This weather glossary contains information on more than 2000 terms,
phrases and abbreviations used by the National Weather Service (NWS)...
the government agency who makes weather forecasts, and issues weather advisories, watches, and warnings, for the United States, and its
territories.

Many of these terms and abbreviations are used by NWS forecasters to communicate between each other and have been in use for many years and
before many NWS products were directly available to the public. It is the purpose of this glossary to aid you in better understanding NWS products.

***

Ice Age
A time of widespread glaciation.

Ice Boom
In hydrologic terms, a floating structure designed to retain ice.

Ice Bridge
In hydrologic terms, a continuous ice cover of limited size extending from shore to shore like a bridge.

Ice Crystals
A barely visible crystalline form of ice that has the shape of needles,
columns or plates. Ice crystals are so small that they seem to be
suspended in air. Ice crystals occur at very low temperatures in a stable atmosphere.

Ice Fog
Same as Freezing Fog; a suspension of numerous minute ice crystals in the
air, or water droplets at temperatures below 0° Celsius, based at the
Earth's surface, which reduces horizontal visibility; also called ice fog.

Ice Gorge
In hydrologic terms, the gorge or opening left in a jam after it has
broken.

Ice Jam
In hydrologic terms, a stationary accumulation that restricts or blocks streamflow.

Ice Nucleus
Any particle that serves as a nucleus in the formation of ice crystals in
the atmosphere.

Ice Pellets
(abbrev. IP) Same as Sleet; defined as pellets of ice composed of frozen
or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard
surfaces. Heavy sleet is a relatively rare event defined as an
accumulation of ice pellets covering the ground to a depth of 1" or more.

Ice Push
In hydrologic terms, compression of an ice cover particularly at the front
of a moving section of ice cover.

Ice Run
In hydrologic terms, flow of ice in a river. An ice run may be light or
heavy, and may consist of frazil, anchor, slush, or sheet ice.

Ice Shove
In hydrologic terms, on-shore ice push caused by wind, and currents,
changes in temperature, etcetera.

Ice Storm
An ice storm is used to describe occasions when damaging accumulations of
ice are expected during freezing rain situations. Significant
accumulations of ice pull down trees and utility lines resulting in loss
of power and communication. These accumulations of ice make walking and
driving extremely dangerous. Significant ice accumulations are usually accumulations of 1" or greater.

Ice Storm Warning
This product is issued by the National Weather Service when freezing rain produces a significant and possibly damaging accumulation of ice. The
criteria for this warning varies from state to state, but typically will
be issued any time more than 1/4" of ice is expected to accumulate in an
area.

Ice Twitch
In hydrologic terms, downstream movement of a small section of an ice
cover. Ice twitches occur suddenly and often appear successively.

Iceberg
A piece of a glacier which has broken off and is floating in the sea.

Icelandic Low
A semi-permanent, subpolar area of low pressure in the North Atlantic
Ocean. Because of its broad area and range of central pressure, it is
an area where migratory lows tend to slow down and deepen. It is
strongest during a Northern Hemisphere winter and early spring, centered
over Iceland and southern Greenland, and is the dominate weather feature
in the area. During the summer, it is weaker, less intense, and might
divide into two parts, one west of Iceland, the other over the Davis
Strait between Greenland and Baffin Island. Then the Azores or Bermuda
High becomes the dominate weather feature in the North Atlantic.

Icing
A coating of ice on a solid object.

Ideal Gas Laws
The thermodynamic laws applying to perfect gases.

IFR
Instrument Flight Rules

Impermeable
Material that does not permit fluids to pass through it.

Impervious
In hydrologic terms, the ability to repel water, or not let water
infiltrate.

IMPL
Impulse- Alternate term for Upper Level System and Shortwave; a general
term for any large-scale or mesoscale disturbance capable of producing
upward motion (lift) in the middle or upper parts of the atmosphere.

Import
In hydrologic terms, water piped or channeled into an area.

IMPT
Important

Impulse
(abbrev. IMPL) Alternate term for Upper Level System and Shortwave; a
general term for any large-scale or mesoscale disturbance capable of
producing upward motion (lift) in the middle or upper parts of the
atmosphere.

in Hg
Inches of Mercury

In-Cloud Lightning
(abbrev. IC) Lightning that takes place within the cloud.

