The Urban Heat Island Effect is why urban areas are warmer than rural areas. It's the altered landscape of a city that helps keep the temperature warmer than the rural areas.
For example, asphalt parking lots, roads, and concrete buildings can retain more heat than rural areas with shade and vegetation. In fact, the EPA estimates that these urban surfaces can be 50 to 90 degrees hotter than the air temperature!
All the concrete and pavement in a city allows most water to runoff. But the water in natural vegetation and trees evaporates and actually cools the surface air. That's actually similar to why we sweat, because the water on our body evaporates which in turn cools the air near our skin. Cities lack the amount of moisture that rural areas have, and is another reason why urban areas are hotter.
Here's more on the urban heat island:
I hope this explains why the St. Louis metro may have higher temperatures and thus a higher heat index as we go through a heat wave this week.
Meteorologist Steve Templeton