wind and unmingled with sunshine, falls so as to render three turns of ploughing possible, there the reaping of good harvest is certain.