Revision of Paris-Urban heat island effect. from Tue, 06/07/2011 - 06:59

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The city of Paris, within its administrative limits largely unchanged since 1860, it has an estimated population of 2.2 million. By the year 2050, it would grow...


As shown in the map, there is large surface area of land is covered by the roads in Paris. The city is highly dense, in some parts it is green, in other, it lacks green spaces. The houses and buildings are compacted and most of them are traditional ones in the center of the city.

As it can be seen as it has less reflectional surfaces in urban fabric. Even all the roofing materials are replaced by green roofs, the UHI effect is reduced by some percentage but the city would be still hotter.

Causes of UHI in Paris


In Paris, cars are highly used even the public transportation are convenient ones. Due to use of cars, the greenhouse gases are released into the atmosphere.

Impervious urban surface

In Paris, roads, pavements are basically impervious surfaces. Asphalts roads have low albedo, so, it absorbs the heat radiation and releases into the atmosphere at night. Even the pavements used are concrete ones in many areas of the Paris. So, these land covers large area which has low albedo and impervious surface, which leads to release of heat and no cooling effect at the night time.


There are lots of parks and green spaces in Paris, but as it would be compared to the infrastructure density, these green spaces are lesser. In some areas it has green surrounding environment in others it is totally deserted. So, the need of vegetations, plants, trees and parks required to be distributed equally in the each corner.

Greenhouse gases

The release of green house gases in the atmosphere have also increased the UHI effect in Paris.

Existing mitigation strategies used in Paris

The use of cooling system. Green walls. Greeneries in buildings.



Albedo or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it. Being a dimensionless fraction, it may also be expressed as a percentage, and is measured on a scale from zero for no reflecting power of a perfectly black surface, to 1 for perfect reflection of a white surface.

Albedo depends on the frequency of the radiation. When quoted unqualified, it usually refers to some appropriate average across the spectrum of visible light. In general, the albedo depends on the directional distribution of incoming radiation. Exceptions are Lambertian surfaces, which scatter radiation in all directions according to a cosine function, so their albedo does not depend on the incident distribution. In practice, a bidirectional reflectance distribution function (BRDF) may be required to characterize the scattering properties of a surface accurately, although the albedo is a very useful first approximation.

The albedo is an important concept in climatology and astronomy, as well as in computer graphics and computer vision. The average overall albedo of Earth, its planetary albedo, is 30 to 35%, because of the covering by clouds, but varies widely locally across the surface, depending on the geological and environmental features.

Atmospheric heat absorption


Energy Transfer in atmosphere

Albedo graph

Albedo percentage of material

Green walls

A green wall is a wall, either free-standing or part of a building, that is partially or completely covered with vegetation and, in some cases, soil or an inorganic growing medium. The vegetation for a green façade is always attached on outside walls; with living walls this is also usually the case, although some living walls can also be green walls for interior use.For living walls there are many methods including attaching to the air return of the building to help with air filtration.

There are two main categories of green walls: green façades and living walls. Green façades are made up of climbing plants either growing directly on a wall or, more recently, specially designed supporting structures. The plant shoot system grows up the side of the building while being rooted in the ground. With a living wall the modular panels are often made of stainless steel containers, geotextiles, irrigation systems, a growing medium and vegetation.

There are three types of Growth Media used in living walls, loose media, mat media and structural media.

Green wall 1

Green wall 2

Green wall 3

Green wall 4

Function of green walls:

Green walls are found most often in urban environments where the plants reduce overall temperatures of the building. "The primary cause of heat build-up in cities is insolation, the absorption of solar radiation by roads and buildings in the city and the storage of this heat in the building material and its subsequent re-radiation. Plant surfaces however, as a result of transpiration, do not rise more than 4–5 °C above the ambient and are sometimes cooler." Living walls may also be a means for water reuse. The plants may purify slightly polluted water (such as greywater) by absorbing the dissolved nutrients. Bacteria mineralize the organic components to make them available to the plants.

Living walls are particularly suitable for cities, as they allow good use of available vertical surface areas. They are also suitable in arid areas, as the circulating water on a vertical wall is less likely to evaporate than in horizontal gardens.

The living wall could also function for urban agriculture, urban gardening, or for its beauty as art. It is sometimes built indoors to help alleviate sick building syndrome.

Green wall diagram

In this image the green walls is not designed to attach on the building but it takes the support of the building to stand in between the pedestrian and the road. This will help to reduce the heat in those areas where building facade cannot be used as green walls and there is no greeneries in surrounding areas.

Green walls

This image is the existing scenario in the Paris.

Cool Roofs

Cool roof is a roofing system that can deliver high solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the building) and high thermal emittance (the ability to radiate absorbed, or non-reflected solar energy). Most cool roofs are white or other light colors. Cool roofs enhance roof durability and reduce both building cooling loads and the urban heat island effect.

Benefits of cool roofs

Most of the roofs in the world (including over 90% of the roofs in the United States) are dark-colored. In the heat of the full sun, the surface of a black roof can increase in temperature as much as 50 °C (122 °F), reaching temperatures of 70 to 90 °C (150-190 °F). This heat increase can contribute to:

-Increased cooling energy use and higher utility bills -Higher peak electricity demand (the maximum energy load, in megawatts, an electric utility experiences to supply customers instantaneously, generally experienced in summer late afternoons as businesses and residences turn up their air conditioners), raised electricity production costs, and a potentially overburdened power grid; -Reduced indoor comfort; -Increased air pollution due to the intensification of the "heat island effect" -Accelerated deterioration of roofing materials, increased roof maintenance costs, and high levels of roofing waste sent to landfills.

cool roofs

Cool roofs albedo percentage

Roof schematic green roofs