1. TEM – Lecture 5 Radiative Heat Flow

2. Radiation Radiation intensity increases with temperature! Every surface above zero K Radiates!

3. https://tfaws.nasa.gov/TFAWS12/Proceedings/Form%20Factors %20Grey%20Bodies%20and%20Radks%20Course.pdf

4. Radiation: Emission and Absorption Stefan Boltzmann Law (Black Body) Radiation from a Real Body: Absorption: Grey Surface

5. Emissivity Coefficients http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html

6. Surface radiate and absorb simultaneously The incident flux of energy depends on the source. If the energy absorbed is higher then the energy radiated, the temperature increases and the radiated flux increases up to a value where both fluxes balance (in the absence of other heat transfer processes). What about solar energy?

7. Solar radiation is uniform over the surface. http://www.itacanet.org/the-sun-as-a-source-of-energy/part- 2-solar-energy-reaching-the-earths-surface/

8. Factors affecting solar radiation Varies daily. Is a function of the hour of the day, Varies seasonally (distance to the sun and inclination of the rotation axis) Depends on the atmospherics particles (dust, clouds).

10. In the Artic it is maximum or zero, because part of the year is dark during the whole day and part of the year there is light during the whole day.

11. Factors affecting solar radiation at the ground level Of the order of 400W/m2 @our latitude

12. Night Radiation (1/2) Would be zero, without atmosphere. Assuming the sky as a surface, its temperature would be zero But there is an atmosphere. The radiation hitting the earth surface depends mostly on: Altitude, the cloud cover, Atmospheric dust (pollution)

13. Night Radiation (2/2) Sky Temperature: Desert, clean sky: T sky = -50 C Cloudy Equatorial Regions: T sky = 20 C Typical Sky Temperature: T sky = 0 C Sky temperature can be measured using an “Infrared Thermometer”, but the value will depend on the orientation of the thermometer.

14. Empirical formulae http://physics.stackexchange.com/questions/153839/what-is-the-temperature-of-the-clear-night-sky-from-the-surface-of-earth

15. Examples

16. Grey Surface surrounded by a closed surface

17. Can one produce Ice in a desert without a freezer? Yes : without other forms of heat transfer, the stable temperature of water with a free surface would be the sky temperature… One needs only a clear atmosphere, no wind and adiabatic container walls.