The concentration of the invisible gas water vapor (H 2 O),
however, varies greatly from place to place, and from time to
time. Close to the surface in warm, steamy, tropical locations,
water vapor may account for up to 4 percent of the atmo-
spheric gases, whereas in colder arctic areas, its concentration
may dwindle to a mere fraction of a percent (see Table 1.1).
Water vapor molecules are, of course, invisible