The featured graph is of average daily soil heat flux as measured by the USDA-ARS-NSTL Flux site from 2006. Positive values indicate a download flux of heat (warming), while negative values indicate a net loss of heat to the atmosphere (cooling). About half of the year the ground is being warmed, while it loses heat for the other half. We are currently in that other half period of the year with our soils heat being "lost" back into the atmosphere and eventually back into space. The average value of this plot is roughly +1 W m^-2, which is within the sensor's sensitivity, and close to zero, which we would expect for a long period of time.

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