Great question and I don’t know. But this graphic will help us understand together:
Here you can see that there’s not much water on Earth. Basically the earth has high and low areas. The low areas have water. To us the oceans seem deep – 7 miles, maybe more, but overall, compared to the size of the planet, it’s really not much water. We can also see from the image above that we really don’t have a lot of air.
That being said, common sense tells us that not all the water is in the atmosphere. Only some, otherwise we’d have no lakes, rivers or ocean! So I’m going to guess – that’s scientifically called an educated guess, not a large percentage of Earth’s water is tied up in the atmosphere.
Now we could research this more with some Google searches, but my answer is just about common sense. I don’t think that much of our total water is in our atmosphere. There’s some, but we still need some left over to fill the lakes, rivers and oceans too.