On Dec 11, 12:18 pm, Robert Clark wrote:
On Dec 10, 8:53 pm, Tudor Hughes wrote:



On Dec 10, 5:42 pm, Robert Clark wrote:

I'm investigating the possibilities of rain or snow on Mars. The
possibility is generally discounted because of the low amounts of
water vapor in the Martian atmosphere, from 10 to 100 precipitable
microns, and around 20 precipitable microns on average.
But I was thinking the amount of precipitation that might fall over
some time period might be much higher than the amount of precipitable
water vapor present.
For instance on Earth the maximum amount of precipitable water vapor
is around 5 to 6 precipitable centimeters, 2 to 3 inches, even over
the oceans. Yet during storms you can have very many inches of rain or
snow fall in a single day.
One way this can happen is easy to imagine, even if the precipitable
water within clouds is also below 6 pr. cm. You could have all the
water in a column fall to the ground, then the cloud move so all the
water in another column falls on the same space on the ground. If the
cloud is large in horizontal extent and moving slowly quite a bit
could fall at the same location on the ground.
However, I was also wondering if is possible that the condensed water
amount in a cloud could itself be much higher than 6 pr. cm. even if
the water vapor content in the surrounding cloud-free area is still at
6 pr. cm and below.
Anyone know if this is possible?

Bob Clark

It doesn't need to be. The cloud (say a large Cb) could be
stationary or quasi-stationary and be receiving a continuous feed of
moist air which then precipitates its water over a small area. At any
one time the cloud may contain say 60 mm precipitable water but there
will be a rapid turnover. In other words, much more than 60 mm
precipitation could have gone into then out of the cloud in a fairly
short time and one could still fairly describe it as the same cloud,
renewing itself all the time. The rainfall depends not so much on the
availability of moisture as the mechanism by which it can be
released. I've no idea whether or not Mars has cyclonic disturbances
(it probably does).

Tudor Hughes, Warlingham, Surrey.- Hide quoted text -

- Show quoted text -

Thanks for that. For those interested here is an image of dense low
lying clouds/fogs on Mars:

http://img.villagephotos.com/p/2005-.../marsiswet.jpg

taken from this ESA (European Space Agency) report:

Adsorption water driven processes on Mars.
D. Möhlmann
FIRST MARS EXPRESS SCIENCE CONFERENCE
21-25 February 2005, ESA/ESTEChttp://sci.esa.int/science-e/www/object/doc.cfm?fobjectid=36779

To me these clouds in the image have the appearance of precipitation
beaing clouds rather than thin cirrus-like clouds that were believed
to be the only type that occur on Mars.

Are there topographical or geographical characteristics that make
an area on Earth subject to a lot of rain? I read that an area in
India during the monsoon season received over a meter of rain in a
single day.

Bob Clark

Where the prevailing wind from an oceanic area is forced to convect by
relief then there is heavy rainfall ie the Indian Monsson, and the
west coast of Britain especially on the west sides of Snowdon and Ben
Nevis. Even Manchester downwind of the Pennines receives a lot of ran.

But Mars does not have any oceans although it does have polar ice
fields. That would suggest that mountain ranges just equator-ward of
the poles would be the place where you might find rain. I suspect
that most surface water/ice on Mars is due to polar frosts.