Inactive Storage Capacity
In hydrologic terms, the portion of capacity below which the reservoir
is not normally drawn, and which is provided for sedimentation,
recreation, fish and wildlife, aesthetic reasons, or for the creation of
a minimum controlled operational or power head in compliance with
operating agreements or restrictions.

Inch-Degrees
The product of rainfall (in inches) multiplied by the temperature (in
degrees Fahrenheit) above freezing. Used as a measure of the snowmelting capacity of rainfall.

Inches of Mercury
(or in Hg) Unit of atmospheric pressure used in the United States. The
name comes from the use of mercurial barometers which equate the height
of a column of mercury with air pressure. One inch of mercury is
equivalent to 33.86 millibars or 25.40 millimeters. See barometric
pressure. First divised in 1644 by Evangelista Torricelli (1608-1647),
an Italian physicist and mathematician, to explain the fundamental
principles of hydromechanics.

To convert millibars (mb) to inches of mercury (in Hg), divide the
millibar reading by 33.86: in Hg = mb / 33.86

Inches of Runoff
In hydrologic terms, the volume of water from runoff of a given depth over
the entire drainage.

INCR
Increase

INDC
Indicate

Index of Wetness
The ratio of precipitation for a given year over the mean annual
precipitation.

Indian Summer
An unseasonably warm period near the middle of autumn, usually following
a substantial period of cool weather.

Indirect Hit
Generally refers to locations that do not experience a direct hit from a tropical cyclone, but do experience hurricane force winds (either
sustained or gusts) or tides of at least 4 feet above normal.

Industrial Consumption
The quantity of water consumed in a municipality or district for
mechanical, trade, and manufacturing purposes, in a given period,
generally one day. The per capita use is generally based on the total population of the locality, municipality, or district.

Infiltration
In hydrologic terms, movement of water through the soil surface into
the soil.

Infiltration Capacity
In hydrologic terms, the maximum rate at which water can enter the soil
at a particular point under a given set of conditions.

Infiltration Index
In hydrologic terms, an average rate of infiltration, in inches per
hour, equal to the average rate of rainfall such as that the volume
of rainfall at greater rates equals the total direct runoff.

Infiltration Rate
In hydrologic terms,

(1) The rate at which infiltration takes place expressed in depth of
water per unit time, usually in inches per hour.

(2) The rate, usually expressed in cubic feet per second, or million
gallons per day per mile of waterway, at which ground water enters an infiltration ditch or gallery, drain, sewer, or other underground conduit.

Inflow Bands
Bands of low clouds, arranged parallel to the low-level winds and moving
into or toward a thunderstorm. They may indicate the strength of the
inflow of moist air into the storm, and, hence, its potential severity. Spotters should be especially wary of inflow bands that are curved in a
manner suggesting cyclonic rotation; this pattern may indicate the
presence of a mesocyclone.

Inflow Jets
Local jets of air near the ground flowing inward toward the base of a
tornado.

Inflow Notch
A radar signature characterized by an indentation in the reflectivity
pattern on the inflow side of the storm. The indentation often is
V-shaped, but this term should not be confused with V-notch. Supercell thunderstorms often exhibit inflow notches, usually in the right
quadrant of a classic supercell, but sometimes in the eastern part of
an HP storm or in the rear part of a storm (rear inflow notch).

Inflow Stinger
A beaver tail cloud with a stinger-like shape.

Influent Seepage
In hydrologic terms, movement of gravity water in the zone of aeration
from the ground surface toward the water table.

Influent Stream
In hydrologic terms, any watercourse in which all, or a portion of the
surface water flows back into the ground namely the, vadose zone, or
zone of aeration.

Infrared Satellite Imagery
This satellite imagery senses surface and cloud top temperatures by
measuring the wavelength of electromagnetic radiation emitted from
these objects. This energy is called "infrared". High clouds are very
cold, so they appear white. Mid-level clouds are somewhat warmer, so
they will be a light gray shade. Low cloud are warmer still, so they
appear as a dark shade of gray or black. Often, low clouds are the same temperature as the surrounding terrain and cannot be distinguished at
all. The satellite picks up this infrared energy between 10.5 and 12.6 micrometer (um) channels.

Initial Detention
In hydrologic terms, the volume of water on the ground, either in
depressions or in transit, at the time active runoff begins.

