# 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/.

 Month Ra Ra Tmax Tmin T̄ W m-2 MJ m-2 °C °C °C Jan 210 563 9.52 -1.61 3.66 Feb 270 676 16.1 2.8 9.5 Mar 348 932 19.7 6.6 13.3 Apr 418 1083 22.9 10.5 16.7 May 465 1246 24.8 14.2 19.4 Jun 482 1250 31.7 20.8 26.0 Jul 471 1260 33.6 23.3 28.1 Aug 429 1150 32.9 21.7 26.7 Sep 367 950 30.4 18.1 23.6 Oct 290 778 26.6 13.8 19.9 Nov 223 578 20.0 7.6 13.6 Dec 192 514 9.94 -1.97 4.03