12.6 Problem Set
- 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?
- 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?
- 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?
- 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 |