Inland freshwater wetlands
In hydrologic terms, swamps, marshes, and bogs found inland beyond the
coastal saltwater wetlands.

Inland Hurricane Warning
Issued for interior counties that sustained winds of 74 mph or greater associated with a hurricane are expected within 24 hours.

Inland Hurricane Watch
Issued for interior counties when sustained winds of 74 mph or greater associated with a hurricane are possible within 36 hours.

Inland Tropical Storm Warning
Issued for interior counties when sustained winds of 39 to 73 mph
associated with a tropical storm are expected within 24 hours.

Inland Tropical Storm Watch
Issued for interior counties when sustained winds of 39 to 73 mph
associated with a tropical storm are possible within 36 hours.

INLD
Inland

Insolation
Incoming solar radiation. Solar heating; sunshine.

Instability
(abbrev. INSTBY)- The tendency for air parcels to accelerate when they
are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for
severe weather - the greater the instability, the greater the potential
for severe thunderstorms.

INSTBY
Instability- The tendency for air parcels to accelerate when they are
displaced from their original position; especially, the tendency to
accelerate upward after being lifted. Instability is a prerequisite for
severe weather - the greater the instability, the greater the potential
for severe thunderstorms.

Instrument Flight Rules
Refers to the general weather conditions pilots can expect at the
surface and applies to the weather situations at an airport during
which a pilot must use instruments to assist take off and landing. IFR conditions for fixed wing aircraft means the minimum cloud ceiling is
greater than 500 feet and less than 1,000 feet and/or visibility is
greater than 1 mile and less than 3 miles.

Instrument Shelter
A boxlike structure designed to protect temperature measuring
instruments from exposure to direct sunshine, precipitation, and
condensation, while at the same time time providing adequate
ventilation.

Intangible Flood Damage
In hydrologic terms, estimates of the damage done by disruption of
business, danger to health, shock, and loss of life and in general all
costs not directly measurable which require a large element of judgment
for estimating.

Interbasin Transfer
In hydrologic terms, the physical transfer of water from one watershed
to another.

Interception
In hydrologic terms, the process by which precipitation is caught and
held by foliage, twigs, and branches of trees, shrubs, and other
vegetation, and lost by evaporation, never reaching the surface of the
ground. Interception equals the precipitation on the vegetation minus streamflow and through fall.

Interception Storage Requirements
In hydrologic terms, water caught by plants at the onset of a rainstorm.
This must be met before rainfall reaches the ground.

Interflow
In hydrologic terms, the lateral motion of water through the upper
layers until it enters a stream channel. This usually takes longer to
reach stream channels than runoff. This also called subsurface storm
flow.

Intermediate Synoptic Times
The times of 0300, 0900, 1500, and 2100 UTC.

Intermittent Stream
In hydrological terms, a stream that flows periodically.

International Date Line
The line of longitude located at 180 degrees East or West (with a few
local deviations) where the date changes by a day. West of the line it
is one day later than east of the line.

Interplanetary Magnetic Field
(abbrev. IMF) In solar-terrestrial terms, the magnetic field carried
with the solar wind.

Interpolate
To estimate a value within an interval between two known values. This
technique is sometimes used with computer models for locations in
between the model's "gridpoints."

Intertropical Convergence Zone
(ITCZ) The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms
near the equator.

Intraseasonal Oscillation
Oscillation with variability on a timescale less than a season. One
example is the Madden-Julian Oscillation.

INTS
Intense

INTSFY
Intensify

INTST
Intensity

Inversion
(abbrev. INVRN) Generally, a departure from the usual increase or
decrease in an atmospheric property with altitude. Specifically it
almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase
occurs. An inversion is present in the lower part of a cap.

INVOF
In the vicinity of

INVRN
Inversion- Generally, a departure from the usual increase or decrease
in an atmospheric property with altitude. Specifically it almost always
refers to a temperature inversion, i.e., an increase in temperature
with height, or to the layer within which such an increase occurs. An
inversion is present in the lower part of a cap.

Ionosphere
A complex atmospheric zone of ionized gases that extends between 50 and
400 miles (80 to 640 kilometers) above the earth's surface. It is
located between the mesosphere and the exosphere and is included as part
of the thermosphere.

