to being dominated by atomic hydrogen and oxygen, with several other molecules being abundant at different altitudes. Therefore, I consider a more general and theoretical approach for diffusion coefficients to be more appropriate here than using measured values that are only appropriate for specific background gases. However, the two approaches lead to very similar diffusion coefficients with values that are typically within a factor of $\sim$1.5 at the altitudes at which molecular diffusion is important.

In Fig. \[fig:Case5\], I show an example simulation for a water vapor atmosphere of an Earth-mass planet orbiting an active solar mass star. This is Case 5 presented in the following section. The strong stellar XUV field heats the gas to such a high temperature that it accelerates away from the planet in the form of a transonic Parker wind, with the point where the atmosphere becomes supersonic being also where the outflow speed exceeds the escape speed. The dissociation of the water molecules takes place very low in the atmosphere, at an altitude of $\sim$100 km, and the upper atmosphere is dominated by atomic hydrogen and oxygen, and their ion equivalents.

Results {#sect:results}
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![ Vertical structures of the hydrodynamically outflowing atmospheres for several 