Table of Contents
The Moroccan Context
Urban Heat Islands and Perceived Temperature
Solution No. 1: Water Evaporation
Solution No. 2: Plant Evapotranspiration
Solution No. 3: Shade in the City
Solution No. 4: Harnessing and Optimizing Winds
Solution No. 5: Colors and Materials
Urgency of Adopting a ''New'' Climatic Urbanism
FIVE SOLUTIONS FOR COOLING MOROCCAN CITIES
The Moroccan Context
According to United Nations projections, the global population is expected to reach approximately 8.5 billion people by 2030 with an estimated urbanization rate of 60%. Demographic dynamics in Morocco confirm this global upward trend, and are projected to reach 43.6 million inhabitants in 2050 compared to 33.8 in 2014, a growth of +22% in 36 years. This increase is accompanied by growth in the urban population since by 2030, 65% of the world's population will live in cities, and this figure is estimated at nearly 75% in 2050[1].
Such growth, including in Morocco, raises significant challenges in terms of energy costs, infrastructure, habitability, and urban climate.
In Morocco, the climate in recent years has been particularly threatened by waves of dry heat and heat waves that have become recurrent with climate change: 48° in Marrakech in July 2012, a record broken in July 2023 with 49.6°. Or even over 50°C in Agadir the same year, or 48°C in Beni Mellal in 2024, a heat wave that caused approximately twenty deaths.
Climate change is multifaceted and manifests itself in various ways, often as a deterioration from the past or as a pessimistic projection: as indicated by the studies of Dennis Meadows, James Hansen, the IPCC, or Don't Look Up... Fortunately, adaptation solutions and alternatives also exist in abundance: drawing inspiration from vernacular and climatic practices or technological advances.
The objective of this article is to explore five solutions or bioclimatic levers for adapting cities to heat waves. The term bioclimatism frequently appears in sustainable development narratives. It involves designing by leveraging site and environmental conditions (sun, wind, etc.) rather than enduring them and compensating through significant energy efforts. In contemporary practice, bioclimatic and technological approaches are complementary. However, bioclimatism, in its adaptation and sobriety, should be predominant!
Urban Heat Islands and Perceived Temperature
The Urban Heat Island (UHI) is defined as a difference in air or surface temperature observed between an urban and peri-urban or rural area, often at night for the air and during the day for the surface[2]. Indeed, the city itself is a source of temperature increase, beyond the global greenhouse effect, through its design and activity. It is possible to identify two main categories of local overheating. The first are due to heat production and retention and are primarily daytime in nature (cars, density, dark albedo), the second are related to limited cooling possibilities and occur mainly at night (cool winds, nocturnal radiation, evaporation, ...)[3].
Note that the consequences of urban heat islands always depend on geographic and climatic context. In cities like Milan, Toulouse, Casablanca or Marrakech, they induce summer discomfort, increased mortality rates, higher costs and excess consumption of cooling energy (particularly air conditioning), etc...[4]
Fig. 1: diagram - principle of the urban heat island (Source Nechfate I.Sakout)
It is observed that in cities, temperatures can be 10 degrees higher compared to the countryside, and this phenomenon is observed mainly at night due to blocked cool winds, or even during the day, temperature differences can reach 8 degrees between a localized heat island (road and dense buildings) and a cool island (park) 500m apart[5].
The distinction between lowering air or surface temperatures in spaces and the thermal comfort perceived by users in the city at any given moment is necessary. Often these two concepts are confused because the boundary between them can be very fine.
When we talk about thermal comfort, we are talking about perceived temperature. There are many indicators of outdoor thermal comfort. For example, the UTCI (Universal Thermal Comfort Index) is among the most comprehensive and widely accepted by the engineering and climatology community to quantify a person's thermal sensation. Air temperature, directly related to the UHI, is then one parameter among those considered by the UTCI.
As mentioned previously, air temperature is a component of thermal comfort. However, the two concepts do not always show the same trend. For example, the color of an outdoor floor can influence air temperature or radiant temperature through radiation or reflection: if the ground is dark, it can overheat in summer and significantly increase air temperature, while if it is light, it does not overheat but reflects the sun onto the person walking on it, and it is the latter who overheats. Therefore, with a light floor exposed to the sun, air temperature is lower, but the risk of thermal stress is greater due to radiation, in other words: a dark floor will heat up more and faster than a light floor.
Fig. 2: diagram - parameters of hygrothermal comfort (Source Nechfate I.Sakout)
Solution No. 1: Water Evaporation
When water evaporates or condenses, it absorbs or releases calories into the ambient air, thus cooling or heating it. Making water evaporate to cool ambient air is an old idea used in buildings such as riads and traditional houses in medinas, which we can experience by approaching a coastline for example.
The psychrometric diagram (diagram below) is a well-known tool in thermal studies for assessing air temperature, absolute and relative humidity of this air[6], as well as the enthalpy contained in the air. In the diagram below, you can see that by evaporating 4g of water in 1 kg of air (equivalent to 0.9 m3 of air at these temperatures), you can decrease the temperature of this air mass by 10°C. Note that in the example illustrated, we start with air likely in several Moroccan cities (such as Marrakech), that is 35°C and 28% relative humidity, to reach air at 25°C and 70% relative humidity. By stopping at evaporation equivalent to 50% relative humidity (optimal humidity comfort), air temperature can drop to 28°C.
Fig. 3: psychrometric diagram and adiabatic cooling (Source Nechfate I.Sakout)
The basins or fountains in Moroccan riads are a perfect manifestation of this principle: the air circulating in the patio cools when it comes into contact with water before entering the surrounding rooms. Other examples, such as Iranian badguirs, can be cited at the building scale; these are wind towers often connected to water basins in the basement. At the urban scale, this solution can be implemented through the presence of water punctually or in a distributed manner to create cool islands, or combined with a wind strategy near a coastal area or an oasis.
Fig. 4: diagram - urban adiabatic cooling (Source Nechfate I.Sakout)
Fig. 5: Lalla Hasna Park, Marrakech Morocco (source Google)
However, water is an increasingly rare resource! Care must be taken to preserve it while avoiding unnecessary evaporation; water could be circulated underground for example and appear when needed to avoid waste. In hot and dry climates such as Marrakech's, such a solution can allow local temperature reduction of up to -10° at the peak of the day (demonstrable on the psychrometric diagram, with a -30% margin to avoid excessive humidity in the air).