There are 2.5 times as many hot days during summer in London as there were 100 years ago – that’s the result I have found while looking at data for Greenwich going back to 1851.
Very hot conditions, where the mean daily temperature is 23.5°C or higher, as defined by the Met Office’s Heat-Health Watch System, have been achieved in this area on 199 days since 1852.
The Met Office, and other meteorological agencies, use 30-year averages to smooth any spikes of hot and cold years – so the result, in my view, is quite remarkable – even though it is just one dataset.
Looking at the data overall it can be seen that while average maxima has decreased by 1.6°C since 1881-1910, minima has increased by 2.1°C.
The decreased maxima is possibly explained by the fact that the sun shines, on average, just over an hour less than it did a century ago. The reason for this could be because of increased air pollution and aircraft contrails?
The increased minima, on the other hand, could be being caused by the fact that vast tracts of suburban front and back gardens are now paved over – along with the loss of the shade of many trees. This, and increased road and house building, acts as a huge radiator, keeping nights warmer than they would have been a century ago.
Experts would question the hybrid nature of my dataset, even though my observing sites are very close to those of Greenwich. It is a shame that many of the first climatological stations, such as Camden Square set up in the early 19th century by George Symons, are not still around as they would now offer an unquestionable insight into just how much London’s climate has warmed.
You could argue that there is no doubt that the warming in these data is man made though, perhaps, paved gardens keeping summer nights warmer would be a lot easier and cheaper to solve than fulfilling the carbon capture policies of many of the world’s governments. But that is an argument for another post.
For now, as the capital’s population continues to increase, it is vitally important that planners get the next generation of property developments right to keep the population cool when a heatwave strikes. Extreme heat severely affects public health, not least the suicide rate – a study has shown that above 18°C, each 1°C increase in mean temperature has associated with a 3.8 and 5.0% rise in suicide and violent suicide respectively. Planners cannot just leave it to energy-hungry air conditioning to bring relief – developments should incorporate plenty of shade and natural cooling in their designs to help counteract the health impacts of future heatwaves.
* There were hot spells before the period in this post but I have excluded these: Luke Howard’s maxima observations from Plaistow, Stratford and Tottenham were taken under non-standard conditions and may be on the high side.
** I have focused my investigation on data recorded at the Royal Observatory, Greenwich. For some reason the data during a couple of years in the 1950s was incomplete – to fill in these gaps I used official data gathered at Kew. The data is completed up to the present day using my own observations taken since 1988.
Wanstead Meteo 
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