Labrador Current—A North Atlantic Ocean current setting southeastward along the east coasts of Baffin Island, Labrador, and Newfoundland.

lagging of tide—The periodic  retardation in the time of occurrence of high and low water due to changes in the relative positions of the Moon and Sun.

Lagrangian measurement—Observation of a current with a device moving with the current.

Speed = 2T – s + p = 29.455,625,3° per solar hour.

latitude—The angular distance between a terrestrial position and the equator measured northward or southward from the equator along a meridian of longitude.

leap year—A calendar year containing 366 days. According to the present Gregorian calendar, all years with the date-number divisible by 4 are leap years, except century years. The latter are leap years when the date-number is divisible by 400.

level of no motion—A level (or layer) at which it is assumed that an isobaric surface coincides with a geopotential surface. A level (or layer) at which there is no horizontal pressure gradient force.

level surface—See geopotential surface as preferred term.

littoral current—A current in the littoral zone such as a long shore or rip current.

littoral zone—In coastal engineering, the area from the shoreline to just beyond the breaker zone. In biological oceanography, it is that part of the benthic division extending from the high water line out to a depth of about 200 meters. The littoral system is divided into a eulittoral and sublittoral zone, separated at a depth of about 50 meters. Also, frequently used interchangeably with intertidal zone.

local epoch—See kappa (6) and epoch (1).

local time—Time in which noon is defined by the transit of the Sun over the local meridian as distinguished from standard time which is based upon the transit of the Sun over a standard meridian. Local time may be either mean or apparent, according to whether reference is to the mean or actual Sun. Local time was in general use in the United States until 1883, when standard time was adopted. The use of local time in other parts of the world has also been practically abandoned in favor of the more convenient standard time.

log line—A graduated line used to measure the speed of a vessel through the water or to measure the velocity of the current from a vessel at anchor. See current line.

long period constituent—A tidal or tidal current constituent with a period that is independent of the rotation of the Earth but which depends upon the orbital movement of the Moon or the Earth. The principal lunar long period constituents have periods approximating a month and half a month, and the principal solar long period constituents have periods approximating a year and half a year.

long period waves (long waves)—Forced or free waves whose lengths are much longer than the water depth. See tidal wave and tsunami.

longitude—Angular distance along a great circle of reference reckoned from an accepted origin to the projection of any point on that circle. Longitude on the Earth's surface is measured on the Equator east and west of the meridian of Greenwich and may be expressed either in degrees or in hours, the hour being taken as the equivalent of 15° of longitude. Celestial longitude is measured in the ecliptic eastward from the vernal equinox. The mean longitude of a celestial body moving in an orbit is the longitude that would be attained by a point moving uniformly in the circle of reference at the same average angular velocity as that of the body, with the initial position of the point so taken that its longitude would be the same as that of the body at a certain specified position in its orbit. With a common initial point, the mean longitude of a body will be the same in whatever circle it may be reckoned.

longshore current—A current paralleling the shore largely within the surf zone. It is caused by the excess water brought to the zone by the small net mass transport of wind waves. Long shore currents feed into rip currents. See progressive wave.

loop of stationary wave—That portion of the oscillating area where the vertical movement is greatest.

Loop Current—A current setting clockwise in the Gulf of Mexico. It enters through the Yucatan Channel from the Caribbean Sea and leaves through the Straits of Florida.

low tide—Same as low water.

low water (LW)—The minimum height reached by a falling tide. The low water is due to the periodic tidal forces and the effects of meteorological, hydrologic, and/or oceanographic conditions. For tidal datum computational purposes, the minimum height is not considered a low water unless it contains a tidal low water.

low water datum (LWD)—(1) The geopotential elevation (geopotential difference) for each of the Great Lakes and Lake St. Clair and the corresponding sloping surfaces of the St. Marys, St. Clair, Detroit, Niagara, and St. Lawrence Rivers to which are referred the depths shown on the navigational charts and the authorized depths for navigation improvement projects. Elevations of these planes are referred to IGLD 1985 and are Lake Superior—183.2 meters, Lakes Michigan and Huron—176.0 meters, Lake St. Clair—174.4 meters, Lake Erie—173.5 meters, and Lake Ontario—74.2 meters. (2) An approximation of mean low water that has been adopted as a standard reference for a limited area and is retained for an indefinite period regardless of the fact that it may differ slightly from a better determination of mean low water from a subsequent series of observations. Used primarily for river and harbor engineering purposes. Boston low water datum is an example.

