185
4
“Rate” Index (Reduction in the Absorption
of Thermal Energy)
In days characterised by anticyclonic conditions in the air and strong stability on the
ground, a vertical temperature profile is established, with Thermal Inversion (thermal inversion on the ground, countryside). The heat produced by the buildings tends
to contrast the vertical thermal inversion, without totally breaking it (thermal inversion in the air, town), creating an air dome whose maximum height corresponds to
the zone with the highest concentration of buildings.
The inversion layer creates a barrier which prevents a redistribution of vertical
air all over the atmosphere layer available. This creates a heat island, which therefore depends on the season, geographic position of the town and its characteristics.
This “blanket” withholds the heat, thus raising the temperature (for instance,
minimum temperatures at night).
Graph showing the Urban Heat Island phenomenon
The presence of materials endowed with little capacity to reflect solar radiation
(solar reflectivity), in conditions of strong solar irradiation, results in high thermal
loads on the surface in general. If this surface has a scarce thermal isolation capacity
(thermal resistance), it tends to warm up and, in its turn, if it does not have a good
capacity to release this heat by irradiation (emissivity), it will take a certain period
of time to return to ambient temperature. Continuing to release heat, even during the
night, the material will affect the rising of the temperature within the heat island.
The capacity of materials to reflect solar radiation (solar reflectivity), to release
the heat absorbed (emissivity) and to isolate thermally (thermal resistance) make it
4
“Rate” Index (Reduction in the Absorption
of Thermal Energy)
In days characterised by anticyclonic conditions in the air and strong stability on the
ground, a vertical temperature profile is established, with Thermal Inversion (thermal inversion on the ground, countryside). The heat produced by the buildings tends
to contrast the vertical thermal inversion, without totally breaking it (thermal inversion in the air, town), creating an air dome whose maximum height corresponds to
the zone with the highest concentration of buildings.
The inversion layer creates a barrier which prevents a redistribution of vertical
air all over the atmosphere layer available. This creates a heat island, which therefore depends on the season, geographic position of the town and its characteristics.
This “blanket” withholds the heat, thus raising the temperature (for instance,
minimum temperatures at night).
Graph showing the Urban Heat Island phenomenon
The presence of materials endowed with little capacity to reflect solar radiation
(solar reflectivity), in conditions of strong solar irradiation, results in high thermal
loads on the surface in general. If this surface has a scarce thermal isolation capacity
(thermal resistance), it tends to warm up and, in its turn, if it does not have a good
capacity to release this heat by irradiation (emissivity), it will take a certain period
of time to return to ambient temperature. Continuing to release heat, even during the
night, the material will affect the rising of the temperature within the heat island.
The capacity of materials to reflect solar radiation (solar reflectivity), to release
the heat absorbed (emissivity) and to isolate thermally (thermal resistance) make it