Outgoing Radiation Under Clear-Sky Conditions

Figure 2, produced by the Earth Radiation Budget Experiment (ERBE) program of the National Oceanic and Atmospheric Administration (NOAA) (Harrison et al, 1988), shows a map of outgoing longwave radiation, in Wm^-2, for the month of April 1985 under clear-sky conditions. Regions colored in red and purple define regions of high amounts of infrared radiation leaving the earth, and green and blue colors denote low IR values. From the Stefan-Boltzmann equation, we can also say that the radiating regions colored red and purple are warmer than those colored green and blue. As expected, the tropical and subtropical regions have the highest outgoing radiation (and temperature) and polar regions have lowest values. Very careful inspection, however, will reveal that some areas in the equatorial regions over land have substantially lower temperatures than adjacent subtropical areas to the north or south. Can you explain this?

Figure 3, also from Harrison et al, 1988, depicts the diurnal range, that is the day-to-night changes, in amount of radiated energy in Wm^-2 under cloud-free skies for April 1985. Note that the range of values is much lower than for the previous photograph. Regions having the largest diurnal variation are generally deserts in the subtropical zones. Having few clouds and low humidity (i.e., very little water vapor for greenhouse gas absorption) in the overlying atmosphere, these regions radiate to outer space directly from their surfaces, which range in temperature from over 60⁰C (333K) during the day to near 10⁰C (283 K) at night. You might use these values in the Stefan-Boltzmann equation to calculate the difference in outgoing radiation for these regions and compare your results with the values of about 60 Wm^-2 given in the photograph. Note that most ocean regions have very low changes in outgoing radiation (and, therefore, temperature) from day to night.

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