NASA boffins have crunched 34 million weather observations collected by the Curiosity rover its two full Martian years trundling about the red planet.
The basics are pretty simple: Mars is reliably cold, dry and windy, while the thin atmosphere means not much heat is retained so air temperature “usually plummets by more than 100 Fahrenheit degrees (55 Celsius degrees) between the afternoon high and the overnight low.”
NASA's analysis also shows that the atmosphere is so thin that air pressure fluctuates seasonally as atmospheric carbon dioxide freezes into Mars' ice caps.
Curiosity is in the Gale Crater at 5° South, but its measurements aren't particularly biased by that location because Mars' northern ice cap is smaller than its Southern cousin. That state of affairs came about because Mars' orbit is elliptical, so the South receives less solar energy than the North.
The southern winter therefore sees more atmospheric CO2 captured and produces lower air pressure, which in turn means the southern winter produces air less conducive to heat capture.
Hence Mars' reliably unpleasant weather.
NASA's cooked up charts to explain this all, embedded below or available here if your browser is hostile to iFrames. ®