On average, large cities are 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 over 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 is presented in the journal “Proceedings” of the US National Academy of Sciences (“PNAS”).
Cities with humid tend to have more local cloud amplification.
University of Alabama research team
“Urbanization significantly changes surface roughness and properties, affecting regional climate and water cycles,” the researchers write. Cities can also contribute to cloud formation. 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.
3.1
percent there are more clouds in July over cities compared to rural areas
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.
Increased 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 from exhaust gases or heating systems, for example, also play a role in cloud formation as condensation nuclei for water droplets, but 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.
