p—Rate of change (as of January 1, 1900) in mean longitude of lunar perigee. p = 0.004,641,83° per solar hour.

p1—Rate of change (as of January 1, 1900) in mean longitude of solar perigee. p1 = 0.000,001,96° per solar hour.

P1—Solar diurnal constituent. See K1. Speed = T – h = 14.958,931,4° per solar hour.

parallax—In tidal work, the term refers to horizontal parallax, which is the angle formed at the center of a celestial body between a line to the center of the Earth and a line tangent to the Earth's surface. Since the sine of a small angle is approximately equal to the angle itself in radians, it is usually taken in tidal work simply as the ratio of the mean radius of the Earth to the distance of the tide-producing body. Since the parallax is a function of the distance of a celestial body, the term is applied to tidal inequalities arising from the changing distance of the tide-producing body.

parallax inequality—The variation in the range of tide or in the speed of a tidal current due to changes in the distance of the Moon from the Earth. The range of tide and speed of the current tend alternately to increase and decrease as the Moon approaches its perigee and apogee, respectively, the complete cycle being the anomalistic month. There is a similar but relatively unimportant inequality due to the Sun, the cycle being the anomalistic year. The parallax has little direct effect upon the lunitidal intervals but tends to modify the phase effect. When the Moon is in perigee, the priming and lagging of the tide due to the phase is diminished and when in apogee the priming and lagging is increased.

parallax reduction—A processing of observed high and low waters to obtain quantities depending upon changes in the distance of the Moon, such as perigean and apogean ranges.

parallel plate intake—Intake of a stilling or protective well with two parallel plates attached below. The plates are typically three times the diameter of the well and are spaced three inches apart. The plates are used to minimize current-induced draw-down (Bernoulli effect) error in water level measurements.

 pelorus—An instrument formerly used on a  vessel in connection with a current line and current pole to obtain the set of the current. In its simplest form, it was a disk about 8 inches in diameter and graduated clockwise for every 5° or 10° . It was mounted rigidly on the vessel, usually with the 0° mark forward and the diameter through this mark parallel with the keel. Bearings were then related to the vessel's compass and converted to true.

perigean tides or tidal currents—Tides of increased range or tidal currents of increased speed occurring monthly as the result of the Moon being in perigee. The perigean range (Pn) of tide is the average range occurring at the time of perigean tides and is most conveniently computed from the harmonic constants. It is larger than the mean range where the type of tide is either semidiurnal or mixed, and is of no practical significance where the type of tide is predominantly diurnal.

perigee—The point in the orbit of the Moon or a man-made satellite nearest to the Earth. The point in the orbit of a satellite nearest to its companion body.

perihelion—The point in the orbit of the Earth (or other planet, etc.) nearest to the Sun.

period—Interval required for the completion of a recurring event, such as the revolution of a celestial body or the time between two consecutive like phases of the tide or tidal current. A period may be expressed in angular measure as 360°. The word also is used to express any specified duration of time.

permanent current—A current that runs continuously and is independent of tides and other temporary causes. Permanent currents include the general surface circulation of  the oceans.

Peru Current—A South Pacific Ocean current setting northward along the west coast of South America. It has sometimes been called the Humboldt Current because an early record of its temperature was taken by the German scientist Alexander von Humboldt in 1802. It has also been called the Peruvian or Chilean Current. The name Corriente de Peru was adopted by a resolution of the Ibero-American Oceanographic Conference at its Madrid-Malaga meeting in April 1935.

phase—(1) Any recurring aspect of a periodic phenomenon, such as new Moon, high water, flood strength, etc. (2) A particular instant of a periodic function expressed in angular measure and reckoned from the time of its maximum value, the entire period of the function being 360°. The maximum and minimum of a harmonic constituent have phase values of 0° and 180°, respectively.

phase inequality—Variations in the tides or tidal currents due to changes in the phase of the Moon. At the times of new and full Moon the tide-producing forces of the Moon and Sun act in conjunction, causing the range of tide and speed of the tidal current to be greater than the average, the tides at these times being known as spring tides. At the times of the quadratures of the Moon these forces are opposed to each other, causing neap tides with diminished range and current speed.

phase lag—Same as epoch (1).

phase reduction—A processing of observed high and low waters to obtain quantities depending upon the phase of the Moon, such as the spring and neap ranges of tide. At a former time this process was known as second reduction. Also applicable to tidal currents.

 pororoca—Brazilian for tidal bore.

PORTS—Physical Oceanographic Real Time System. A national system of current, water level, and other oceanographical and meteorological sensors telemetering data in real- ime to central locations for storage, processing, and dissemination. Available to pilots, mariners, the U.S. Coast Guard, and other marine interests in voice or digital form. First introduced in Tampa Bay.

potential, tide-producing—Tendency for particles on the Earth to change their positions as a result of the gravitational interactions between the Sun, Moon, and Earth. Although gravitational attraction varies inversely as the square of the distance of the tide producing body, the resulting potential varies inversely as the cube of the distance.

predicting machine—See tide predicting machine.

pressure gauge—A water level gauge that is operated by the change in pressure at the bottom of a body of water due to the rise and fall of the water level. See gas purged pressure gauge.

pressure gradient force, horizontal—The horizontal component of the product of the specific volume and the rate of decrease in pressure with distance.

pressure sensor—A pressure transducer sensing device for water level measurement. A relative transducer is vented to the atmosphere and pressure readings are made relative to atmospheric pressure. An absolute transducer measures the pressure at its location. The readings are then corrected for barometric pressure taken at the surface.

primary control tide station—A tide station at which continuous observations have been made over a minimum of 19 years. Its purpose is to provide data for computing accepted values of the harmonic and nonharmonic constants essential to tide predictions and to the determination of tidal datums for charting and for coastal and marine boundaries. The data series from this station serves as a primary control for the reduction of relatively short series from subordinate tide stations through the method of comparison of simultaneous observations and for monitoring long-period sea level trends and variations. See tide station, secondary control tide station, tertiary tide station, and subordinate tide station (1).

primary tidal bench mark—See bench mark.

prime meridian—The meridian of longitude which passes through the original site of the Royal Observatory in Greenwich, England and used as the origin of longitude. Also known as the Greenwich Meridian.

priming of tide—The periodic acceleration in the time of occurrence of high and low waters due to changes in the relative positions of the Sun and Moon.

progressive wave—A wave that advances in distance along the sea surface or at some intermediate depth. Although the wave form itself travels significant distances, the water particles that make up the wave merely describe circular (in relatively deep water) or elliptical (in relatively shallow water) orbits. With high, steep, wind waves, a small overlap in the orbital motion becomes significant. This overlapping gives rise to a small net mass transport. See long shore current and rip current. Progressive waves can be internal, traveling along a sharp density discontinuity, such as the thermocline, or in a layer of gradually changing density (vertically).

protective well—A vertical pipe with a relatively large opening (intake) in the bottom. It is used with the air acoustic ranging sensor and electronic processing (filtering) technique to minimize the nonlinear characteristics of the stilling well. Its purpose is also to shield the sensing element from physical damage and harsh environment. Unlike a stilling well, damping of high frequency waves is not a critical requirement. See stilling well.

pycnocline—A layer in which the density increases significantly (relative to the layers above and below) with depth.