This gas in the atmosphere is most influential in day to day weather changes

Water vapors

Explanation:

Water vapors can change the Earth atmosphere on daily basis. This is because of the reason that the relative proportion of water vapors in the atomosphere changes more significantly than any other gases (e. g. oxygen, carbon dioxide, ozone) whose proportion doesn't change much on daily basis. Further, water vapod can hold thermal energy and carbon particles to act as a aerosoles, which ultimately results in the rainfall. Therefore, the proportion of water vapors influences weather on daily basis whereas changes in the proportion of other gases is a slow process and is generally studied under the umbrella of climate change.