Cities create their own microclimates and their sites are almost always warmer, compared to a nearby rural location, than if the city were not there. The effect on temperatures measured in the city has been termed the ‘urban heat island’ (UHI). In this study we assess the strength of the UHI within and around London based on monthly and seasonal maximum, mean and minimum temperatures. The purpose of the study is not to look at UHI extremes on individually conducive days, but to show that the effect has a certain magnitude on monthly and seasonal averages, and assess whether this magnitude of UHI for Central London has increased over the twentieth century compared to rural locations around London (Jones London is possibly the longest-studied UHI of any city; Luke Howard (1833) was probably the first scientist to suggest that the temperature recorded in a city was likely to be higher than that in the surrounding countryside. The major studies of urbanrelated warming in London since then have all recognized the importance of homogeneous temperature series and the need to look at long time series, as opposed to the effects on individual days or short periods. Moffitt (1972) compared the long Kew (KEW) record with that from Rothamsted (ROTH), remarking that this rural site is unlikely to be affected by any development at Harpenden (a mile to the northeast of the site). A study was conducted (in 1925) concerning a site move that took place in 1915. Although the study is no longer available, a letter in the station metadata files states that there was good agreement between readings at the two sites and that there was no regular pattern in the minor variations (Moffitt, 1972). Some additional discussion of this site move is given by Tyrrell (1972) and Moffitt (1973). ROTH is used as one of the three key sites in England for the Central England Temperature (CET) series, and no adjustments to its data have been made (Parker, 1992). Moffitt (1972) concluded that KEW warmed relative to ROTH between 1878 and 1968 by about 0.8 degC, slightly more in winter and summer and slightly less in spring and autumn. Moffitt (1972) also showed that KEW (for annual average between the stations, assuming a standard lapse rate of 0.6 degC per 100 metres. This value will be important later when we assess the magnitude of Central London’s UHI. We will also assess whether this value should be applied equally to maximum and minimum temperatures. Later work by Lee (1992) and Wilby (2003) used the more central London site of St James’s Park (SJP) and replaced the ROTH rural record with Wisley (WIS). The periods of record were reduced to 1962–1989 and 1958–1998, respectively, and the emphasis changed to the magnitude of UHI. Lee (1992) considered whether the urbanrelated warming trend had increased over his period of analysis, but was only able to find that minimum temperatures had risen while maxima decreased at SJP compared to WIS with no overall change in mean temperatures. Wilby (2003) found similar results to Lee (1992) and correlated counts of daily extreme UHI values (SJP warmer than WIS by more than 4 degC) against a number of synoptic indices. In this study we will use an additional site in Central London (London Weather Centre, LWC) and one in a more suburban location (Heathrow Airport, LHR), and use the full length of digitally available daily and monthly average records.