Urban—rural humidity differences in London

Abstract Measurements of vapour pressure taken four times daily over a 10‐year period are used to investigate diurnal and seasonal patterns of urban—rural absolute humidity differences in London. The urban atmosphere is found to be more humid than the rural at night in all months and during the day in winter and spring, while during summer days, the urban atmosphere is less humid. Patterns of urban excesses and deficits of humidity can be explained by reference to urban and rural energy balances and sources of moisture. A good relationship exists between heat island intensity and urban—rural vapour pressure differences.