Natural air grain drying is governed by the principle of Equilibrium Moisture Content (EMC): the moisture content that grain will attain if exposed to air of a specific relative humidity and temperature for a long enough duration.
Each grain will have a different EMC based on the moisture of the outside air. For example the outside air today could have the potential to add 10 per cent moisture to wheat, but only seven per cent to canola. From this a range can be determined at which optimum drying will occur.
If the EMC of the air delivered to the grain, including fan and heater warming, is lower than the grain EMC, moisture will be released from the grain to the surrounding air and drying will occur. If the EMC of the air delivered to the grain is greater than the grain EMC, the grain will absorb moisture, and re-hydrating will occur. †