Christians, please give me a proper scientific answer?

No, your reasoning is way off. Why? Because you do not know you Scriptures. It has never rained nor had there been a cloud in the sky before the flood. There was water under the crust, on the crust, and above the atmosphere. The plants were watered by dew and the rising of the water from under the ground, but not from the sky! So there could never have been a rain bow before the flood.