This page discusses that at the MER rover landing sites the maximum
Summer-time air temperatures can exceed 30 C, 86 F, while the
temperature swings from day to night reach a 100 C , 180 F, change:

Extreme Planet Takes Its Toll.
June 12, 2007
Like Sun Belt retirees who complain about cold weather, NASA's Mars
rovers are becoming less tolerant of temperature changes with age.
http://marsrovers.jpl.nasa.gov/spotlight/20070612.html

On this same page is discussed the observations of frost on the
Opportunity rover.
It had been known for some time that daytime, Summer temperatures can
exceed the melting point of water at equatorial locations on Mars but
it was believed the equator was desiccated.
We now have visual evidence that the equator on Mars is not
desiccated, in addition to the GRS/HEND instrument readings on Mars
Odyssey showing this.
The errors of the report of "puddles" at the Opportunity rover site
have been legitimately criticized. But the observation of frost on the
Opportunity rover occurred 3 years ago.
Is not the refusal to investigate the possibility of liquid water at
the Opportunity site when the environmental conditions suggest it
should be possible an even greater error of omission by the scientific
community at large?

Bob Clark