- 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.
- 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.
- 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.
- 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 deg.
- 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
at about 30 to 40 deg. N and S latitude.
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.
- 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.
- 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.
- 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.
- 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.
- 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 ().
- 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.
