FIGURE I I. I Winter-Spring thermal index for Iceland 1501—1801 based on content analysis of historical documents (annals, correspondence, and diaries). Isolated reports of very cold or very mild seasons are also shown (Ogilvie, 1992).

As with all proxy data, historical observations need to be calibrated in some way, in order to make comparisons with recent data possible. This is commonly done by utilizing early instrumental data that may overlap with the proxy record, to develop an equation relating the two data sets. Thus, Bergthorsson (1969) regressed observations of sea-ice frequency off the coast of Iceland with mean annual temperatures during the nineteenth century, and then used the resulting equation to reconstruct long-term temperature fluctuations over the preceding 300 yr from sea-ice observations. Similarly, observations of Dutch canal freezing frequencies were calibrated with instrumentally recorded winter temperature data by de Vries (1977) (Fig. 11.2). The calibration equations were then used to reconstruct paleotempera-tures prior to the period of instrumental records in each area.

In many locations, precipitation occurrence (daily frequency) was recorded by diarists together with remarks on rainfall intensity, rather than rainfall totals. Several studies have shown that statistically significant relationships exist between rainfall frequency (and/or rainfall duration) and daily rainfall amounts, enabling estimates of daily rainfall to be made, and then for monthly and seasonal totals to be accumulated. In the lower Yangtze River valley of eastern China, for example, local officials maintained detailed weather observations during the Qing Dynasty (1636-1910). This set of records (Qing Yu Lu, the "Clear and Rain Records") provide daily records on sky conditions, wind directions, precipitation type (rain or snow, light, heavy, torrential, etc.) and the duration of precipitation events (Wang and Zhang, 1988). By relating precipitation totals to precipitation frequency and intensity during recent decades (using instrumental data), Wang and Zhang (1992) were able to establish regression equations that could then be applied to the historical data to reconstruct past precipitation amounts. Others have noted that rainfall frequency (number of rainy days and/or the length of rainy episodes) is often inversely related to temperature because cloud cover tends to reduce direct radiation, and rainfall itself will lead to evaporative cooling. W.C. Wang et al. (1992) applied this line of reasoning to the Qing Yu Lu in order to extend the instrumental record of summer temperature in Beijing from 1855 back to 1724. Mikami (1992b) also noted the strong relationship between the number

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