6 December, 2000
In order to reconstruct the climate going back several thousand years, Gary Clow needs a precise temperature profile from the base to the surface of the ice sheet. The temperature logs from the main borehole reach from the base at 1000 meters up to 70 meters. The four shallow boreholes provide the temperatures for the top section of the ice and firn. However, the temperature curve for the top 20 meters is not smooth. This is due to seasonal temperature differences.
Summer and winter temperatures at Siple Dome vary by about 30 degrees Celsius (54F). These differences diffuse slowly down through the firn. They are visible in the temperature data to a depth of about 20 meters. In order to smooth out the temperature curve, the average or mean annual temperature of the surface snow must be determined. This will provide information about the climate for the last 50 to 100 years.
Finding the mean annual temperature involves recording or logging the temperature of the surface for one year. Setting up this part of the project has been my responsibility. We have five instruments for recording temperature. They are called Hobo temperature loggers. I learned how to use them when I visited Gary in Colorado last June.
A computer is used to start or "launch" the data loggers. They can record temperatures many times a day for up to several years. Temperature data is obtained inside the unit and at a sensor on the end of a six-foot wire. This allows you to record temperatures at different snow depths. We decided to place the Hobos at a depth of two meters with the external sensor at one meter.
A depth of two meters was chosen because the Hobo battery is only rated to a temperature of -30C (-22F). At this depth, the temperature should not drop below that point. I also constructed blocks of insulating foam to surround the instrument.
I placed a unit at each of the four drill sites. An additional Hobo was put in at 32km south with the sensor at a depth of three meters. Sarah Das also installed four Hobo loggers. So we have one temperature logger about every 10 km (6mi) from 30km north to 40km south. In addition to providing the mean annual surface temperature, the data will also provide information about climate variability across Siple Dome.
When Gary Clow and Bob Hawley return next year for the project's final season, they will be able to dig up the Hobo units and download the temperature data that will have been collected for an entire year.
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