National Weather Service United States Department of Commerce

Weather Glossary: I's

Ice Boom
A floating structure designed to retain ice.
Ice Bridge
A continuous ice cover of limited size extending from shore to shore like a bridge.
Ice Crystals (IC)
A fall of unbranched (snow crystals are branched) ice crystals in the form of needles, columns, or plates. They are also referred to as Diamond Dust.
Ice Fog
Occurs when the temperature is much below freezing and water vapor condenses directly as ice crystals (sublimation). Its formation is the same as radiation fog except the air temperature must be very cold. It occurs mostly in Arctic regions, but it is not unknown in middle latitudes during the cold season.
Ice Gorge
The gorge or opening left in a jam after it has broken.
Ice Jam
A stationary accumulation that restricts or blocks streamflow.
Ice Pellets (PL)
Precipitation of transparent and translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2" (5 mm), or less. Ice Pellets bounce when they contact the ground. It is sometimes called "Sleet". There are two main types:
  1. Hard grains of ice consisting of frozen raindrops, or largely melted and refrozen snowflakes.
  2. Pellets of snow encased in a thin layer of ice which have formed from the freezing, either of droplets intercepted by the pellets, or of water resulting from the partial melting of the pellets.
Intensity of sleet based on rate of accumulation or by visibility
Ice Pellet Intensity Rate-of-fall Visual Estimation
6-Minutes One Hour
Light < 0.01"
(< 0.25 mm)
Up to 0.10"
(Up to 2.5 mm)
Scattered pellets that do not completely cover an exposed surface regardless of duration. Visibility is not affected.
Moderate 0.01" to 0.03"
(0.25 to 0.76 mm)
0.11" to 0.30"
(2.8 to 7.6 mm)
Slow accumulation on the ground. Visibility reduced by ice pellets to less than 7 statute miles (11 kilometers).
Heavy > 0.03"
(> 0.25 mm)
> 0.30"
(> 7.6 mm)
Rapid accumulation on ground. Visibility reduced by ice pellets to less than 3 statute miles (5 kilometers).
Ice Push
Compression of an ice cover particularly at the front of a moving section of ice cover.
Ice Run
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
On-shore ice push caused by wind, and currents, changes in temperature, etc.
Ice Storm

It is usually 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 the loss of power and communications.

These accumulations of ice make walking and driving extremely dangerous. Significant ice accumulation are accumulations of ¼" (6 mm) 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 vary from state to state.
Ice Twitch
Downstream movement of a small section of an ice cover. Ice twitches occur suddenly and often appear successively.
Material that does not permit fluids to pass through it.
The ability to repel water, or not let water infiltrate.
Water piped or channeled into an area.
See upper level system.
Inactive Storage Capacity
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.
In-Cloud Lightning (IC)
Lightning that takes place within the cloud.
The product of inches of rainfall multiplied the temperature in degrees above freezing (Fahrenheit Scale), used as a measure of the snowmelt capacity of rainfall.
Inches of Runoff
The volume of water from runoff of a given depth over the entire drainage basin.
Inclined Staff Gage
A staff gage that is placed on the slope of a stream bank and graduated so that the scale reads directly in vertical depth.
Index of Refraction
See refractive index.
Index of Wetness
The ratio of precipitation for a given year over the mean annual precipitation.
Indian Summer
A warm, calm spell of weather occurring in autumn, especially in October and November. Usually follows a substantial period of cool weather.
Indirect Flood Damage
Expenditures made as a result of the flood (other than repair) such as relief and rescue work, removing silt and debris, etc.
Indirect Flood Damage
Expenditures made as a result of the flood (other than repair) such as relief and rescue work, removing silt and debris, etc.
Movement of water through the soil surface into the soil. The infiltration rate is a function of surface wetness soil texture, surface residue cover, irrigation application or precipitation rate, surface topography and other factors.
Infiltration Capacity
The maximum rate at which water can enter the soil at a particular point under a given set of conditions.
Infiltration Capacity Curve
A graph showing the time-variation of infiltration capacity. A standard infiltration capacity curve shows the time-variation of the infiltration rate which would occur if the supply were continually in excess of infiltration capacity.
Infiltration Index
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
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 another underground conduit.
Inflow Bands (or Feeder 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
Movement of gravity water in the zone of aeration from the ground surface toward the water table.
Influent Stream
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 (IR) 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. This imagery can be used both during the day and night.

Initial Detention
The volume of water on the ground, either in depressions or in transit, at the time active runoff begins.
Initial Loss
In hydrology, rainfall preceding the beginning of surface runoff. It includes interception, surface wetting, and infiltration unless otherwise specified.
Initial Moisture Deficiency
The quantity, usually expressed in depth of water in inches upon a unit area, by which the actual water content of a given soil zone (usually the root zone) in such area is less than the field capacity of such zone at the beginning of the rainy season. Also called Initial Water Deficiency.
Initial Water Deficiency
The quantity, usually expressed in depth of water in inches upon a unit area, by which the actual water content of a given soil zone (usually the root zone) in such area is less than the field capacity of such zone at the beginning of the rainy season. Also called Initial Moisture Deficiency.
Inland Freshwater Wetlands
Swamps, marshes, and bogs found inland beyond the coastal saltwater wetlands.
Incoming solar radiation. Solar heating; sunshine.
Instability (Unstable Air)
A state of atmosphere in which the vertical distribution of temperature allows rising, warm air to continue to rise and accelerate. This kind of motion is conducive for thunderstorm development. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms. See lifted index and sounding.
Instantaneous Unit Hydrograph
The theoretical, ideal, unit hydrograph that has a infinitesimal duration.
Instream Use
The use of water that does not require withdrawal or diversion from its natural watercourse; for example, the use of water for navigation, recreation, and support of fish and wildlife.
Intangible Flood Damage
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
The physical transfer of water from one watershed to another.
Intercepting Drain
A drain constructed at the upper end of the area to be drained, to intercept surface or ground water flowing toward the protected area from higher ground, and carry it away from the area. Also called Curtain Drain.
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
Water caught by plants at the onset of a rainstorm. This must be met before rainfall reaches the ground.
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.
Interflow Runoff
The parts of runoff, caused by precipitation and/or snowmelt, that enters the ground and moves in upper levels of the soil mantle above the water table, heading towards the streams.
Intermediate Zone
The subsurface water zone below the root zone and above the capillary fringe.
Intermittent Stream
A stream that flows periodically. Compare perennial stream.
International SIGMET (SIGnificant METeorlogical Information)

