The bow is a rainbow.
The pre-flood world was very different. It's not clear there was rain before the flood.
2:5 And every plant of the field before it was in the earth, and every herb of the field before it grew: for the LORD God had not caused it to rain upon the earth, and there was not a man to till the ground.
2:6 But there went up a mist from the earth, and watered the whole face of the ground.
Genesis 1:6 tells us there was water above the sky. This could have been a layer of clear ice keeping the atmosphere in and under greater pressure. This would explain how some larger dinosaurs (dragons) could breathe despite having relatively small nostrils, and dragonflies could grow with wingspans of over two feet (see Meganeura). (Insects breathe by diffusion which limits their size.) This layer was destroyed in the flood and is no longer there.
Genesis 1:6 And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
Genesis 7:11 In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.
The tear around the earth (see fault line in the mid atlantic and others) is a witness to the breaking open of the fountains of the great deep. The windows of heaven could be a reference to the layer of ice above the sky.
There were also no seasons before the flood, as seasons only came after the flood:
Genesis 8:22 While the earth remaineth, seedtime and harvest, and cold and heat, and summer and winter, and day and night shall not cease.
The catastrophe that caused the flood may have also caused the tilt in the earths axis that gives us our seasons. This would explain why we find large numbers of animal fossils under the Arctic (including, for example, camels).
The flood was a great catastrophe that gave rise to our current day geology and weather systems. There is no reason to believe there were conditions for a rainbow to form before the flood.
I believe the bow was a rainbow and the earth only had the right conditions for one to form after the flood.