QUESTION:
In many Hubble-Images you can see water-ice clouds, similar to the
cirrus clouds on earth. But why then is it not raining on Mars?

ANSWER from Bob Haberle on June 14, 1999:
The short answer is because it's too cold. Average temperature is 220K
(-63F). At these temperatures water cannot exit as liquid. Therefore, it
cannot rain. (Rain is defined as precipitation with liquid water). It can,
however, snow. The clouds you refer too are indeed like cirrus clouds here
on earth. These are composed of ice crystals. On earth cirrus clouds are not
precipitating clouds and this is probably true for the Martian version as
well. The main limitation for precipitation on Mars is the limited amount of
water vapor in its atmosphere.