Soil and ground (here defined as beyond the depth of weathering or soil) temperatures can be superior to surface temperatures as indicators of climatic trends, because the ''noise'' common to the latter has been effectively reduced or removed. As a result, the 12.8 m ground temperatures recorded routinely at the University of Minnesota St. Paul campus climatological observatory since 1963 can provide valuable information on climatic trends.

The observatory, located in the midst of land that has been in agricultural test plots since 1885, was instrumented with copper-constantan thermocouples buried at various depths in the soil and ground. The two air temperature measurements, with which the 12.8 m ground temperature is compared, are made with maximum and minimum glass thermometers housed in standard temperature shelters.

The mean air temperature increases of 0.049 ¿C and 0.035 ¿C per year, 1963--1990, at the observatory and the nearby Eastern Minnesota station, respectively, compare favorably with the 12.8 m ground temperature mean increase of 0.041 ¿C per year. Due to the brevity of the record and to its approximate coincidence with the most recent northern hemisphere warming period, the extrapolation of these rates beyond the record period must be done with care. ¿ American Geophysical Union 1993