1 Discuss on the relation between the measured data beam rad

1. Discuss on the relation between the measured data (beam radiation and diffuse radiation) and the weather data provided by the CWB online database.

2. Define these three terms: a) emission; b) reflection; and c) scattering.

3. If a glass cover (transmissivity = 0.93) is placed on the top of the two sets of pyrometer, what do you expect on the averaged results individually?

4. Why is a solar water heater generally installed with a glass cover on the top to be insulated from the outside air? Compare the solar energy collecting efficiencies of the solar water heaters with and without a glass cover, respectively.

Solution

2.

Emissivity is a measure of how strongly a body radiates at a given wavelength; it ranges between zero and one for all real substances. A gray body is defined as a substance whose emissivity is independent of wavelength. In the atmosphere, clouds and gases have emissivities that vary rapidly with wavelength. The ocean surface has near unit emissivity in the visible regions.

Reflection occurs when a wave hits the interface between two dissimilar media, so that all of or at least part of the wave front returns into the medium from which it originated.

Scattering is a physical process that causes radiation to deviate from a straight trajectory. We saw this in the introductory sections on reflection: If there were microscopic irregularities in the surface we would get diffuse instead of specular reflection. The same goes for radiation passing through a transparent medium: If there are non-uniformities like particles, bubbles, droplets, density fluctuations etc, some of the radiation will deviate from its original trajectory.