To enable bulk handling and storage of seed, they are dried to reduce their
moisture content. To dry seed, moisture moves from the seed into the
surrounding air. Moisture in the air is measured as relative humidity (RH,
the percentage of the total moisture which the air could hold at that
temperature). Seed are hygroscopic; i.e., they can lose moisture to the
surrounding air if the air is dryer than the RH which is in equilibrium with the moisture content of the surface of the seed. At a certain level, the seed
moisture content will be in equilibrium with the air’s RH, so there is no
moisture loss or gain in the seed. Conversely, seed will gain moisture if the
surrounding air is damper than the equilibrium level with moisture content
of the seed.