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Concepts: Designing Urban Landscape

The average Albedo for the planet Earth including its atmosphere is 30%

Albedo is the proportion of the radiation that is reflected by a surface. It is the measure of the reflective nature of an object means how reflective a surface is. The more reflective a surface is the higher the albedo value. Dark colour objects absorb most of the solar radiation and hence have a low albedo, whereas light-coloured ones have a high albedo, as they reflect most of the radiation. Ice and white clouds have high albedos. Albedo is also affected by Emissivity of an object. When electromagnetic radiation is incident on a surface, it is either absorbed causing the surface to get hot or it is reflected. The ratio of reflected to incident radiation is called Albedo.

Reflectivity: the percentage of incident light or solar radiation that is reflected.

Emissivity: the rate at which absorbed energy is radiated from an object. It is the ratio of the energy radiated by an object to the energy radiated by a black body of the same temperature.

0 to 1 scale (or in percentage): 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that reflects all incident radiation. The albedo for snow is 90% (0.9) since it reflects most of the radiation incident on it. On the other hand, a dark canopy forest has a low albedo of 10% (0.1).

What if? Earth’s average surface temperature is currently about 15 °C or 288K. If Earth were frozen entirely (and hence be more reflective), the average temperature of the planet would drop below −40 °C. In contrast, if the entire Earth was covered by water, a so-called ocean planet the average temperature on the planet would rise to almost 27 °C.

0 K = -273 degree C  AND  273 K = 0 degree C

Therefore to convert K (Kelvin) to degree C (celsius scale) add 273.

To convert degree C to K add 273.

 

Black Body Radiation:

A black object absorbs all wavelengths and if heated, it will emit all wavelengths too. However, not all frequencies will be intense. The hotter the radiating body is, the shorter the wavelength. Because of the atomic structure, solids can absorb different wavelengths of radiations.

Stefan- Boltzmann Law: As the temperature of a blackbody increases, the intensity of radiation from the surface of the body also increases.

Coating City streets white

Lesson learned:

Increasing albedo and reducing black body radiation may help us in reducing the UHI effect.

Solid concrete objects, dark tarmac and asphalt act as blackbody and intensify radiation.

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