An artifact?

This is a satellite image from last Thursday. If you recall, we had a strong front pass and snow squalls in the District. This cold air was derived from Canada. In this infrared (IR) enhanced satellite, you can see a green colored mass over Ontario. However, there are no clouds there. This is the IR reporting a very cold mass in Canada. The interesting phenomenon here is that when looking at the Great Lakes, the IR returns disappear. Also of interst is that if you look in Minnesota, the green returns have a "spillage effect" around the west side. While, I find it hard to believe the lakes have such a drastic effect, one can see the upslope snow in the Appalachain Mtns. I wonder if the IR is in fact driven by the context of the underlying ground. If a weather geek knows the answer, please let me know. I am curious about how the IR returns are actually calculated, i.e. is it in context of the underlying ground temperatures?