Urban microclimate: More clouds over big cities than in the countryside

Science Cloudy to overcast

Why the city sky is often full of clouds

Sunflowers bloom in front of the European Central Bank and the Frankfurt banking skyline

Cloud panorama over the Frankfurt bank skyline

Source: picture alliance / greatif

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”).

also read

Hamburg water strategy combo

“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.

also read

Various phases of a total lunar eclipse, blood moon, astrophotography

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.

also read

In more affluent neighborhoods like Berlin-Kreuzberg, more trees than anywhere else ensure a pleasant climate

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.

You can listen to our WELT podcasts here

In order to display embedded content, your revocable consent to the transmission and processing of personal data is required, since the providers of the embedded content as third-party providers require this consent [In diesem Zusammenhang können auch Nutzungsprofile (u.a. auf Basis von Cookie-IDs) gebildet und angereichert werden, auch außerhalb des EWR]. By setting the switch to “on”, you agree to this (which can be revoked at any time). This also includes your consent to the transfer of certain personal data to third countries, including the USA, in accordance with Art. 49 (1) (a) GDPR. You can find more information about this. You can withdraw your consent at any time via the switch and via privacy at the bottom of the page.

“Aha! Ten minutes of everyday knowledge” is WELT’s knowledge podcast. Every Tuesday, Wednesday and Thursday we answer everyday questions from the field of science. Subscribe to the podcast at Spotify, Apple Podcasts, deezer, Amazon Music, Google Podcasts or directly by RSS feed.

See more here

See also  Can the Montréal Agreement stop biodiversity loss? - Knowledge

Leave a Reply

Your email address will not be published. Required fields are marked *