The fraction of the total atmospheric pressure contributed by water vapor, however, is small and is at most a few tens of millibars at the Earth's surface.
(The fraction of the total atmospheric pressure; is; small) (The fraction of the total atmospheric pressure; is at most; a few tens of millibars at the Earth's surface)
The fraction of the total atmospheric pressure The fraction of the total atmospheric pressure
is is at most
small
a few tens of millibars at the Earth's surface