2008 GLOS Mooring deployment
Air temperature in the spring and summer tends to be higher than the surface water temperature (see plot below). In this case, the surface waters are warmed by the air (and the air is cooled, as residents of Duluth are aware of any time the wind blows off the lake). Later in the year, when the lake has warmed up and the seasons turn, the air will typically be colder than the water, and will cool off the water.
Here's what the sensor looks like; there's a small thermometer inside the vanes shown here; the vanes are to keep sunlight off the sensor, and to keep airflow over the sensor moving. Sorry about the odd perspective.