Read here and here. Besides the normal night time warming from radiating buildings and parking lots, there are urban heat island (UHI) impacts that affect coastal waters, watershed evaporation, and soil. As these multiple studies indicate, UHI is huge, diverse, and has a larger impact than CO2 (read the Barrows, Alaska study in the first linked article).
"So what have we learned about the urban heat island effect from data obtained in North America? We've learned that it is large and growing in large-and-growing cities, as well as in small towns. Given these undeniable facts, it is presumptuous in the extreme to believe that the global surface air temperature record of the last few decades has been adequately adjusted for small-town and large-city heat island effects; and we can thus be fairly confident that the true warming of the planet has likely been far less than what has been claimed by essentially all assessments of the phenomenon conducted to date."
The California example: (click on image to enlarge)
"the three researchers found that "most [California] regions showed a stronger increase in minimum temperatures than with mean and maximum temperatures," and that "areas of intensive urbanization showed the largest positive trends, while rural, non-agricultural regions showed the least warming." In fact, they report that the Northeast Interior Basins of the state actually experienced cooling. Large urban sites, on the other hand, exhibited rates of warming "over twice those for the state, for the mean maximum temperature, and over five times the state's mean rate for the minimum temperature.""