This NWS aviation product advises of weather potentially hazardous to all aircraft. The purpose of this information is to advise pilots of the occurrence or expected occurrence of en-route weather phenomena which may affect the safety of aircraft operations. Criteria for Domestic and International SIGMETs are similar, however the format, contractions, and wording used are different.

International SIGMETs are issued for oceanic areas adjacent to the United States by a Meteorological Watch Office (MWO). The National Weather Service has MWOs at Anchorage, AK, Guam Island in the Pacific Ocean, Honolulu, HI, Kansas City, MO, and the Tropical Prediction Center in Miami, FL.

International SIGMET criteria are:

  • Thunderstorms,
  • Tornadoes,
  • Lines of thunderstorms,
  • Embedded thunderstorms,
  • Large areas of thunderstorms,
  • Large hail,
  • Tropical cyclone,
  • Severe icing,
  • Severe or extreme turbulence,
  • Dust storms and sandstorms lowering visibilities to less that three miles (4.8 km), and
  • Volcanic Ash

These SIGMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3,000 square miles (4,800 square kilometers) at any one time.

International SIGMETs are issued for 12 hour periods for volcanic ash events, 6 hours for hurricanes and tropical storms and 4 hours for all other criteria. If conditions persist beyond the forecast period, the SIGMET is updated and reissued.

Intertropical Convergence Zone (ITCZ)
The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere.
Inundation Map
A map delineating the area that would be inundated in the event of a dam failure.

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.

This occurs when warm air sits over cold air, possibly trapping moisture and pollutants in the surface air layer. An inversion is present in the lower part of a cap.

An acronym for "in vicinity of".
An acronym for Infrared. See Infrared Satellite Imagery.
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.
Irrigated Area
The gross farm area upon which water is artificially applied for the production of crops, with no reduction for access roads, canals, or farm buildings.
The controlled application of water to arable lands to supply water requirements not satisfied by rainfall.
Irrigation Efficiency
The percentage of water applied that can be accounted for in soil moisture increase for consumptive use.
Irrigation Requirement
The quantity of water, exclusive of precipitation, that is required for crop production. It includes surface evaporation and other economically unavoidable wastes.
A line of equal change in atmospheric pressure during a specified time period.
Isentropic Analysis

It is 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 1,000 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.

The forecaster can also induce stability the atmosphere. When these surfaces are closely packed together, the atmosphere is very stable. When these surfaces are far apart, the atmosphere is unstable. This type of analysis helps to forecast the freezing level and icing, clear air turbulence, and overrunning convection.

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.
Lines of equal barometric pressure as shown on a weather map.
Isobaric Chart
Same as a constant pressure chart.
Isobaric Process
Any thermodynamic change of state of a system that takes a place at constant pressure.
Isobaric Surface
A surface along which the atmospheric pressure is everywhere equal.
A contour (or surface) of equal depth in a body of water. It is plotted on a bathymetric chart. It is also a contour or surface of equal depth of the water table below the ground surface.
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.
A contour of constant Doppler velocity values.
A line on a chart connecting points of equal dew point.
A line (or surface) connecting points of equal or constant salinity in water bodies or groundwater.
Same as a contour depicting vertical height of some surface above a datum plane.
A line on a weather map connecting points receiving equal sunlight.
A line on a weather map connecting points receiving equal precipitation amounts.
A National Weather Service convective precipitation descriptor for a 10% chance of measurable precipitation (0.01"/0.25mm). Isolated is used interchangeably with few. See Precipitation Probability (PoP).
Isolated Storm
An individual cell or a group of cells that are identifiable and separate from other cells in a geographic area.
A line on a weather map connecting constant thickness (layer of atmosphere).
A line on a weather map connecting points of equal wind speed.
A line on a weather map connecting points of equal temperature.
Of equal or constant temperature with respect to either space or time.
Isothermal Atmosphere
An atmosphere in hydrostatic equilibrium in which the temperature is constant with altitude and in which, the pressure decreases exponentially upward.
Isothermal Layer
Any layer where the temperature is constant with altitude, such that the temperature lapse rate is zero. Specifically, the approximately isothermal region of the atmosphere immediately above the tropopause.
Isothermal Process
Any thermodynamic change of state of a system that takes place at constant temperature.
Having the same characteristics in all directions, as with isotropic antennas. Directional or focused antennas are not isotropic.
Acronym for Intertropical Convergence Zone.