Ionospheric Storm
A disturbance in the F region of the ionosphere, which occurs in
connection with geomagnetic activity.

IPV
Improve

IR
Infrared Satellite Imagery

Iridescence
Brilliant spots or borders of colors in clouds, usually red and green,
caused by diffraction of light by small cloud particles. The phenomenon
is usually observed in thin cirrus clouds within about 30° of the sun
and is characterized by bands of color in the cloud that contour the
cloud edges.

Iridescent Clouds
Clouds that exhibit brilliant bright spots, bands, or borders of colors, usually red and green, observed up to about 30 degrees from the sun. The coloration is due to the diffraction with small cloud particles
producing the effect. It is usually seen in thin cirrostratus,
cirrocumulus, and altocumulus clouds.

Irrigation
In hydrologic terms, the controlled application of water to arable lands
to supply water requirements not satisfied by rainfall.

Irrigation Requirement
In hydrologic terms, the quantity of water, exclusive of precipitation,
that is required for crop production. It includes surface evaporation
and other economically unavoidable wastes.

Isallobar
A line of equal change in atmospheric pressure during a specified time
period.

Isentropic Analysis
A way in the forecaster can look at the atmosphere in 3-dimensions
instead of looking at constant pressure surfaces (such as the 850 mb,
700 mb, 500 mb, etc.) which are in 2-dimensions. In this analysis method,
the forecaster looks at constant potential temperature (the temperature
that it would take if we compressed or expanded it adiabatically to the pressure of 1000 mb) surfaces. Air parcels move up and down these
surfaces; therefore, the forecaster can see where the moisture is located
and how much moisture is available.

Isentropic Lift
Lifting of air that is traveling along an upward-sloping isentropic
surface.

Isentropic lift often is referred to erroneously as overrunning, but
more accurately describes the physical process by which the lifting
occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include
elevated convection in the form of embedded thunderstorms.

Isentropic Surface
A two-dimensional surface containing points of equal potential
temperature.

Isobar
A line connecting points of equal pressure.

Isobaric Chart
A weather map representing conditions on a surface of equal atmospheric pressure. For example, a 500 mb chart will display conditions at the
level of the atmosphere at which the atmospheric pressure is 500 mb.
The height above sea level at which the pressure is that particular
value may vary from one location to another at any given time, and
also varies with time at any one location, so it does not represent a
surface of constant altitude/height (i.e., the 500 mb level may be at
a different height above sea level over Dallas than over New York).

Isobaric Process
Any thermodynamic change of state of a system that takes a place at
constant pressure.

Isobath
In hydrologic terms, an imaginary line on the earth's surface or a
line on a map connecting all points which are the same vertical
distance above the upper or lower surface of a water-bearing formation
or aquifer.

Isochrone
A line on a chart connecting equal times of occurrence of an event. In
a weather analysis, a sequence plotted on a map of the frontal positions
at several different observation times would constitute a set of
isochrones.

Isodop
A contour of constant Doppler velocity values.

Isodrosotherm
A line connecting points of equal dew point temperature.

Isoheight
Same as a contour depicting vertical height of some surface above a
datum plane.

Isohel
A line on a weather map connecting points receiving equal sunlight.

Isohyet
A line connected points of equal precipitation amounts.

ISOL
Isolate(d)

Isolated
A National Weather Service convective precipitation descriptor for a
10 percent chance of measurable precipitation (0.01 inch). Isolated is
used interchangeably with few.

ISOLD
Isolated

Isopleth
A broad term for any line on a weather map connecting points with equal
values of a particular atmospheric variable (temperature, dew point,
etc.). Isotherms, isotachs, etc. are all examples of isopleths.

Isotach
A line connecting points of equal wind speed.

Isotherm
A line connecting points of equal temperature.

Isotropic
Having the same characteristics in all directions, as with isotropic
antennas. Directional or focused antennas are not isotropic.

Issuance Time
The time the product is transmitted.

ITCZ
Inter-tropical Convergence Zone. The region where the northeasterly and southeasterly tradewinds converge, forming an often continuous band of
clouds or thunderstorms near the equator.

