h—Rate of change (as of January 1, 1900) in mean longitude of the Sun. h = 0.041,068,64° per solar hour.

half-tide level—Same as mean tide level.

halocline—A layer in which the salinity changes significantly (relative to the layers above and below) with depth.

harmonic analysis—The mathematical process by which the observed tide or tidal current at any place is separated into basic harmonic constituents.

harmonic analyzer—A machine designed for the resolution of a periodic curve into its harmonic constituents. Now performed by electronic digital computer.

harmonic constants—The amplitudes and epochs of the harmonic constituents of the tide or tidal current at any place.

harmonic constituent—See constituent.

harmonic function—In its simplest form, a quantity that varies as the cosine of an angle that increases uniformly with time. It may be expressed by the formula:

y = A cos at

in which y is a function of time (t), A is a constant coefficient, and a is the rate of change in the angle at.

harmonic prediction—Method of predicting tides and tidal currents by combining the harmonic constituents into a single tide curve. The work is usually performed by electronic digital computer.

harmonic reduction—Same as harmonic analysis.

harmonic tide plane—Same as Indian spring low water.

head—The difference in water level at either end of a strait, channel, inlet, etc.

head of tide—The inland or upstream limit of water affected by the tide. For practical application in the tabulation for computation of tidal datums, head of tide is the inland or upstream point where the mean range becomes less than 0.2 foot. Tidal datums (except for mean water level) are not computed beyond head of tide.

high tide—Same as high water.

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

high water, full and change (HWF&C)—Same as establishment of the port.

high water inequality—See diurnal inequality.

high water interval (HWI)—See lunitidal interval.

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

high water mark—A line or mark left upon tide flats, beach, or along shore objects indicating the elevation of the intrusion of high water. The mark may be a line of oil or scum on along shore objects, or a more or less continuous deposit of fine shell or debris on the foreshore or berm. This mark is physical evidence of the general height reached by wave run up at recent high waters. It should not be confused with the mean high water line or mean higher high water line.

higher high water (HHW)—The highest of the high waters (or single high water) of any specified tidal day due to the declinational effects of the Moon and Sun.

higher low water (HLW)—The highest of the low waters of any specified tidal day due to the declinational effects of the Moon and Sun.

Humboldt Current—Same as Peru Current.

hydraulic current—A current in a channel caused by a difference in the surface elevation at the two ends. Such a current may be expected in a strait connecting two bodies of water in which the tides differ in time or range. The current in the East River, New York, connecting Long Island Sound and New York Harbor, is an example.

hydrographic  datum—A datum used for referencing depths of water and the heights of predicted tides or water level observations. Same as chart datum. See datum.