I was surprised to find the actual answer to this
question. It is quite a bit more than I thought.
Clouds are composed of water and ice crystals.
(As an aside, it's also pretty cool how clouds
stay "up" in the air. The water droplets in them
are so small that they don't fall through air
because they weigh so little that gravity isn't
strong enough on them to pull them down through
the air.) According to this thunderstorms
source, the average thunderstorm contains 1.1
million gallons of water. That is just a
single thunderstorm though.
NPR video gives a weight for an average hurricane:
108 billion pounds of water.
In gallons, that is 12,940,000,000 gallons.
That's hard to think about, so let's scale it.
Wembley stadium, one of the largest in the world
stadium), could be completely filled from
floor to dome over 43 times by the average
hurricane. That's a lot of water.