Why is it warmer near the lake than it is inland?

Dear Tom,
Why is it warmer near the lake than it is inland?

Karina Monarrez,

Dear Karina,
There are two reasons for higher temperatures at the lakefront during the winter. The first is the “heat island effect.” An urban area has higher average temperatures than its rural surroundings due to its greater absorption, retention and generation of heat than rural areas. Furthermore, the heat island effect in Chicago is greatest in the inner city and near the lake. Secondly, Lake Michigan in the winter is a source of heat. Its relatively warm water (“warm” compared to colder wintertime air temperatures) keeps temperatures over the lake and at the lakefront higher than inland readings. In the summer, however, lake water temperatures are usually cooler than inland air temperatures and daytime readings at the lakefront will be lower.