How do water and air temperatures relate?

How do water and air temperatures relate?

Background: Lake Superior, the largest lake in North America, is located on the US and Canadian border. The climate in the Lake Superior region is a lot like northern Maine, so it is very cold in the winter and will get quite hot in the summer. 

In the graph, the solid gray line represents the air temperature throughout the year and the dotted black line is the surface temperature of the lake water. The Y-axis is the temperature, and the X-axis is the day of the year where January 1st is 1 and December 31st is 365.


Describe what the graph shows about the difference between water and air temperatures at different times of the year. When is the water warmer than the air and when is it colder? 

In what ways do daily air temperatures differ from daily surface water temperatures? Why do you think this is the case?