Microclimate & Thermal Comfort

The figure above shows  5 main elements, which need to be considered in microclimate design:

Air temperature and humidity form the base condition and are relatively constant over a local urban landscape. This might sound contradictory, but while the surface temperature of an asphalt parking lot in the sun is much higher than in the shade of a tree close by, the air temperature 1,5m above it will be largely the same. The heat radiating from the asphalt is quickly dispersed evenly in the atmosphere. The same goes for water which evaporates on a hot surface. This fact means that air temperature and humidity can’t effectively be modified, except for isolated and very localized circumstances.

However the amount of sunshine, wind or radiated heat that a person is exposed to is greatly influenced by the landscape and consequently affect how a person feels in urban public spaces:

Solar radiation is the main source of heat, controlling it through shading structures or trees is a main modification of the local microclimate. Understanding when (time of year and day) and where (areas with a lot of solar exposure) shade is necessary and useful is crucial to improve thermal comfort. The amount of warming in various parts of the landscape can be modified by placement of landscape elements. However it is much easier to decrease warmth input , than to amplify it.

🠊 Principle: Sun is the main source of heat. The main priority in summer is minimizing solar radiation.

Wind provides the main form of cooling by convection, in other words, by transporting heat away from surfaces and mixing the warm air with cooler air from outside of the city. Generally speaking, the effect of wind-chill on a person is bigger in winter, than in summer, due to the bigger temperature difference between the person’s skin and the air temperature. It is much easier to protect from wind, than funnel wind for ventilation.

🠊 Principle: Wind is the main source of cooling. The main priority in winter is reducing wind. 

Terrestrial radiation depends on the surface materials and on how much sun the surface is exposed to. Therefore shading can prevent that heat is stored in the surface materials and radiated as terrestrial radiation. If shading is not probable, surfaces can be picked so they reflect more solar radiation and absorb less of it (high albedo). Even better are surfaces which are not sealed and can therefore store and evaporate moisture, this means that heat is transferred into  latent heat rather than radiated as sensible heat. It is important to note that terrestrial radiation will only have a significant effect if the energy emitted by the surfaces surrounding a person is higher than the energy emitted by a person. So if someone is surround by cool surfaces it a person will cool down, if the surfaces are hot the person will be warmed. This is important to consider especially because depending on the season one or the other might be preferred.

Solar radiation, terrestrial radiation and wind and how they are affected by the landscape provide us strategies with which the microclimate can be modified and designed to be comfortable to be in.

🠊 Principle: Thermal comfort of people = Heat added by sun and terrestrial radiation - heat carried away by wind and terrestrial radiation

🠊 Principle: Winter microclimate design 🠊 reduce wind speed, maximize solar & terrestrial radiation

🠊 Principle: Summer microclimate design 🠊 minimize solar & terrestrial radiation, allow wind ventilation

Applying these principles is straightforward if a space is mainly used in winter or summer. However if a space should be comfortable all year these microclimate design principles contradict each other and even make things worse in other seasons. This is why we need temporary, flexible and adaptable microclimate interventions in highly seasonal climates like Malmö! See our intervention overview and proposals to get inspiration on how to do that.