Why the city sky is often full of clouds
Those who like the firmament blue and clear should rather move to the country. Researchers have now discovered that it is often overcast over cities. Under certain conditions in particular, more clouds form over metropolises – with consequences for the urban microclimate.
GLarge cities are, on average, more cloud-covered than rural areas. This is shown by a US study, for which satellite images of 447 cities in the US from the years 2002 to 2020 were evaluated. The results can be transferred to German and European cities, said Petra Fuchs from the German Weather Service (DWD).
According to the results, cloud cover in cities during the day increased by an average of 3.1 percent compared to rural areas in July, and by as much as 5.8 percent at night in June. The study by a group led by Leiqiu Hu from the University of Alabama in Huntsville in the journal “Proceedings” presented to the US National Academy of Sciences (“PNAS”).
“Urbanization significantly changes surface roughness and properties, affecting regional climate and water cycles,” the researchers write. Cities can also contribute to cloud formation.
More cloud formation in humid, cool regions
To learn more about this, the researchers first looked at cloud patterns at different times of the year and times of day. In July, around 80 percent of the cities surveyed show a statistically clear effect on cloud cover during the day, and even 90 percent on cloud cover at night.
“Cities with humid climates tend to show more local cloud enhancement during the warm season compared to dry regions,” it said. In cooler and humid regions, cities tend to have more cloud formation during the day in winter.
In January, the sky over cities is only 1.8 percent more overcast than over the countryside, at night the conditions are even reversed, when there are slightly fewer clouds over the city than over the countryside.
The researchers explain the observations primarily with the fact that buildings and streets in a city heat up more during the day than rural areas. This causes more warm air to rise, resulting in a wake of cooler and wetter air from the surrounding countryside, which warms and rises as well.
Influences on urban microclimate
The effect is significantly stronger at night in summer, because buildings and streets store the sun’s heat better than the ground and plants and emit it as thermal radiation at night. For mountain towns and coastal towns, the environment has further influences on the urban microclimate; near the sea a weaker effect during the day, but a stronger effect at night.
More cloud cover is likely to reduce solar radiation over warmer urban areas and consequently reduce the existing contrast in surface warming between urban and rural areas, the scientists suspect.
Particles originating from exhaust gases or heating systems, for example, also play a role in cloud formation as condensation nuclei for water droplets; however, these were not examined in the study
Petra Fuchs from the DWD said that the different aspects of cloud formation in urban areas are well reflected in the study. The fact that warmer air can absorb more moisture and lead to increased cloud formation can also be observed in the course of climate change.
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