Question:

How does Kiln-Drying differ from Air-Drying lumber?

 Simplified Answer:

Both methods start the same; the lumber ends are sealed and stacked with spacers between each log for air follow. The pile is covered on the top and remains outside until moisture content lessens to 45% +/-, from here the two methods differ. Kiln-Drying is done mechanically by induced heat and fans in a controlled environment for a prolong period of time to reach moisture content around 10 % +/-. This can be achieved in several weeks' time-based on-board thickness. On the other hand, Air-Drying lumber method is dependent upon natural air flow and outdoor temperature. This can take more than a year to reach (MC) level for furniture building. In Wisconsin I can achieve (MC) level of 12 % +/- at 70 degrees for final drying. 

 This difference comes down to drying time and storage space between the two methods. Air-Drying method takes years and storage space for drying lumber supply that is ready for wood working. Both methods work but have to be done correctly or you have a lot of firewood.

  I use the Air-Drying method for the reason the lumber process remained environmentally friendly which is in line with salvaging trees and turning them into a renewable resource.