One: energy is energy. UV, visible light, IR, or, "heat," in the sense of molecular motions of solids, liquids, or gases are all energy.ishag said:So, my question is if we are reflecting the sunlight back to the atmosphere, will it convert to heat and increase the temperature of atmosphere further increasing UHI?
Would you be able to see a vast white sunlit roof from a long way up? How come?ishag said:The reflected light from white roof can interact with gas and vapour molecules in the atmosphere and re-emit the radiation. Even though we are using white paint to reduce the temperature inside the building, it will eventually result in an increase in the atmospheric temperature.
Is this correct?
We need to separate the visible and IR portions in the discussion.ishag said:I am sorry, I didn't understand your question.
I have this doubt because I have read that usage of cool roofs including white painted roofs will help in decreasing urban heat island effect. Cool roofs have high solar reflectance and thermal emittance. The reflected and emitted radiation are going back to the atmosphere. Will this increase the overall atmospheric temperature considering we are using the cool roofs in a vast area.
Then how come visible light reaches us down here on Earth from the sun? Why doesn't most of it get converted to heat in the atmosphere?ishag said:Will the visible light heat the atmosphere?
I think it would. after all, its energy. The photons absorbed by molecules in the atmosphere are converted into thermal motion.
right, but at the same wavelength?ishag said:If the IR resonates well with the molecules in the material, then the molecules vibrates, the temperature of the material increases. It also re-emits the radiation.
Quite so. So we need to modify our split into visible and IR a little. Instead, we should split the bands into those which interact strongly with the atmosphere and those that don't.ishag said:Are all bands of IR treated equally by the atmosphere?
I don't think so. They interact with the atmosperic gases based on the paricular frequency. Wether it resonates with the molecular vibration of particular molecule.
Right. Contrast this with black body absorption and re-emission.ishag said:For the IR, let's say the white roof also reflects a high proportion of that. Will the wavelength of the reflected light be changed at all?
No. During reflection, wavelength doesn't change. It just bounces back.
The urban heat island effect refers to the phenomenon where urban areas experience higher temperatures compared to surrounding rural areas. This is due to the modification of land surfaces by human activities, such as construction, which leads to an increase in heat absorption and retention.
High solar reflectance, also known as albedo, refers to the ability of a surface to reflect sunlight. Surfaces with high solar reflectance can help mitigate the urban heat island effect by reflecting a significant amount of solar radiation instead of absorbing it, thus reducing the overall temperature of the area.
Emittance refers to the ability of a surface to emit thermal radiation. Surfaces with high emittance can release heat more efficiently, thus reducing the overall temperature of the area. This is important in urban areas where surfaces with low emittance, such as dark pavements and buildings, contribute to the urban heat island effect by retaining heat.
There are several strategies for increasing solar reflectance and emittance in urban areas. These include using light-colored or reflective materials for pavements, roofs, and buildings, incorporating green spaces and trees to provide shade and cooling, and implementing cool roof technologies that use reflective coatings to increase solar reflectance and emittance.
The urban heat island effect can have negative impacts on human health and the environment. Higher temperatures can lead to heat-related illnesses and discomfort, particularly for vulnerable populations such as the elderly and young children. It can also increase energy demand for cooling, leading to higher greenhouse gas emissions. In the environment, the urban heat island effect can disrupt ecosystems, reduce air quality, and contribute to the formation of urban smog.