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- Hide quoted text -

- Show quoted text -

It's a pretty good rule that all precipitation is caused by
rising moist air, which is cooled by expansion. One obvious cause is
a range of mountains. The area in India you are referring to could
either be the Western Ghats or the hills of Assam where a moist
airflow is forced to rise over them during the monsoon and the
rainfall can be tremendous (eg Cherrapunji, with about 11 metres/yr).
Even where I live has 30% more rainfall than central London, nearly
all the excess being due to altitude. I hope this doesn't all sound a
bit patronising but I sense you are a relative beginner weatherwise.
I very much doubt that there's enough moisture in the Martian
atmosphere for there to be any precipitation of water despite the
presence of some pretty high mountains (eg Olympus Mons, about 20 km
above the surrounding areas).

Tudor Hughes, Warlingham, Surrey, 556 ft.