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Weather Glossary - I

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ICAO

Abbreviation for International Civil Aviation Organisation.

Ice Pellets

Precipitation in the form of pellets of ice with diameter 5mm or less.

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, usualy 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 inflow band with a stinger-like shape.

Infrared satellite image

Infrared (IR) satellite images are a picture of cloud cover, using the infrared spectrum rather than the visible. IR satellite images indicate the temperature of the cloud-top, by measuring the heat radiating from the clouds. The colder the cloud-top, the brighter the cloud will be on the resulting satellite image. Severe thunderstorms normally have very cold tops, so will show up as bright spots on images.

In general, the clouds will be colder than the land or ocean, so the clouds will be easy to identify. In winter months the land surface can reach temperatures similar to the top of low cloud cover, making it difficult to distinguish cloud from the land. Low cloud can also occasionally be difficult to identify on IR images.See also Satellite images.

Inshore

The coastal waters zone adjacent to the coastline within which the majority of small craft operate and which is usually within 5 to 10 nautical miles of the coastline.

Insolation

Incoming solar radiation. Solar heating, sunshine.

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. See lifted index and sounding.

Intensity of precipitation

Slight or light:

  • Rain: Individual drops easily identified, puddles form slowly, small streams may flow in gutters.
  • Drizzle: Can be felt on the face but is not visible. Produces little runoff from roads or roofs. Generally visibility is reduced, but not less than 1000 m.
  • Snow: Small sparse flakes. Generally visibility is reduced, but not less than 1000 m.
  • Hail: Sparse hailstones of small size, often mixed with rain.

Moderate:

  • Rain: Rapidly forming puddles, down pipes flowing freely, some spray visible over hard surface.
  • Drizzle: Window and road surfaces streaming with moisture. Visibility generally between 400 and 1000 m.
  • Snow: Large numerous flakes and visibility generally between 400-1000 m.
  • Hail: particles numerous enough to whiten the ground.
  • Heavy:
  • Rain: falls in sheets, misty spray over hard surfaces, may cause roaring noise on roof.
  • Drizzle: Visibility reduced to less than 400 m.
  • Snow: Numerous flakes of all sizes. Visibility generally reduced below 400 m.
  • Hail: A proportion of the hailstones exceed 6 mm diameter.

Inter Tropical Convergence Zone

A relatively narrow zone of persistent thunderstorms in tropical waters. It marks the meeting or convergent point of winds originating from both the northern and southern hemispheres. The ITCZ usually lies poleward of the Equatorial trough.

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.

Inversions are common in winter when there is a large anticyclone present. Subsiding air in the anticyclone warms as it descends and produces a layer of warmer air around 1000-2000m above the surface. This inversion is strengthened at night due to radiational cooling of the lowest levels of the atmosphere.

Inversions can trap pollutants in the lower parts of the atmosphere, leading to poor visibility, especially in winter when wood fires increase the levels of particles in the atmosphere.

An inversion is present in the lower part of a cap. See sounding.

Isallobar

The line of equal change in atmospheric pressure during a certain time period. It marks the change in pressure tendency.

Isentropic lift

Lifting of air that is travelling 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 rain, 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 surfaces

A surface of equal pressure. Isobaric surfaces are used in upper level charts where geopotential heights are contoured to decribe the upper level features. These charts are typically produced at standard levels such as 850hPa, 700hPa, 500hPa etc. In additional to geopotential height, these can charts are also used to display a variety of parameters such as streamlines, vorticity, moisture, temperature and so on.Also known as a constant pressure surface.

Isodrosotherm

A line connecting points of equal dew point temperature.

Isohyet

A line connecting points of equal precipitation amounts.

Isopleth

General term for a line connecting points of equal value of some quantity. Isobars, isotherms, etc. all are examples of isopleths.

Isotach

A line connecting points of equal wind speed.

Isotherm

A line connecting points of equal temperature.

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