Cities may reduce the surface humidity, because rainfall is drained away and not re-evaporated into the air. Thus measurements at St Louis (Missouri) showed reductions of 1.5 g/kg compared with the suburbs, and the difference was 3.5 g/kg at Mexico City at 2 p.m. in July. This leads to a lower RH, aggravated by urban heating (Section 3.7).
On the other hand, there can be a locally increased humidity within oasis-like cities such Figure 610 Distribution of monthly mean mixing ratio (g/ as Alice Springs, because of water brought to kg) near the surface in January and July in southern Africa. the city or pumped from underground. The
mixing ratio averages 0.8 g/kg higher in Alice Springs than at Ayers Rock, in similar country but remote from any city. Likewise, it is about 0.3 g/kg higher in the centre of Johannesburg than in the countryside around.
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