Looks like Spring really is on the way:
The blue trace shows the groundwater temperature at the inlet pipe. The minimum values should be pretty close to the actual ground temperature about four feet down, which seems roughly constant at 37 °F for March.
The black trace comes from a datalogger tucked in the dirt under the concrete patio, so it feels some air temperature variations, too.
The red trace comes from the datalogger dangling in the well pit in the back yard, directly under one of the vent holes, so it’s recording the air temperature in a below-grade chamber.
The green trace shows the attic air temperature, which is strongly influenced by sun on the (white) asphalt shingle roof. That said, the air temperature gets a lot lower than any of the below-grade loggers; it’s fair to say they’re recording something fairly close to the actual ground temperature.
The Smell of Molten Projects in the Morning 