12.6 Problem Set

1. Under daylight conditions, the human eye exhibits maximum sensitivity to light at a wavelength of 555 nm. At what temperature (K) would a blackbody emit light with a peak intensity at this wavelength?
2. The sun has a radius of approximately 6.96 x 108 m and an average surface temperature of 5,780 K. Assume it has an emissivity of 0.990. What is the total radiant flux from the sun in W m^-2 and in W?
3. Imagine the Earth is on the surface of a sphere whose radius is equal to the sun-Earth distance, 1.50 x 1011 m. If the total radiant flux from the sun (W) calculated above is evenly distributed over the surface of that imaginary sphere, what is the average flux in W m^-2?
4. Use the Hargreaves method to estimate monthly ET₀ for Arcadia, Oklahoma, USA in the year 2016, given the data below. Use a spreadsheet and report only the annual total (mm). Assuming a latent heat of vaporization of 2.465 MJ kg^-1, one MJ m^-2 is equivalent to 0.4057 mm of evaporated water.

Table 12‑1. Extraterrestrial radiation (Ra), mean temperature ( ), mean of daily maximum temperature (Tmax), and mean of daily minimum temperature (Tmin) near Arcadia, Oklahoma, USA in 2016. Ra estimates obtained from https://www.nrel.gov/midc/solpos/solpos.html and temperatures obtained from the Oklahoma Mesonet, Spencer site, http://www.mesonet.org/.