Hn-Hz

 

HNLC

In biological oceanography, an abbreviation for high nutrient-low chlorophyll regions.

 

HNRS

Abbreviation for HOMS National Reference Center, the name given to national centers for the HOMS system. More information can be found at the HNRS Web page.

 

HOC

Acronym for hydrophobic organic compound.

 

Holdridge system

A bioclimatic classification scheme that is applied on several hierarchical levels, the first of which involves relating three climatic variables to vegetation types. The variables are ABT, APETR, and APPT. These variables, together with a frost line, intersect to form 36 hexagons within a triangular coordinate system, each of which defines a bioclimate named for a vegetation type. See Holdridge (1947) and Prentice (1990).

The Holdridge (1947) system describes potential vegetation as a function of biotemperature and humidity provinces. This allows the creation of large-scale maps of potential vegetation distribution from global-scale mean climatic data as in Emanuel et al. (1985).

 

Holocene

The most recent geological epoch, lasting from 8000 Ya - present. Also the latest epoch in the Quaternary period.

 

holothurian

A member of the class Holothuroidea.

 

Holothuroidea

A class of marine invertebrates in the phylum Echinodermata. These are elongate and bilaterally symmetrical animals with a leathery skin and a diffuse skeleton of calcite spicules, commonly called sea cucumbers. Holothurians are mainly benthic with only those of the order Pelagiothurida being planktonic. There are 650 or so species of these, with some living in abyssal regions. See the Introduction to the Holothuroidea at the U.C. Museum of Paleontology.

 

homeostasis

Active control to maintain relatively constant conditions.

 

homogeneous nucleation

See GPC.

 

homosphere

One of two atmospheric layers in a scheme based on the homogeneity of composition. This is the atmospheric layer consisting of a homogeneous mixture of nitrogen, oxygen and argon along with variable concentrations of trace gases. The upper boundary of the homosphere is located at around 80-90 km, above which sits the heterosphere, the other layer in this two-layer scheme. This is also called the neutral atmosphere.

 

HOMS

Acronym for the Hydrological Operational Multipurpose System, a system developed by the HWRP of the WMO for the transfer of technology in operational hydrology. This technology is usually in the form of descriptions of hydrological instruments, technical manuals or computer programs. See the HOMS Web page for more information.

 

HOPE

Abbreviation for a primitive equation ocean circulation model developed and used at the DKRZ.

 

Hopen-Bjornoya Current

See Pfirman et al. (1994).

 

Hopf bifurcation

More later.

 

HOT

Acronym for Hawaii Ocean Time-series, a program for making repeated observations of the hydrography, chemistry and biology at a station north of Hawaii since October 1988. The objective of this research is to provide a comprehensive description of the ocean at a site representative of the central North Pacific Ocean. See the HOT Web site.

 

Hough functions

The eigenfunctions of a linearized form of the governing equations of motion on a sphere, i.e. Laplace's tidal equations, as first discovered by Hough (1898). Each Hough mode is a function of latitude and longitude and has three components: (1) a zonal (eastward) wind component, (2) a meridional (northward) wind component, and (3) a geopotential component. A distinct horizontal scale and frequency is associated with each mode, and the modes are orthogonal over the sphere in the continuous case. They are sometimes divided into two classes: (1) low-frequency Rossby-Hough modes that tend to satisfy the geostrophic relation and (2) higher frequency Hough modes that correspond to inertia-gravity waves. They can also be thought of as a generalized Fourier series in which the basis functions are the normal modes of a resting atmosphere. See Daley (1991).

 

hour angle

The angle that the hour circle of a heavenly body makes with the observer's meridian at the celestial pole. It is measured positively westwards from the meridian from 0 to 24 hr, and it denoted in hours, minutes and seconds.

 

hour circle

The great circle passing through the celestial poles and a heavenly body, cutting the celestial equator at 90 .

 

Hoxton, Walter

See Peterson et al. (1996).

 

HPC

Abbreviation for Hydrological Processes and Climate, an EOS project to provide a description and a better understanding of the physical processes that contribute to the maintenance and variability of the global hydrologic and energy cycles. The three main objectives involve understanding the physical mechanisms of atmospheric hydrologicl processes and their interaction with the dynamics and radiative properties of the atmosphere, the role of hydrologic processes in large-scale ocean/atmosphere/land interaction leading to natural fluctuations of the global climate system over a variety of time scales, and the role of land surface processes in the global hydrologic cycle. See the HPC Web site.

 

HPCC

Acronym for High Performance Computing and Communications, a program to extend U.S. techological leadership in high performance computing and computer communications and provide wide dissemination and application of the technologies to speed the pace of innovation and to improve the national economic competitiveness, national security, education, health care, and the global environment. It has components in several government agencies, including ARPA, EPA, NASA, NOAA, and NSF.

 

HPLC

Abbreviation for High Pressure Liquid Chromatography.

 

horse latitudes

The belts of variable, light winds and fine weather associated with the subtropical anticyclones The name originated with the historical sailing practice of throwing the horses being transported to America or the West Indies overboard when these latitudes were reached and the light winds caused the voyage to be overly extended.

 

HRC

Abbreviation for Highly Reflective Clouds. The identification of this type of cloud is used to infer decadal variability of precipitation over the oceans from space-based instruments.

 

HRDI

Abbreviation for High Resolution Doppler Imager.

 

HRIR

Abbreviation for High Resolution Infrared Radiometer.

