In 2019 the UK used 46.5 billion litres of hydrocarbon road fuel [0]. Let's say it all has diesel's energy density of 36.9 MJ/litre. That's 1.71585e+18 joules p

In 2019 the UK used 46.5 billion litres of hydrocarbon road fuel [0]. Let's say it all has diesel's energy density of 36.9 MJ/litre. That's 1.71585e+18 joules per year total. Let's pretend fuel use per capita is constant across the country. On 2018 numbers, London's 8.9 million people [4] out of the UK's 66.44 [5] gives it about 13% of the total, so 2.299e+17 joules. If you get 286kJ from burning one mole of hydrogen gas, London would have to burn 803 gigamoles per year, making the same quantity of water in the process. If water weighs 18.0153 g/mol [3], that's 14.478 million tonnes of water per year. If that all fell as rain, then assuming water weighs a tonne per cubic metre, spread out over London's 1,737.9 km2 [4], that would be an extra 0.0228 mm of rain per day. In July, Greenwich sees an average of 1.1 mm of rain per day [6], so that's on the order of 2% extra rainfall. This calculation is extremely approximate, and i have neglected to account for a the water from the combustion of hydrocarbon fuels. Rain is not the only way water gets involved in weather - i'm not sure how to estimate the effect on humidity, for example. And the effect will be bigger for denser and drier cities (Dubai!). It would be interesting to see a proper analysis of this. [0] https://www.ukpra.co.uk/en/about/facts-and-figures https://www.ukpra.co.uk/en/about/facts-and-figures [1] https://www.acea.be/news/article/differences-between-diesel-and-petrol https://www.acea.be/news/article/differences-between-diesel-... [2] https://personal.utdallas.edu/~metin/Merit/MyNotes/energyScience2.pdf https://personal.utdallas.edu/~metin/Merit/MyNotes/energySci... [3] https://en.wikipedia.org/wiki/Molar_mass https://en.wikipedia.org/wiki/Molar_mass [4] https://en.wikipedia.org/wiki/London https://en.wikipedia.org/wiki/London [5] https://en.wikipedia.org/wiki/Demography_of_the_United_Kingdom https://en.wikipedia.org/wiki/Demography_of_the_United_Kingd... [6] https://en.wikipedia.org/wiki/Climate_of_London https://en.wikipedia.org/wiki/Climate_of_London