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.
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.
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.
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.
