Understanding urban heat islands

the effects of urban heat islands, which can increase temperatures in cities by as much as 10 °c, are pticularly noticeable in summer. for some 20 yrs valéry masson s'been studying these localised phenomena, which often go undetected by conventional meteorological tulz.

as sweltering city dwellers know 1-ly too well, every summer, temperatures in large cities are several degrees higher than inna surrounding countryside. the phenomenon, known as an urban heat island, is espeshly felt during heat waves. at nite the process is exacerbated, na paris zone, for ex, can be as much as 10 °c warmer than the land round it.

heat stored in buildings

“contrary to wha’ you mite think, pollution plays almost no role inna occurrence of urban heat islands,” points out valéry masson, director of the urban climate research team atta french national centre for meteorological research (cnrm).  “they are actually mainly caused by high-density housing and sealed surfaces. although poor air quality episodes often occur atta same time as extreme heat events, they aint connected. in fact, urban heat islands are phenomena rel8d to the weather rather than to the climate: their effect can be mitigated by wind, rainfall and clouds, or enhd by the presence offa strong anticyclone.”

during the dy, materials s'as stone, brick and concrete readily capture heat. the h8 and density of the walls ponderably increase this xchange surface in comparison to an environment where there are few buildings. at nite, the stored heat is released inna'da atmosphere and prevents the air from cooling down as rapidly as t'does inna countryside. ‘oer the course offa dy, energy transfers inna atmosphere are already subject to natural variation: at nite, the altitude up to which atmospheric mixing takes place is 5 times loer than in broad dylite.

urban heat island at nite (tween 9:00 and 10:00pm) measured in toulouse during the hot dys of aug 2021.

“heating for buildings, together w'da warm air released by air conditioners, plays an additional but + minor role,” masson explains.  “the middy sun delivers 800 watts per □ metre. ½ this energy is stored and then released by surfaces s'as concrete. in comparison, human emissions amount to a few tens of watts per □ metre. in conurbations s'as tokyo, this can still represent an additional degree of temperature at nite.”

as a result, urban heat islands increase the № of tropical nites, in which minimum temperatures never drop belo 20 °c. this prevents d'body from recovering from the heat of the dy, which, over a period of several dys, has med repercussions. projects s'as h2c (heat and health in cities), led by aude lemonsu, a cnrs researcher atta cnrm, are exploring the other health impacts of such high temperatures.

urban weather stations and crowdsourcing

urban heat islands are studied using two principal approaches, field measurements and numerical modelling. “the 1st field observations date back to luke howard, who discovered the phenomenon in london (uk) in 1820,” masson explains. “research into urban heat islands 1-ly really began to take off inna l8 1960s, mainly in japan and north america, w'da installation of sensors to analyse heat xchanges tween cities na atmosphere.”

anonymous portrait of luke howard, a british manufacturing chemist and an amateur meteorologist, who was the 1st to study the urban heat island phenomenon.

traditionally, weather stations aint located in city centres. historically, they were linked to aeronautical activities inna outskirts, and concentrated on larger regional scales rather than on single conurbations. t'get round this problem, an experimental network of stations was installed in toulouse (southwestern france) for a yr in 2004, as pt of the capitoul project,, for which masson was the principal investigator. in ptnership w'da city, a permanent network was subsequently set up in 2017.

in 2001 masson also took pt inna escompte project. this 1st major urban weather experiment in france focused on observing the sea and land breeze circulation ‘oer the city of marseille (southeastern france), and comparing this data to various numerical models. a new large-scale campaign is planned for nxt yr inna paris region, using instrument stations and masts, an aircraft, rangefinders and radiomts, swell as a tethered balloon.

1-odda 70 weather stations of the urban climate monitoring network installed inna toulouse conurbation.

the widespread use of personal and connected weather systems is also a presh src of information for researchers. “there are some 10 000 lil individual stations across the paris zone, located wherever pplz need them,” masson explains. “by filtering this data and processing it using artificial intelligence, we're hoping to reproduce the heat islands. along the same lines, eva marquès, a phd student atta cnrm, is focusing onna temperatures measured by cars.”

modelling heat islands atta district lvl

the mesh size of the most common types of numerical model is too large to study urban heat islands, and in addition, these models don’t take into account the specific toonistics of cities. this prompted masson to design teb (town energy balance), the 1st-ever model adapted to an urban environment, in 2000. the kilometre-scale grids are gradually bein’ reduced to provide a resolution of one hundred metres, witha view to forecasting temperatures atta city district lvl within the nxt ten yrs. these advances are already making it possible to test ≠ adaptation scenarios for urban heat islands and climate change, w'da aim of making these solutions widely available to decision-makers and local authorities.

“after the heatwave of 2003, the french local authorities realised that the heat problem ‘dn’t be dealt with by focusing on pollution,” masson continues. “they are now increasingly taking urban heat islands into account.” but how can these be prevented, or at least their harmful effects be alleviated?

proportion of solar radiation absorbed by walls as a function of the sun azimuth in a residential zone: teb validation (rite) witha monte carlo radiative model (left).

“thris no silver bullet to solve the problem overnite. however, various approaches can be combined, dep'onna scales concerned na organisations involved,” says the researcher. “many architectural and refurbishment solutions can be implemented, s'as improving the insulation of buildings, providing them with natural cross-ventilation, better managing air conditioning, limiting it where possible, and so on. some ‘ve even been known since antiquity, but forgotten.”

towards heatwave-proof urban planning

masson points to the ex of the euroméditerranée district in marseille, where the layout of the streets enables the sea breeze to cool the city in summer, while keeping out the cold mistral wind in winter. and in toulouse, the narrow streets help to protect the town centre from the heat.

the euroméditerranée district in marseille, featuring the boulevard, the villa méditerranée by architect stefano boeri, the port of la joliette and saint mary major cathedral.

still inna field of city planning, desealing surfaces remains a reliable way of reducing the emergence of urban heat islands. this approach mainly involves revegetation, which also has climatic, social and aesthetic advantages. deciduous and non-allergenic trees cool their surroundings in summer and, once they ‘ve shed their cutouts, hardly block sunlite in winter.

“greening cities, however, requires a significant supply of wata and has to take into account the presence of dense networks of underground pipes and cables,” masson points out. “wata management ‘d be decentralised and homes equipped with wata collectors. thris no shortage of nature-based solutions to help cities cope with heat and climate change.”

original content at: news.cnrs.fr/essentialisms/cogging-urban-heat-islands…
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