 

HRIS

Abbreviation for High Resolution Imaging Spectrometer.

 

HRTWN

Abbreviation for Hawaii Regional Tsunami Warning Network.

 

HSSTD

Abbreviation for Historical Sea Surface Temperature Dataset.

 

Huggins bands

Spectral bands in which solar radiation is weakly absorbed by ozone in the atmosphere. These bands exist between 3000 and 3600 Angstroms. These have more structure than the similar Hartley bands. See Liou (1992).

 

Humboldt Current

See Peru Current.

 

humidity mixing ratio

The ratio of the mass of water vapor in a sample of moist air to the mass of dry air with which it is associated.

 

humidity provinces

A term used in the Holdridge scheme for describing potential vegetation as a function of temperature and precipitation. The humidity provinces are calculated as the ratio of potential evapotranspiration to precipitation.

 

HURL

Acronym for Hawaii Underwater Research Laboratory.

 

hurricane

More later. See Gray (1979).

 

Huygen's principle

The observation that every point along a wave front acts like a generator of new spherical secondary waves such that the wave front position after some time interval is located at the tangent to all of the new waves.

 

HWRP

Abbreviation for the Hydrology and Water Resources Program, a WMO program to apply hydrology to meet the needs for sustainable development and use of water and related resources, for the mitigation of water-related disasters, and to effective environmental management at national and international levels. More information can be found at the HWRP Web site.

 

hydrochlorofluorocarbon (HCFC)

A class of chemicals being used to replace CFCs since they deplete stratospheric ozone to a much lesser extent than CFCs. These have ODPs ranging from 0.01 to 0.1. The production of these chemicals is being gradually phased out, with those with the highest ODPs going first. These will be replaced by HFCs or some other types of chemicals.

 

hydrofluorocarbon (HFC)

A class of chemicals being used to replace CFCs. They do not contain chlorine or bromine and therefore do not deplete ozone in the stratosphere. These all have an ODP value of zero but can have high GWP values.

 

hydrodynamic modulation

A type of modulation of backscattered ripple waves measured by SAR that arises through variations in the energy of the Bragg backscattering ripples causes by hydrodynamic interactions betwen the short ripple waves and the longer waves. See Komen et al. (1996).

 

hydrographic theorem

An expression, developed using the continuity equation, for water budget estimates in two-layer channels with different salinity values in each layer. It was first developed by Knudsen. See Dietrich (1963).

 

hydrography

The study of the physical features of water bodies like oceans and lakes (in analogy to geography being the study of the physical features on land). Oceanic features of interest include the location and spatial extent of water masses as identified by their characteristic properties such as salinity, temperature and micronutrient concentrations. Early systematic attempts at applying hydrography to the oceans were the core layer method and the isopycnal method in the 1920s and 1930s by Wust, Iselin, Montgomery, Defant and others, and variants of these methods are still used today to provide a first-order general classification of the waters of the world ocean. Much care, however, should be taken when attempting to use the results of these mostly static classification methods to understand the dynamical aspects of the ocean (although the latter is much more closely related to dynamical fields). This is best exemplified by the classic apothegm "the hydrographer's ocean is much smoother than the dynamicist's ocean".

 

hydrological cycle

More later.

 

hydrology

More later.

 

hydrometeor

Any condensed water particle in the atmosphere of size much larger than individual water molecules, e.g. fog, cloud, some hazes, rain and snow.

 

hydrophytes

Plants adapted to live in open fresh water or permanently waterlogged soils.

 

hydrosphere

This consists of all water in the liquid phase distributed on the Earth, including the oceans, interior seas, lakes, rivers, and subterranean waters.

 

hydrostatic approximation

In this strictest sense, this involves replacing the static pressure in the hydrostatic equation with the thermodynamic pressure, with the equation being exact in a fluid at rest. This approximation changes the determination of the vertical velocity w from a prognostic to a diagnostic process, with the w thus determined being precisely that needed to maintain hydrostatic equilibrium.

 

hydrostatic equation

An equation relating the vertical pressure gradient to the vertical distribution of density in a fluid (atmosphere or ocean) at rest. It is given by

where is the static pressure, z the vertical coodinate, g the gravitational acceleration, and the density.

 

hydrostatic pressure

See static pressure.

 

HydroWire

A site on the Web that functions as an electronic newsletter and communication link for the aquatic sciences community. See the HydroWire web site.

 

hygrophytes

Plants which exercise little or no control over water loss.

 

hypermorphosis

In heterochrony, this is a type of peramorphosis that occurs by extending the juvenile growth period by a delay in the onset of sexual maturation. Early juvenile development proceeds at the same rate as in the ancestor.

 

hypolimnion

The layer of water below the thermocline in a fresh water lake, as opposed to the epilimnion.

 

hypsometric curve

A plot of the percentage of elevation and depth distribution on the continents and oceans.

 

hypsometric equation

In meteorology, a relation stating that the thickness of an atmospheric layer bounded by two isobaric surfaces is proportional to the mean temperature of that layer and the pressure change across it, i.e.

where is the average virtual temperature between heights and , a is equal to (where is the gas constant for dry air), and and are the pressures at the two levels. See Salby (1992) and Stull (1995).

 

hypsometry

The study of the elevation and depth distribution on the continents and oceans.

 

hysteresis

More later.

 

hythergraphs

Plots of precipitation and temperature or humidity and temperature, usually by month.