Imaging at the Opportunity rover landing site in Meridiani Planum confirms observations using the HEND instrument on Mars Observer that water/ice is deposited near the equator seasonally on Mars:

Space Sciences
Whoa! Frost on the solar panels?
Posted by Robert Clark on 12/14/2004 7:32:38 AM
In Reply to: Sabkha or playa, take your pick... posted by Nick Hoffman on 12/13/2004 6:23:08 PM
http://habitablezone.com/space/messages/360805.html

However, the HEND instrument shows the greatest amount is deposited during southern Summer:

47 - EVIDENCE OF THE SEASONAL REDISTRIBUTION OF WATER IN THE SURFICIAL MARTIAN REGOLITH BASED ON ANALYSIS OF THE HEND MAPPING DATA.
R.O. Kuzmin, E.V. Zabalueva, I.G. Mitrofanov, M.L.Litvak, A.V. Parshukov,
V.Yu.Grin'kov, W. Boynton, R.S. Saunders.
"As it well seen from fig.1b,c,d, two distinctive “hollows” of neutrons flux reduction have been appeared in the northern hemisphere during northern summer at Ls=130°-170° and in first half of northern winter at Ls=270°-330°, being extended from high to low latitudes. At that, later “hollow” (Ls=270°-330°) is characterized by much stronger reduction of the neutrons flux and it traces from northern polar region up to low latitudes in the southern hemisphere. The first “hollow” is related with periods of the northern middle summer, while the second one – with of the southern middle summer. In both case the residual polar caps serve as main source of the water in the Martian atmosphere."
p. 2
http://www.geokhi.ru/~planetology/theses/47_kuzmin_et_al.pdf

Opportunity landed just barely after this time in southern Summer at about LS 340° (LS stands for solar longitude and indicates Mars position in its orbit.)
However, it is notable that Spirit did land near the end of the time period of Ls=270°-330°. Spirit is closer to the southern pole and this may explain how could experience deposition during this period while apparently not during the current northern Summer. Then the controversial indications of mud at the Spirit landing site early in the mission may indeed have been indications of this summer-time water deposition.
Opportunity has observed clouds during the current water deposition period and since the amount of atmospherically deposited water is greater during southern Summer, we would expect the cloud density to be even greater then. Indeed it could be of sufficient density to allow precipitation which could reach the ground as rain.
The next Ls=270°-330° period begins in August, 2005.

Come on Opportunity!

Bob Clark