Genesis 7:12

And the rain was upon the earth forty days and forty nights.

And the rain was upon the earth. х geshem (H1653), differing from maaTaar (H4306), rain in general, denotes unusual and continuous torrents of rain; cf. Job 37:6; Zechariah 10:1].

Forty days and forty nights. In all ages and countries there have been idiomatic customs in the use of what may be called representative numbers, where a definite is put for an indefinite quantity. A Greek who wished to express the notion of a great but undetermined number said, a 'myriad,' or ten thousand; a Roman, 'six hundred;' and in like manner an Oriental, 'forty.' The 'forty thieves,' the 'forty martyred monks of the convent of El Arbaim,' not to speak of a similar use of this numeral in various passages of Scripture are examples of a known and definite number being used to express only the idea of many. It is evident, however, that, although the word may occur in a very general sense elsewhere, it is not so employed in this narrative; because the progress and duration of the deluge are marked with extraordinary precision, and that it must be interpreted here as denoting literally forty days.