Genèse 8:5

And the waters went and diminished until the tenth month, the month Tammuz. In Tammuz, in the first of the month, the heads of the mountains were seen.

AND THE WATERS DECREASED CONTINUALLY UNTIL THE TENTH MONTH. This verse is to be interpreted by transposition: the waters decreased continually until the tops of the mountains were seen in the tenth month, which is the month of Tammuz. Scripture thus informs us that during seventy-three days the waters decreased fifteen cubits. But we do not know the amount of the original decrease which enabled the ark to rest since Scripture did not find it necessary to inform us either of the number of cubits that the ark was submerged in the waters or of the amount of the decrease.
In the matter of the floating of the ark, it appears to me that because the waters flowed from the deeps and were hot, as our Rabbis have said, the ark therefore floated upon the face of the waters. Were it not for that, it would have sunk on account of its weight for there were many living things in it, as well as a great deal of food and drink. But as soon as the waters subsided from their flowing or from their heat and they decreased on account of the wind, the ark at once entered into the midst of the waters due to the weight of its load, and it rested on the mountain.
In the opinion of the commentators, the Scriptural accounts concerning the total of one hundred and fifty days during which the waters prevailed, the resting of the ark, the visibility of the tops of the mountains and the succeeding forty days — all these events we know by way of prophecy for Scripture so informs us, but Noah knew only that he felt that the ark had rested, and he waited a period of time which, in his opinion, was sufficient for the waters to abate.
Now according to our words, as well as those of our Rabbis and all commentators, the mountains of Ararat, which are among the highest mountains under the heavens, had fifteen cubits of water above their summit. Therefore this difficulty is to be posed: it is known that the Greek mountain Olympus is very much higher than they, and the land of Ararat, which is near Babylon, lies in the lower part of the globe! Perhaps we should then say that the decrease of waters which took place on the seventeenth day of the seventh month was very much more than fifteen cubits, and at first the tops of the high mountains were seen, not the mountains of Ararat, and it just happened that the ark was in the land of Ararat during the seventh month and it rested on the tops of those mountains.
Now Noah, from the time the rains ceased, would open and close the window at …

והמים היו הלוך וחסור, the waters kept receding. This construction is not unique, as we encounter it in Ezekiel 1,34 והחיות רצוא ושוב.

עד החודש העשירי, this does not mean that during this month the waters had already receded completely; what happened was that the waters receded progressively until relatively low mountain tops became visible. The process continued further. The “tenth” month described was the month of Av, seeing it was the tenth, counting from the month of Marcheshvan when the rains had started falling.