In many tropical and sub-tropical regions, sun drying remains the preferred method of grain drying, mostly for economic reasons. Traditional sun drying has changed little over the centuries. The grain is spread on mats or paved ground in layers of 5- to 15-cm thickness and is exposed to the ambient conditions. The grain is stirred intermittently, usually is covered at night, and dries adequately in 2 to 4 days.
The fixed costs of sun drying are low (except if a special drying floor has to be constructed). However, sun drying is an unreliable process because it is weather-dependent. Also, the solar radiation changes with the season and the time of day, and the flux density is low.
Notwithstanding its disadvantages, it is possible to produce dried grain of superior quality if sun drying is practiced competently. Of particular importance are the proper selection of the maximum layer thickness and the initial moisture content of the grain, and the recognition that during certain periods of the year (i.e., the wet season) adequate sun drying of grain is not feasible.
Notwithstanding the widespread use of the sun drying of grains in the developing world, few controlled scientific experiments have been conducted. Therefore, no general recommendations valid for every region can be made for the sun drying of grains.
Physics of Sun Drying.The sun drying of grain is affected by the solar radiation, the ambient air temperature, the ambient relative humidity, the wind velocity, the soil temperature, the grain-layer thickness, and the grain type. Simulation of the process  has resulted in a better understanding of sun drying, in particular of the effects of initial moisture content, layer thickness, and season on the drying process (see Figs. 1.21–1.23). The specific information in the three figures pertains to Jakarta (Indonesia), but the trends are valid for other locations.
Figure 1.21. Average moisture content during sun drying of rice at different initial moisture contents.
Figure 1.22. Rice-grain moisture content during sun drying
with grain-bed depths of 5, 8, 10, and 15 cm.
Figure 1.23. Moisture content of rice during sun drying in July and March.