ITWAS
Integrated Terminal Weather System
--- SBBSecho 3.13-Win32
* Origin: The Thunderbolt BBS - tbolt.synchro.net (618:250/33)

This weather glossary contains information on more than 2000 terms,
phrases and abbreviations used by the National Weather Service (NWS)...
the government agency who makes weather forecasts, and issues weather advisories, watches, and warnings, for the United States, and its
territories.

Many of these terms and abbreviations are used by NWS forecasters to communicate between each other and have been in use for many years and
before many NWS products were directly available to the public. It is the purpose of this glossary to aid you in better understanding NWS products.

***

Ice Age
A time of widespread glaciation.

Ice Boom
In hydrologic terms, a floating structure designed to retain ice.

Ice Bridge
In hydrologic terms, a continuous ice cover of limited size extending from shore to shore like a bridge.

Ice Crystals
A barely visible crystalline form of ice that has the shape of needles,
columns or plates. Ice crystals are so small that they seem to be
suspended in air. Ice crystals occur at very low temperatures in a stable atmosphere.

Ice Fog
Same as Freezing Fog; a suspension of numerous minute ice crystals in the
air, or water droplets at temperatures below 0° Celsius, based at the
Earth's surface, which reduces horizontal visibility; also called ice fog.

Ice Gorge
In hydrologic terms, the gorge or opening left in a jam after it has
broken.

Ice Jam
In hydrologic terms, a stationary accumulation that restricts or blocks streamflow.

Ice Nucleus
Any particle that serves as a nucleus in the formation of ice crystals in
the atmosphere.

Ice Pellets
(abbrev. IP) Same as Sleet; defined as pellets of ice composed of frozen
or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard
surfaces. Heavy sleet is a relatively rare event defined as an
accumulation of ice pellets covering the ground to a depth of 1" or more.

Ice Push
In hydrologic terms, compression of an ice cover particularly at the front
of a moving section of ice cover.

Ice Run
In hydrologic terms, flow of ice in a river. An ice run may be light or
heavy, and may consist of frazil, anchor, slush, or sheet ice.

Ice Shove
In hydrologic terms, on-shore ice push caused by wind, and currents,
changes in temperature, etcetera.

Ice Storm
An ice storm is used to describe occasions when damaging accumulations of
ice are expected during freezing rain situations. Significant
accumulations of ice pull down trees and utility lines resulting in loss
of power and communication. These accumulations of ice make walking and
driving extremely dangerous. Significant ice accumulations are usually accumulations of 1" or greater.

Ice Storm Warning
This product is issued by the National Weather Service when freezing rain produces a significant and possibly damaging accumulation of ice. The
criteria for this warning varies from state to state, but typically will
be issued any time more than 1/4" of ice is expected to accumulate in an
area.

Ice Twitch
In hydrologic terms, downstream movement of a small section of an ice
cover. Ice twitches occur suddenly and often appear successively.

Iceberg
A piece of a glacier which has broken off and is floating in the sea.

Icelandic Low
A semi-permanent, subpolar area of low pressure in the North Atlantic
Ocean. Because of its broad area and range of central pressure, it is
an area where migratory lows tend to slow down and deepen. It is
strongest during a Northern Hemisphere winter and early spring, centered
over Iceland and southern Greenland, and is the dominate weather feature
in the area. During the summer, it is weaker, less intense, and might
divide into two parts, one west of Iceland, the other over the Davis
Strait between Greenland and Baffin Island. Then the Azores or Bermuda
High becomes the dominate weather feature in the North Atlantic.

Icing
A coating of ice on a solid object.

Ideal Gas Laws
The thermodynamic laws applying to perfect gases.

IFR
Instrument Flight Rules

Impermeable
Material that does not permit fluids to pass through it.

Impervious
In hydrologic terms, the ability to repel water, or not let water
infiltrate.

IMPL
Impulse- Alternate term for Upper Level System and Shortwave; a general
term for any large-scale or mesoscale disturbance capable of producing
upward motion (lift) in the middle or upper parts of the atmosphere.

Import
In hydrologic terms, water piped or channeled into an area.

IMPT
Important

Impulse
(abbrev. IMPL) Alternate term for Upper Level System and Shortwave; a
general term for any large-scale or mesoscale disturbance capable of
producing upward motion (lift) in the middle or upper parts of the
atmosphere.

in Hg
Inches of Mercury

In-Cloud Lightning
(abbrev. IC) Lightning that takes place within the cloud.

