Question

which atmospheric gas plays the biggest role in weather and forms clouds? water vapor carbon dioxide methane oxygen

Explanation:

Water vapor is a greenhouse gas that undergoes phase changes (evaporation, condensation) which are crucial for weather processes. Clouds form when water vapor condenses around tiny particles. Carbon dioxide, methane, and oxygen do not have such a direct and significant role in cloud - formation and weather dynamics as water vapor.

Answer:

water vapor