A new study indicates that "urbanization"... the process of replacing natural landscapes or agricultural land with paved streets and concrete structures... seems to make temperatures in the areas higher. [source] Well, duh!
Haven't you ever heard something like, "Lows tonight in the 50s; cooler in the outlying areas?" Replace a naturally cooled system with a giant heat sink and what do you get? Come on, the answer isn't that hard. Now put your weather stations inside those heat sinks and what do you get? Really, it isn't that hard.
Now, reflect upon the fact that this urbanization process has increased remarkably over the past century and that weather stations are increasingly within the areas affected by these giant heat sinks and what do you suppose happens to the weather data? No, I won't explain it further. You're smart enough to figure that out.