Pertaining to, or accomplished in, half a day, or twelve hours; occurring twice every day.

Pertaining to, or traversed in, six hours, or in half the time between the rising and setting of a heavenly body; as, a semidiurnal arc.

Having a period or cycle of approximately one-half of a TIDAL DAY (12.4 hours). The predominating type of TIDE throughout the world is semidiurnal, with two high waters and two low waters each TIDAL DAY. The TIDAL CURRENT is said to be semidiurnal when there are two flood and two EBB periods each day. See Figure 11.