Inter-decadal variability in daily rainfall at Durham (UK) since the 1850s

Records of daily rainfall at Durham (UK) are analysed from 1850 to the present. The most notable wet period is the 1870s, followed by a protracted period of below-average rainfall. Other than the 1870s, summers tended to be much wetter than winters during the latter part of the 19th century. In the 20th century, summers had become drier and winters wetter. To analyse large daily totals, three thresholds, 15, 22.6, and 25 mm, are used. For all three indices, the frequency of large falls was highest from the 1860s to the 1880s, followed by a low frequency in the 1900s. Since then, the general trend has been for totals to gradually increase. There is a clear division for the annual total rainfall, number of heavy falls (≥ 15 mm), and proportion of total rainfall provided by heavy falls in the winter (154 mm, 147 days, 12%, respectively) and spring (140 mm, 180 days, 16%) seasons in comparison to the summer (181 mm, 329 days, 28%) and autumn (186 mm, 304 days, 23%) seasons. However, since 1990, winter total rainfall (164 mm) has overtaken the summer total (154 mm). Furthermore, since 1990, heavy falls of rain in spring (23 days) and the proportion of total rainfall (26%) have overtaken summer heavy rain (17 days) and the proportions of rainfall in both summer (24%) and autumn (24%). In terms of heavy rainfall, winters have seen only a small increase in its relative importance, whereas its relative importance declined from the 1960s to the mid-1990s for summers. The widely recognised increase in the winter: summer ratio of rainfall since the 1960s is clearly evident. Overall, there has been a narrowing of the differences between seasons in terms of the frequency of heavy falls of rain in the 1990s.