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instrumentation and observing protocols has proven to be a major challenge.

To appreciate the issues involved in comparing estimates of surface and lower tropospheric temperature trends, it is necessary to have at least a rudimentary understanding of these three kinds of measurements and the uncertainties inherent in each of them. Chapters 6, 7, and 8 present this basic background information, and the final chapter (9) discusses the issues involved in making comparisons between the different kinds of measurements. Collectively, these last four chapters of the report are the basis for the findings and recommendations presented in chapters 3 and 4.break