Cities are Changing Cloud Formations

Mark Altaweel

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Clouds play a vital role in our climate system by aiding in the cooling of the Earth and supplying moisture. The formation of clouds can be disrupted by urban environments due to the influences of urban surfaces and the release of aerosols.

The exact nature of urban environment interaction with cloud formation is still poorly understood and generally not well considered in climate modeling applied today. Now, scientists seek to better understand how our cities and towns affect cloud formation with the results potentially being important for future climate understanding. 

Using satellite imagery to analyze cloud patterns over urban areas

In a recent study, over 447 cities in the contiguous US were studied using satellite-derived cloud patterns created over two decades using diurnal/nocturnal and seasonal data. Researchers looked at how cloud patterns changed both from day-to-night and from summer to winter.

Researchers counted the frequency of cloud-enhanced days relative to the total number of days of the period of study. Cities were studied across various geographies and periods, with results showing generally cloudier days in the late spring and summer months in urban settings.

A view of dark clouds over some urban buildings with blue sky in the background.
Urban areas tend to increase the formation of clouds. Photo: Caitlin Dempsey.

Regional moisture is encouraged through evapotranspiration into the atmosphere and through moisture transport by means of large-scale circulations.

In many cities, the thermodynamic differences between urban and rural areas, coupled with surface roughness, generate updrafts and mixing that elevate moisture over cities to cloud condensation layers. This effect is further amplified at night due to stronger urban heating and a probable increase in moisture content.

The spatial differences though in the US are clear; cloud enhancement are stronger over the eastern and southeastern contiguous United States. Enhancements in night-time cloud cover are usually observed in the cooler northern US cities and along the west coast.

A view of houses along Hermosa Beach with a blue sky with no clouds.
A cloudless day on a day in February in Hermosa Beach, California. Photo: Caitlin Dempsey.

Moreover, regions with higher precipitation seem to experience more urban-enhanced cloud activity during the night.

From measured cloud differences, stronger urban surface heating appears to be associated with a 1.5% daytime cloud enhancements; nocturnal enhancements appear to manifest a 2.3% increase in clouds due to surface heating with a 2.8% increase as affected by urban size.

Overall, moist and warm climates have generally stronger local cloud enhancement.

In the winter, cities also affect cloud cover, but this is much weaker than the summer months. Interestingly, arid cities show the strongest signal in cloud enhancements in the winter than wetter cities.

Relatively higher temperature in cities may suppress fog formation, creating what are called fog holes in areas where fog forms except in the urban setting. This has been observed in the Indo-Gangetic Plains. In the spring and autumn months, signals were closer between urban settings and rural areas, showing less effect of urban environments on cloud formation.[1]

The winter months are complex, as regional temperature patterns that interact with moisture conditions affect cloud formation over urban settings. This is why cooler and generally wetter regions tend to have more urban clouds during daytime in the winter months.

Clouds hang in the sky over this view across the suburban Silicon Valley with a grassy meadow with a few oak trees in the foreground.
Clouds hang over the Silicon Valley on March of 2015. Photo: Caitlin Dempsey.

Temperature seems to be a more significant determinant, as seen in the northern cities throughout the contiguous US.

Cities on the west coast may exhibit considerable fluctuation in cloud cover. The impact on clouds during the daytime is relatively mild, but the influence at night is more pronounced.

Cities weaken land breezes

Cities appear to weaken land breezes that helps urban surface heating to be enhanced at night with the drier and cooler areas enhancing their cloud cover. Overall, it is clear that urban settings have a major impact on cloudiness across the contiguous US.

Inland cities experience enhanced cloudiness in the summertime during the day. In the winter, regions also show enhanced daytime and reduced nighttime cloud influences.

In general, moisture and heat energy appear to be the main factors that affect cloud coverage in urban settings. Given the effects on clouds, it is evident that urban settings play an important role in moisture.

Aerosols from urban places are also found to potentially modify the microphysical properties of cloud droplets. The findings also shed light on why numerous climate models have occasionally struggled to accurately simulate urban cloud cover in cities like London.[2]

Considering the significant role of clouds in climate and climate modeling, which will become increasingly crucial for predicting future climate change effects, it’s essential that we gain a better understanding of urban-cloud formation processes and improve data collection.

To improve our comprehension of how our cities influence cloud formations, it’s necessary for studies to that cover different parts of the Earth, since there’s substantial variation due to geographic and overall climatic differences.

As urban and paved areas expand across regions experiencing high population growth, the impact of urban environments on climate will increase in influence on cloud patterns.

References

[1]    For more on how urban environments affect cloud formation in the contiguous United States, see:  Vo TT, Hu L, Xue L, et al. (2023) Urban effects on local cloud patterns. Proceedings of the National Academy of Sciences 120(21): e2216765120. DOI: 10.1073/pnas.2216765120.

[2]    For more on how a city like London affects its cloud cover in the summertime months, see:  Theeuwes NE, Boutle IA, Clark PA, et al. (2022) Understanding London’s summertime cloud cover. Quarterly Journal of the Royal Meteorological Society 148(742): 454–465. DOI: 10.1002/qj.4214.

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About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.

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