Inactive Storage Capacity
In hydrologic terms, the portion of capacity below which the reservoir
is not normally drawn, and which is provided for sedimentation,
recreation, fish and wildlife, aesthetic reasons, or for the creation of
a minimum controlled operational or power head in compliance with
operating agreements or restrictions.

Inch-Degrees
The product of rainfall (in inches) multiplied by the temperature (in
degrees Fahrenheit) above freezing. Used as a measure of the snowmelting capacity of rainfall.

Inches of Mercury
(or in Hg) Unit of atmospheric pressure used in the United States. The
name comes from the use of mercurial barometers which equate the height
of a column of mercury with air pressure. One inch of mercury is
equivalent to 33.86 millibars or 25.40 millimeters. See barometric
pressure. First divised in 1644 by Evangelista Torricelli (1608-1647),
an Italian physicist and mathematician, to explain the fundamental
principles of hydromechanics.

To convert millibars (mb) to inches of mercury (in Hg), divide the
millibar reading by 33.86: in Hg = mb / 33.86

Inches of Runoff
In hydrologic terms, the volume of water from runoff of a given depth over
the entire drainage.

INCR
Increase

INDC
Indicate

Index of Wetness
The ratio of precipitation for a given year over the mean annual
precipitation.

Indian Summer
An unseasonably warm period near the middle of autumn, usually following
a substantial period of cool weather.

Indirect Hit
Generally refers to locations that do not experience a direct hit from a tropical cyclone, but do experience hurricane force winds (either
sustained or gusts) or tides of at least 4 feet above normal.

Industrial Consumption
The quantity of water consumed in a municipality or district for
mechanical, trade, and manufacturing purposes, in a given period,
generally one day. The per capita use is generally based on the total population of the locality, municipality, or district.

Infiltration
In hydrologic terms, movement of water through the soil surface into
the soil.

Infiltration Capacity
In hydrologic terms, the maximum rate at which water can enter the soil
at a particular point under a given set of conditions.

Infiltration Index
In hydrologic terms, an average rate of infiltration, in inches per
hour, equal to the average rate of rainfall such as that the volume
of rainfall at greater rates equals the total direct runoff.

Infiltration Rate
In hydrologic terms,

(1) The rate at which infiltration takes place expressed in depth of
water per unit time, usually in inches per hour.

(2) The rate, usually expressed in cubic feet per second, or million
gallons per day per mile of waterway, at which ground water enters an infiltration ditch or gallery, drain, sewer, or other underground conduit.

Inflow Bands
Bands of low clouds, arranged parallel to the low-level winds and moving
into or toward a thunderstorm. They may indicate the strength of the
inflow of moist air into the storm, and, hence, its potential severity. Spotters should be especially wary of inflow bands that are curved in a
manner suggesting cyclonic rotation; this pattern may indicate the
presence of a mesocyclone.

Inflow Jets
Local jets of air near the ground flowing inward toward the base of a
tornado.

Inflow Notch
A radar signature characterized by an indentation in the reflectivity
pattern on the inflow side of the storm. The indentation often is
V-shaped, but this term should not be confused with V-notch. Supercell thunderstorms often exhibit inflow notches, usually in the right
quadrant of a classic supercell, but sometimes in the eastern part of
an HP storm or in the rear part of a storm (rear inflow notch).

Inflow Stinger
A beaver tail cloud with a stinger-like shape.

Influent Seepage
In hydrologic terms, movement of gravity water in the zone of aeration
from the ground surface toward the water table.

Influent Stream
In hydrologic terms, any watercourse in which all, or a portion of the
surface water flows back into the ground namely the, vadose zone, or
zone of aeration.

Infrared Satellite Imagery
This satellite imagery senses surface and cloud top temperatures by
measuring the wavelength of electromagnetic radiation emitted from
these objects. This energy is called "infrared". High clouds are very
cold, so they appear white. Mid-level clouds are somewhat warmer, so
they will be a light gray shade. Low cloud are warmer still, so they
appear as a dark shade of gray or black. Often, low clouds are the same temperature as the surrounding terrain and cannot be distinguished at
all. The satellite picks up this infrared energy between 10.5 and 12.6 micrometer (um) channels.

Initial Detention
In hydrologic terms, the volume of water on the ground, either in
depressions or in transit, at the time active runoff begins.

