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

(Also called ice-crystal fog, frozen fog, frost fog, frost flakes, air hoar, rime fog, pogonip.) A type of fog, composed of suspended particles of ice; partly ice crystals 20 to 100 micron in diameter, but chiefly (especially when dense) ice particles about 12–20 micron in diameter, formed by direct freezing of supercooled water droplets with little growth directly from the vapor. It occurs at very low temperatures, and usually in clear, calm weather in high latitudes. The sun is usually visible and may cause halo phenomena. Ice fog is rare at temperatures warmer than -30°C, and increases in frequency with decreasing temperature until it is almost always present at air temperatures of -45°C in the vicinity of a source of water vapor. Such sources are the open water of fast-flowing streams or of the sea, herds of animals, volcanoes, and especially products of combustion for heating or propulsion. At temperatures warmer than -30°C, these sources can cause steam fog of liquid water droplets, which may turn into ice fog when cooled (see frost smoke). See ice-crystal haze, arctic mist.

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. A Winter Storm Warning is issued for sleet or a combination of sleet and snow based on total accumulation which is locally defined by area.

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 ¼" 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.

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.

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

Inundation

The process of covering normally dry areas with flood waters.

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

IP

Ice pellets (sleet). See "PL"

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

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