# Urban heat effect: high solar reflectance and emittance

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1. Jul 20, 2015

### ishag

1. The problem statement, all variables and given/known data
"The urban heat island (UHI) effect refers to the phenomenon of a metropolitan or built up area which
is significantly warmer than its surrounding areas." Using white paint in roofs increases the solar reflectance and decrease the thermal intake into a building.
Question:-
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?

2. Jul 20, 2015

### Bystander

One: energy is energy. UV, visible light, IR, or, "heat," in the sense of molecular motions of solids, liquids, or gases are all energy.
Two: what "heats" the atmosphere in your scenario?

3. Jul 20, 2015

### ishag

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?

4. Jul 21, 2015

### haruspex

Would you be able to see a vast white sunlit roof from a long way up? How come?

5. Jul 21, 2015

### ishag

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.

6. Jul 21, 2015

### haruspex

We need to separate the visible and IR portions in the discussion.

Will the visible light heat the atmosphere? If you think it will, answer my earlier question.

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? If the IR had instead fallen on a surface which absorbs it, what would have happened to that energy subsequently?
Are all bands of IR treated equally by the atmosphere?

7. Jul 21, 2015

### ishag

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.

Would you be able to see a vast white sunlit roof from a long way up? How come?
White reflects all the wavelengths in the viscible range. bust can i see it? I do not know. Please explain this.

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 doesnt change. It just bounces back.

If the IR had instead fallen on a surface which absorbs it, what would have happened to that energy subsequently?
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.

Are all bands of IR treated equally by the atmosphere?
I dont think so. They interact with the atmosperic gases based on the paricular frequency. Wether it resonates with the molecular vibration of particular molecule.

8. Jul 21, 2015

### haruspex

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?

right, but at the same wavelength?

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.
Right. Contrast this with black body absorption and re-emission.