Inland freshwater wetlands
In hydrologic terms, swamps, marshes, and bogs found inland beyond the
coastal saltwater wetlands.

Inland Hurricane Warning
Issued for interior counties that sustained winds of 74 mph or greater associated with a hurricane are expected within 24 hours.

Inland Hurricane Watch
Issued for interior counties when sustained winds of 74 mph or greater associated with a hurricane are possible within 36 hours.

Inland Tropical Storm Warning
Issued for interior counties when sustained winds of 39 to 73 mph
associated with a tropical storm are expected within 24 hours.

Inland Tropical Storm Watch
Issued for interior counties when sustained winds of 39 to 73 mph
associated with a tropical storm are possible within 36 hours.

INLD
Inland

Insolation
Incoming solar radiation. Solar heating; sunshine.

Instability
(abbrev. INSTBY)- The tendency for air parcels to accelerate when they
are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for
severe weather - the greater the instability, the greater the potential
for severe thunderstorms.

INSTBY
Instability- The tendency for air parcels to accelerate when they are
displaced from their original position; especially, the tendency to
accelerate upward after being lifted. Instability is a prerequisite for
severe weather - the greater the instability, the greater the potential
for severe thunderstorms.

Instrument Flight Rules
Refers to the general weather conditions pilots can expect at the
surface and applies to the weather situations at an airport during
which a pilot must use instruments to assist take off and landing. IFR conditions for fixed wing aircraft means the minimum cloud ceiling is
greater than 500 feet and less than 1,000 feet and/or visibility is
greater than 1 mile and less than 3 miles.

Instrument Shelter
A boxlike structure designed to protect temperature measuring
instruments from exposure to direct sunshine, precipitation, and
condensation, while at the same time time providing adequate
ventilation.

Intangible Flood Damage
In hydrologic terms, estimates of the damage done by disruption of
business, danger to health, shock, and loss of life and in general all
costs not directly measurable which require a large element of judgment
for estimating.

Interbasin Transfer
In hydrologic terms, the physical transfer of water from one watershed
to another.

Interception
In hydrologic terms, the process by which precipitation is caught and
held by foliage, twigs, and branches of trees, shrubs, and other
vegetation, and lost by evaporation, never reaching the surface of the
ground. Interception equals the precipitation on the vegetation minus streamflow and through fall.

Interception Storage Requirements
In hydrologic terms, water caught by plants at the onset of a rainstorm.
This must be met before rainfall reaches the ground.

Interflow
In hydrologic terms, the lateral motion of water through the upper
layers until it enters a stream channel. This usually takes longer to
reach stream channels than runoff. This also called subsurface storm
flow.

Intermediate Synoptic Times
The times of 0300, 0900, 1500, and 2100 UTC.

Intermittent Stream
In hydrological terms, a stream that flows periodically.

International Date Line
The line of longitude located at 180 degrees East or West (with a few
local deviations) where the date changes by a day. West of the line it
is one day later than east of the line.

Interplanetary Magnetic Field
(abbrev. IMF) In solar-terrestrial terms, the magnetic field carried
with the solar wind.

Interpolate
To estimate a value within an interval between two known values. This
technique is sometimes used with computer models for locations in
between the model's "gridpoints."

Intertropical Convergence Zone
(ITCZ) The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms
near the equator.

Intraseasonal Oscillation
Oscillation with variability on a timescale less than a season. One
example is the Madden-Julian Oscillation.

INTS
Intense

INTSFY
Intensify

INTST
Intensity

Inversion
(abbrev. INVRN) Generally, a departure from the usual increase or
decrease in an atmospheric property with altitude. Specifically it
almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase
occurs. An inversion is present in the lower part of a cap.

INVOF
In the vicinity of

INVRN
Inversion- Generally, a departure from the usual increase or decrease
in an atmospheric property with altitude. Specifically it almost always
refers to a temperature inversion, i.e., an increase in temperature
with height, or to the layer within which such an increase occurs. An
inversion is present in the lower part of a cap.

Ionosphere
A complex atmospheric zone of ionized gases that extends between 50 and
400 miles (80 to 640 kilometers) above the earth's surface. It is
located between the mesosphere and the exosphere and is included as part
of the thermosphere.

