A knowledge of downward atmospheric radiation at the Earth's surface is useful in studies of solar energy, natural air conditioning, and agriculture. This radiation is difficult to measure, but it can be estimated from the surface air temperature, with corrections for (a) radiation from clouds, and (b) variations in the air temperature lapse rate near the surface. It can also be calculated using upper air data and the emissivities of the radiating gases in the atmosphere (mainly water vapor).
Such calculations show that in Thailand the mean downward atmospheric radiation under a clear sky ranges from a minimum of 323W/m2 at night in winter at Chiang Mai to a maximum of 430W/m2 during the day in spring at Bangkok. The effective clear sky temperatures are on the average 12.4°C less than the surface air temperatures.
Under average conditions of cloudiness the effective sky temperatures in Thailand at 7:00am are typically 15°C to 20° lower than the daily maximum air temperatures, and are about 8°C lower than the mean minimum air temperatures at night. Detailed studies suggest that nocturnal cooling could give some natural air conditioning in Chiang Mai, but elsewhere in Thailand nocturnal cooling would not be effective.
Full details of the methods and results of these calculations of downward atmospheric radiation in Thailand are given in the publications listed below.
By R. H. B. Exell, 1998. King Mongkut's University of Technology Thonburi.