Radcliffe Meteorological Station
Annual Meteorological Data
The following annual means and totals of the principal meteorological elements are available:
- Mean air temperatures
- Mean maximum air temperatures
- Mean minimum air temperatures
- Total Precipitation
- Potential evapotranspiration (1815-1996)
- Number of rain days
- Hours of bright sunshine
This webpage presents the time series of temperature, precipitation and potential evapotranspiration, the variables which are central to climatic research. Over the recent past, rainfall patterns in Oxford have been characterised by large deviations from the long-term averages. A number of dry spells has been registered, most notably between 1988 and 1992, in the summer of 1995, and in 1996-97. The unusual rainfall patterns have been accompanied by exceptionally mild temperatures. The Radcliffe temperature time series confirms that the post-1986 decade is the warmest on record by a considerable margin. An important consequence of the combination of rainfall deficits and exceptionally high temperatures is a high evaporative demand. In the last decade, potential evaporation losses have been persistently above average. As a major loss of water from the drainage basin, evaporation controls soil moisture contents, ground water recharge and streamflow. The most important economic implications of increasing evaporation are depressed runoff and potential shortage of water resources. The anticipated increase in temperature due to the greenhouse effect has led to the concern that the subsequent increase in evaporation may affect water availability in the future.
Two interesting facts are apparent from the Radcliffe temperature time series:
- In the 20th century temperatures were generally higher than in the 19th century. This is thought to be an effect of the urban heat island. With the development of the city, more open spaces were built up. This process produced radical changes in the nature of the surface and atmospheric properties of the city. Differences in surface materials, drainage characteristics, sources of heat, configuration of surfaces and pollution acted to change all aspects of the climate of the city, air temperature being affected particularly strongly. Urban heat island effect is the most common example of inadvertent climatic modification.
- The post-1986 decade was the warmest decade on record. This fact may be an effect of the interannual climatic variability, or it may be suggestive of climate change. Only time will tell!
Over the recent past, rainfall patterns in Oxford have been characterised by large deviations from the long-term averages. A number of dry spells has been registered, most notably between 1988 and 1992, in the summer of 1995 (this was the second driest summer on record since 1767; only 1818 had less rain), and in 1996-97. The droughts of the 1780s and early 1800s were much more prolonged and more extreme than the contemporary ones (and than the droughts of the early 1890s and early 1900s). A peculiar feature of the present rainfall patterns is that during the fifteen years ending in 1995, on average, the winter rainfall totals were approximately 20% higher than the summer rainfall totals, while in the previous years no such difference was observed. In other words, we may be now experiencing dry summers which form lengthy periods of rainfall deficiencies.
Water is removed from the surface into the atmosphere by two processes: evaporation, which occurs from the 'free' water surface, and transpiration, which occurs when moisture is removed from plant leaves through their stomata. In meteorological context these processes are treated together, and the combined process is called evapotranspiration. Potential evapotranspiration (PET) is evapotranspiration which occurs from a moist, green crop completely covering the surface. It represents the atmospheric demand for water irrespective of the previous moisture conditions. As a major loss of water from the drainage basin, PET controls soil moisture contents, ground water recharge and streamflow. The ability of the atmosphere to draw moisture from the surface is enhanced by higher temperatures which increase the atmospheric moisture-holding capacity. During the last ten years PET values were persistently above average in Oxford. This correlates well with positive temperature departures. The PET time-series was estimated using the Thornthwaite method (E. M. Shaw. 1994. Hydrology in Practice, Chapmann & Hall).