Ionospheric Storm
A disturbance in the F region of the ionosphere, which occurs in
connection with geomagnetic activity.

IPV
Improve

IR
Infrared Satellite Imagery

Iridescence
Brilliant spots or borders of colors in clouds, usually red and green,
caused by diffraction of light by small cloud particles. The phenomenon
is usually observed in thin cirrus clouds within about 30° of the sun
and is characterized by bands of color in the cloud that contour the
cloud edges.

Iridescent Clouds
Clouds that exhibit brilliant bright spots, bands, or borders of colors, usually red and green, observed up to about 30 degrees from the sun. The coloration is due to the diffraction with small cloud particles
producing the effect. It is usually seen in thin cirrostratus,
cirrocumulus, and altocumulus clouds.

Irrigation
In hydrologic terms, the controlled application of water to arable lands
to supply water requirements not satisfied by rainfall.

Irrigation Requirement
In hydrologic terms, the quantity of water, exclusive of precipitation,
that is required for crop production. It includes surface evaporation
and other economically unavoidable wastes.

Isallobar
A line of equal change in atmospheric pressure during a specified time
period.

Isentropic Analysis
A way in the forecaster can look at the atmosphere in 3-dimensions
instead of looking at constant pressure surfaces (such as the 850 mb,
700 mb, 500 mb, etc.) which are in 2-dimensions. In this analysis method,
the forecaster looks at constant potential temperature (the temperature
that it would take if we compressed or expanded it adiabatically to the pressure of 1000 mb) surfaces. Air parcels move up and down these
surfaces; therefore, the forecaster can see where the moisture is located
and how much moisture is available.

Isentropic Lift
Lifting of air that is traveling along an upward-sloping isentropic
surface.

Isentropic lift often is referred to erroneously as overrunning, but
more accurately describes the physical process by which the lifting
occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include
elevated convection in the form of embedded thunderstorms.

Isentropic Surface
A two-dimensional surface containing points of equal potential
temperature.

Isobar
A line connecting points of equal pressure.

Isobaric Chart
A weather map representing conditions on a surface of equal atmospheric pressure. For example, a 500 mb chart will display conditions at the
level of the atmosphere at which the atmospheric pressure is 500 mb.
The height above sea level at which the pressure is that particular
value may vary from one location to another at any given time, and
also varies with time at any one location, so it does not represent a
surface of constant altitude/height (i.e., the 500 mb level may be at
a different height above sea level over Dallas than over New York).

Isobaric Process
Any thermodynamic change of state of a system that takes a place at
constant pressure.

Isobath
In hydrologic terms, an imaginary line on the earth's surface or a
line on a map connecting all points which are the same vertical
distance above the upper or lower surface of a water-bearing formation
or aquifer.

Isochrone
A line on a chart connecting equal times of occurrence of an event. In
a weather analysis, a sequence plotted on a map of the frontal positions
at several different observation times would constitute a set of
isochrones.

Isodop
A contour of constant Doppler velocity values.

Isodrosotherm
A line connecting points of equal dew point temperature.

Isoheight
Same as a contour depicting vertical height of some surface above a
datum plane.

Isohel
A line on a weather map connecting points receiving equal sunlight.

Isohyet
A line connected points of equal precipitation amounts.

ISOL
Isolate(d)

Isolated
A National Weather Service convective precipitation descriptor for a
10 percent chance of measurable precipitation (0.01 inch). Isolated is
used interchangeably with few.

ISOLD
Isolated

Isopleth
A broad term for any line on a weather map connecting points with equal
values of a particular atmospheric variable (temperature, dew point,
etc.). Isotherms, isotachs, etc. are all examples of isopleths.

Isotach
A line connecting points of equal wind speed.

Isotherm
A line connecting points of equal temperature.

Isotropic
Having the same characteristics in all directions, as with isotropic
antennas. Directional or focused antennas are not isotropic.

Issuance Time
The time the product is transmitted.

ITCZ
Inter-tropical Convergence Zone. The region where the northeasterly and southeasterly tradewinds converge, forming an often continuous band of
clouds or thunderstorms near the equator.

ITWAS
Integrated Terminal Weather System
--- SBBSecho 3.14-Win32
* Origin: The Thunderbolt BBS - Little Rock, Arkansas (618:250/33)