low water equinoctial springs—Low water springs near the times of the equinoxes. Expressed in terms of the harmonic constants, it is an elevation depressed below mean sea level by an amount equal to the sum of the amplitudes of the constituents M2, S2, and K2.

low water inequality—See diurnal inequality.

low water interval (LWI)—See lunitidal interval.

low water line—The intersection of the land with the water surface at an elevation of low water.

lower high water (LHW)—The lowest of the high waters of any specified tidal day due to the declinational effects of the Moon and Sun.

lower low water (LLW)—The lowest of the low waters (or single low water) of any specified tidal day due to the declinational effects of the Moon and Sun.

lower low water datum (LLWD)—An approximation of mean lower low water that has been adopted as a standard reference for a limited area and is retained for an indefinite period regardless of the fact that it may differ slightly from a better determination of mean lower low water from a subsequent series of observations. Used primarily for river and harbor engineering purposes. Columbia River lower low water datum is an example.

lowest astronomical tide—As defined by the International Hydrographic Organization, the lowest tide level that can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions.

lunar cycle—An ambiguous expression which has been applied to various cycles associated with the Moon's motion. See Callippic cycle, Metonic cycle, node cycle, and synodical month.

lunar day—The time of the rotation of the Earth with respect to the Moon, or the interval between two successive upper transits of the Moon over the meridian of a place. The mean lunar day is approximately 24.84 solar hours in length, or 1.035 times as great as the mean solar day.

lunar interval —The difference in time between the transit of the Moon over the meridian of Greenwich and a local meridian. The average value of this interval, expressed in hours, is 0.069 L, where L is the local longitude in degrees, positive for west longitude and negative for east. The lunar interval equals the difference between the local and Greenwich interval of a tide or current phase.

lunar month—Same as synodical month.

lunar nodes—The points where the plane of the Moon's orbit intersects the ecliptic. The point where the Moon crosses in going from south to north is called the ascending node and the point where the crossing is from north to south is called the descending node. References are usually made to the ascending node which, for brevity, may be called the node.

lunar tide—That part of the tide on the Earth due solely to the Moon as distinguished from that part due to the Sun.

lunar time—Time based upon the rotation of the Earth relative to the Moon. See lunar day.

lunation—Same as synodical month.

lunicurrent interval—The interval between the Moon's transit (upper or lower) over the local or Greenwich meridian and a specified phase of the tidal current following the transit. Examples are strength of flood interval and strength of ebb interval, which may be abbreviated to flood interval and ebb interval, respectively. The interval is described as local or Greenwich according to whether the reference is to the Moon's transit over the local or Greenwich meridian. When not otherwise specified, the reference is assumed to be local. For a and b markings, see lunitidal interval.

lunisolar tides—Harmonic tidal constituents K1, and K2, which are derived partly from the development of the lunar tide and partly from the solar tide, the constituent speeds being the same in both cases. Also, the lunisolar synodic fortnightly constituent MSf.

lunitidal interval—The interval between the Moon's transit (upper or lower) over the local or Greenwich meridian and the following high or low water. The average of all high water intervals for all phases of the Moon is known as mean high water lunitidal interval and is abbreviated to high water interval (HWI). Similarly, mean low water lunitidal interval is abbreviated to low water interval (LWI). The interval is described as local or Greenwich according to whether the reference is to the transit over the local or Greenwich meridian. When not otherwise specified, the reference is assumed to be local. When there is considerable diurnal inequality in the tide, separate intervals may be obtained for the higher high waters, lower high waters, higher low waters, and lower low waters.

These are designated respectively as higher high water interval (HHWI), lower high water interval (LHWI), higher low water interval (HLWI), and lower low water interval (LLWI). In such cases, and also when the tide is diurnal, it is necessary to distinguish between the upper and lower transit of the Moon with reference to its declination. Intervals referred to the Moon's upper transit at the time of its north declination or the lower transit at the time of south declination are marked a. Intervals referred to the Moon's lower transit at the time of its north declination or to the upper transit at the time of south declination are marked b.