from The American Heritage® Dictionary of the English Language, 5th Edition.
- adjective Of, relating to, occurring, or performed during half a day.
- adjective Occurring or coming approximately once every 12 hours, as the tides.
- adjective Of or relating to the arc described by a celestial body between its meridian passage and its points of rising or setting.
from The Century Dictionary.
- Pertaining to or accomplished in half a day (either twelve hours or six hours); continuing half a day.
- In entomology, partly diurnal; flying in twilight; crepuscular; specifically, of or pertaining to the Semidiurna.
from the GNU version of the Collaborative International Dictionary of English.
- adjective Pertaining to, or accomplished in, half a day, or twelve hours; occurring twice every day.
- adjective Pertaining to, or traversed in, six hours, or in half the time between the rising and setting of a heavenly body.
from Wiktionary, Creative Commons Attribution/Share-Alike License.
- adjective Twice
daily(usually of tides).
from Wiktionary, Creative Commons Attribution/Share-Alike License
Most places in the ocean usually experience two high tides and two low tides each day (semidiurnal tide), but some locations experience only one high and one low tide each day (diurnal tide).
Apalachicola Bay is in an area of transition between the semidiurnal tides of southwest Florida and the diurnal tides of northwest Florida.
Also the tides there are diurnal rather than semidiurnal.
Tidal range at spring tides is probably about 1 m, and tides are semidiurnal.
The semidiurnal tidal forces vanish at the Pole and are very small over the entire Arctic Ocean.
As a consequence the semidiurnal portion of the tide wave in these regions is almost wholly derived from the tides in the Atlantic Ocean.
Even though a semidiurnal signature, that is two peaks per day -- one in early morning and one in the late afternoon -- was observed in 2007; in 2008, the variation was diurnal or one peak per day (see figure).